Foraging patterns of kestrels and shrikes andtheir relation to an optimal foraging model

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Authors Mills, Gregory Scott

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© 1979

GREGORY SCOTT M I L L S

A L L RIGHTS RESERVED

FORAGING PATTERNS OF KESTRELS AND SHRIKES AND THEIR RELATION TO AN OPTIMAL FORAGING MODEL

byGregory Scott Mills

A Dissertation Submitted to the Faculty of theDEPARTMENT OF ECOLOGY AND EVOLUTIONARY BIOLOGY

In Partial Fulfillment of the Requirements For the Degree ofDOCTOR OF PHILOSOPHY

In the Graduate CollegeTHE UNIVERSITY OF ARIZONA

19 7 9

Copyright 1979 Gregory Scott Mills

THE UNIVERSITY OF ARIZONA

GRADUATE COLLEGE

I hereby recommend that this dissertation prepared under my

direction by ___________ Gregory Scott Mills______________________

entitled FORAGING PATTERNS OF KESTRELS AND SHRIKES AMD THEIR

RELATION TO AN OPTIMAL FORAGING MODEL__________________

be accepted as fulfilling the dissertation requirement for the

degree of ______________ Doctor of Philosophy_____________________

Dissertation Director Date

As members of the Final Examination Committee, we certify

that we have read this dissertation and agree that it may be

presented for final defense.

C }j-Iv ______ __f z /s ^ y 'T 'S

... u^u_ . ' v. . ' Lz-v ________ Jijzc* 7 g____l j . l U L . r ̂ ??

Final approval and acceptance of this dissertation is contingent on the candidate's adequate performance and defense thereof at the final oral examination.

STATEMENT BY AUTHOR

This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library*

Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made* Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the copyright holder*

SIGNED; 5

ACKNOWLEDGMENTS

I would like to thank Ho R=, Pulliam, Co R= Tracy, Jc Re Silli- man, W0 Ao Calder, Stephen Mo Russell, and, especially, Jo Ho Brown for their comments and suggestions concerning the ideas presented in this papero Steve Sutherland made valuable contributions to the concept of optimal perch height and Tom Caraco kindly shared his ideas on risk aversiono Jo Ho Brown and Ao Co Gibson made valuable contributions to the preparation of the manuscripto I thank the staff and officers of The Research Ranch for their consent and aid in some aspects of this study0

TABLE OF CONTENTS

PageLIST OF ILLUSTRATIONS o o o o o o o o o o o o o o o o o o vLIST OF TABLES © © © © © © © © © © © © © © © © © © © © © vxABSTRACT © © o o © © © © © © © © © © © © © © © © © © © © © vxx

1© INTRODUCTION © © © © © © © © © © © © © © © © © © © © © © © I2© PATTERNS OF HUNTING FROM PERCHES © © © ............. © © 4

An Equation for Net Energy Gain © © © © © © © © © © © 4Metiiods © o © © © © © © © © © © © © © © © © © © © © © ^Patch Choxce © o © © © © © © ©©© © © © © © © © © © © 6

Geometry of Hunting from Perches © © © © © © © © © 7Optxmal Perch Hexght © © © © © © © © © © © © © © © 12Predictions and Tests © © © © © © © © © © © © © © 19

Movement Between Patches © © © © © © © © © © © © © © © 27Allocation of Time in Patches © © © © © © © © © © © © 30Opt xmal Dxet © © © © © © © © © © © © © © © © © © © © © 3 *̂Comparison of Foraging Patterns of Kestrels andShrxkes o o © © © © © © © © © © © © © © © © © © © © 43

Concurrent Goals © © © © © © © © © © © © © © © © © © o 44Conclusxons © o o © © © © © © © © © © © © © © © © © © 46

3© PATTERNS OF HUNTING WHILE HOVERING © © © © © © © © © © © © 48Methods © © © © © © © © © © © © © © © © © © © © © ©• © 49Advantages of Hunting While Hovering © © © © © © © © © 49Costs of Hoverxng © © © © © © © © © © © © © © © © © © 31Hoverxng Hexght o o © © © © © © © © © © © © © © © © © 39Optimal Wind Speed for Hovering © © © © © © © © © © © 61Hoverxng Txme © © © o © © © © © © © © © © © © © © © © 66Hovering as an Alternate Hunting Technique © © © © © © 68

LIST OF REFERENCES © © © © © © o © © © © © © © © © © © © © 71

iv

LIST OF ILLUSTRATIONS

Figure Pagelo Geometric considerations of perches and vegetation „ = = 82= Relative areas visible to a perched bird showing effects

of grass height and density 0 = 0 0 0 0 0 0 = o b o o = 103= Approximate increase in visible area with increasing

hunting height 0000. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 114o Approximate prey encounter rate with increasing perch

height o 0 0 0 0 0 o 0 0 - 0 0 0 00 00 o o o o 00 0 0 0 155o Effects of increasing height on net energy gain per

attack 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 l660 Rate of net energy gain as a function of hunting height. 177o Index of grasshopper abundance in months of August

through December 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 , 00 2^80 Success rates and lengths of giving-up times for shrikes

(A) and kestrels (B) as functions of season 0 0 0 0 0 0 339= Two possible mechanisms for threshold renewal 0 0 0 0 0 4110o The effect of air speed on the power required to fly 0 o 9211= Hovering effort as a function of wind speed 0 = 0 0 = 0 9812= Hovering height as a function of wind speed at 2. m = « = 69

v

LIST OF TABLES

Tablelo Effects of perch height on the distances traveled to

prey for kestrels and shrikes » o o . < , » . o o . . o o o2= Perch height related to time of year o o =, 0 « 0 , o = o-3= The relation between perch height and wind speed4.e. Effects of perch height on distances traveled between

p e r c h e s o o o o o o o o o o o o o o o o o o o o o o o b o

5® Effects of wind speed on distance traveled betweenperches 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

60 The effects of perch height on giving-up time forkestrels and shi* ikze s o o o o o o o o 00 0.0.00 00 o o

7o Distances to prey at different times of year 0 0 o o 0080 Effects of distance to prey on success rates of kestrels

and shr r k e S o o o o o o o o o o 0 0 0 0 0 0 0 0 0 0 0 0 09 o Effects of time on perch on success rates of kestrels

and shrikes 00 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 010o Effects of time since last prey capture on success rate

of kestrels 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 011o Comparison of prey types' and rates of prey capture from

perches and hovers 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 01 2 o Calculations of Vmp for the American Kestrel o o o 0 0 o

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ABSTRACT

Although considerable literature on optimal foraging theory exists, few field tests have been conducted* To make such tests, winter foraging patterns of American Kestrels (Falco sparverius) and loggerhead Shrikes (Lanins ludovicianus) were observed in southeastern Arizona to compare actual patterns with predictions of an optimal foraging model developed for predatory ground-hunting birds* The model is developed from considerations of foraging theory, energetics, and perch and vegetational characteristics that influence vision of the predator* Two hunting techniques are analyzed; hunting from perches by kestrels and shrikes, and hunting while hovering by kestrels*

Analysis of hunting from perches includes patch selection, movement between patches, allocation of time in patches, and prey selection* For kestrels and shrikes, patch selection primarily involves selection of a perch* Considerations of factors affecting hunting from perches predict the existence of an optimal hunting height which increases with decreasing prey abundance and increasing prey size* When comparable prey decrease in abundance, kestrels and shrikes hunt more often from higher perches* Selection of perch height is also affected by wind; birds perch lower at high wind velocities* Kestrels and shrikes appear to minimize time and energy spent traveling between patches; they nearly always forage unidirectionally and travel greater distances between high perches than low ones* Givirig-up times, i*e*,

vii

times spent in patches where no prey were attacked, appear to be determined in part by previous hunting times; giving-up time correlated better with the previous three hunting times than just the last one0 Prey selection appears to be strongly influenced by three factors: distance from perch, evaluation of probability of success, and size and type of preyo Success rate decreases with hunting time„ The interpretation is that a threshold of prey selectivity diminishes with timeo Such a diminishing threshold could account for partial preferences in diets0 Contrary to predictions of some optimal foraging models, prey selectivity appeared to increase with decreasing prey densityo An explanation for this pattern may be that birds minimize variance in food intake by avoiding riskso

' Analysis of hunting while hovering primarily concerns the energetics of hovering flight and their effects on the utilization of this foraging method,. Hovering allows kestrels to hunt in areas without suitable perches, but the relatively high energetic costs restrict its use to times of favorable wind speeds,. The optimal wind speed at which to hover is apparently equal to the air speed at which flight is least costly= Most hovering occurs when optimal wind speed and optimal hunting height coincide; when they do not, kestrels appear to adopt a compromise between the two„ Because wind speed increases with height, hovering height decreases as wind speed increases« Duration of individual hovers from which prey was not attacked was affected by time of year, duration of the previous hover from which prey was attacked, and wind speed* Rate of energy intake is greater when

hovering than when hunting from perches» Hovering appears to be an important alternative foraging" strategy for some species of birds at times of favorable environmental conditions^

CHAPTER 1

INTRODUCTION

Optimal foraging theory shows great promise for providing a better understanding of animal behavior and community structure (Pyke, Pulliam and Charnov 1977), but relatively few studies have fully assessed its application in natural systems* In most papers, Optimal foraging has been treated only theoretically on a strategic level, e * go, Schoener 1971 and Charnov 1973= The scarcity of field tests may be in part due to difficulties in translating theory on a strategic level to testable predictions on a tactical level* On a strategic level, terms are often vaguely defined and it is possible to focus on only one variable while others are ignored* On a tactical level, terms must be defined more precisely and many variables that potentially affect an animal's behavior must be considered simultaneously* Another problem that may contribute to the scarcity of field tests of foraging theory is the difficulty in selecting a system where an animal can be observed for extended periods*

A crucial part of optimal foraging models is identification of an animal's goal (Schoener 1971, Charnov 1973, Pyke et al* 1977)= Although the choice of goal may affect the overall time budget of an animal, many goals ultimately reduce to the prediction that an animal

' should attempt to maximize net energy intake while foraging* To do

1

this, an animal must make a number of choices, Charnov (1973) identified a hierarchy of such choices: a habitat in which to hunt, apatch within that habitat, a foraging method to use in the patch, and prey types to be pursued. Although I believe that such a hierarchy, is a useful tool for analyzing foraging behavior, I do not believe that the four choices must occur in the order listed. In particular, foraging method may be determined before habitat or patch selection occurs because particular kinds of animals may be constrained by evolutionary adaptations which restrict their range of foraging methods.

In this paper I construct a tactical model for some aspects of foraging of ground-hunting predatory birds from considerations of perch and vegetation characteristics, energetic costs, and ideas from optimal foraging literature. Foraging of these birds provides a good system to test foraging theory because complicating variables are minimized, terms can be operationally defined, and foraging activities are easily observed. The model is developed assuming that these birds are attempting to maximize net energy intake while foraging. This goal appears to be appropriate for predatory birds, and all foraging behaviors in this study were predicted from this assumption. However, some data collected during this study suggest that prey selection may also be influenced by minimizing variance in energy intake. In most cases predictions of foraging behavior generated from both goals are the same, and, therefore, discrimination between the two is not usually critical. Concurrent goals, such as avoiding predation, or maintaining territories do not appear to significantly affect foraging behavior of

these birds,, A more thorough discussion of these factors is presented later in this paper»

Qualitative predictions of the model developed are tested and, in many cases, verified in the field with foraging patterns of American Kestrels (Falco sparverius) and Loggerhead Shrikes (Lanins ludovicianusX Predictions are based primarily on foraging theory and bonsiderations of flight energetics and geometric properties of hunting from perches, but were also biased by known information of kestrel biology0 Some predictions were changed during the course of the study in light of new considerations, but all predictions were a priori in the sense that they were made before the extensive data were analysed«, These predictions can also be treated as hypotheses and tested independently by other investigators working with other organises or in different habitatso

Pyke et al= (1977) have divided foraging theory into four categories: diet, patch choice, allocation of time in patches, and patterns of movement between patcheso In Chapter 2 of this paper, foraging method, hunting from perches, is treated as a constant while behaviors associated with patch choice, allocation of time in patches, and patterns of movement between patches are examinedo I also analyze some aspects of diet, specifically quality evaluation of prey by distance and capture success rate* In Chapter 3, I analyze factors influencing the choice between two foraging methods for ground-hunting predatory birds, hunting while hovering and hunting from perches^Patch choice and allocation of time in patches for birds hunting while hovering are also examined®

CHAPTER 2

PATTERNS OF HUNTING FROM PERCHES

An Equation for Net Energy GainRate of net energy gain of a bird hunting from perches can be

represented by the equationsEg = E/A ° A/t - RMR - C/t (1)

where Eg is net rate of energy gain; E/A is the net energy gained per attack; A/t is the attack rate; RMR is resting metabolic rate, here defined as all the energy required to hunt from a perch including thermoregulation; and C/t is the rate of energy expended changing perches when no prey are attacked,,

Net energy gained per attack (E/A) is a function of other variables such that:

where fs is the frequency of success (success rate), e is mean energy content of prey attacked, and a is the mean energy expended in making an attack including costs to fly to the ground and return to a perch0

Similarly, attack rate (A/t) is a function of other variables' such that:

where Pp is the proportion of prey encountered that are attacked and N/t is the encounter rate with prey over the entire foraging bouto

E/A = fs(e) - a (2)

A/t = Pp (N/t) (3)

• 5To increase net energy intake, a bird can increase E/A or A/t

or decrease KMR or C/t, E/A can be increased by increasing fs, or e, orby decreasing ao Attack rate can be increased by increasing N/t or Ppo Because these variables are interrelated and tradeoffs occur between some, the exact combination of values that results in a maximum net energy gain depends on the relative values of eacho Because many of the terms cannot be measured, these equations will not be evaluated numerically but used to provide an understanding of the factors that affect hunting from percheso Qualitative predictions and analyses of foraging behaviors can then be madeo

MethodsObservations of foraging kestrels and shrikes were made in the

grasslands of southeastern Arizona from September 1975 to March 1977° Although some data were collected throughout the year, most observations were made in fall and winter monthso Most data were collectedbetween 0900 and 1500 h0 Birds were watched with lOx binoculars or a 15-60x telescope from a parked vehicle0

Data taken on foraging birds included perch height, distance traveled between perches, distance to prey, success rate of attacks, and time spent hunting on percheso A bird was considered to be foraging when it showed active signs of searching the groundo Except for a few times in early fall, birds appeared to forage almost constantly„ Time spent in nonforaging activities (such as preening) was subtracted from the time on perches* Most birds were followed as long as possible *

Times were measured with a stopwatch and data were recorded on a portable tape recorder and transcribed later. Heights and distances were estimated visually but were calibrated periodically by taking precise measurements. Wind speeds were measured with a Dwyer handheld wind meter.

In one area perches consisting of poles (agave stalks) 3 to 5 m high were erected on three successive fenceposts spaced 3 m apart such that perch height increased from 2 (fenceposts) to 5 m at approximately one meter intervals. Only 2 such units were erected, but 13 others of perches 2, 3, and 4 m high and 4 units of 2 and 3 m poles were also constructed serially in the same area.

No quantitative study of prey populations was conducted but grasshoppers were censused along a 1750 m route in grassland habitat. Kestrel diets were monitored by analysis of pellets found at roosts.

Patch ChoiceAlthough the term "patch" has been widely used in the litera

ture, it is often ambiguous and poorly defined. For perch hunting birds, a patch can be operationally defined as the area that can be hunted from a perch; thus, time in a patch and movement between patches are easily measured.

Net caloric intake can be increased by foraging in patches where encounter rate with prey (N/t) is high. For birds hunting from perches, encounter rate is a function of prey availability and area hunted. Prey availability is some function of prey density, prey type, vegetational structure, and weather. One way encounter rate can be

increased is by hunting in areas where prey availability is highero In a fine-grained situation, patches must be visited for prey availability to be assessed^ Thus, variations in prey availability would have little effect on patch selection, although it would contribute significantly to habitat selection* Much of the area in which kestrels and shrikes foraged appeared to be homogeneous so that birds probably could not assess prey availability before visiting patches*

Encounter rate can also be increased by hunting a larger area* Area hunted can be increased by hunting in habitats with little vegetation so visibility is increased, and by using higher perches. But increasing perch height also increases foraging costs and handling time of prey. The following analysis of the geometry of perches and vegetative structure on the terms of equations (1) and (2) suggests that there exists an optimal height from which to hunt arid that patches should be chosen on the basis of perch height and vegetative structure.

Geometry of Hunting from PerchesFigure 1 is a model that provides a basis for estimating the

relative area of ground that is visible from a perch, where h equals perch height, g is the. average height of grass clumps or other vegetation, d is the average distance between these clumps, and y is the distance from a given clump to the base of the perch (y is a multiple of d). There is a distance, x, behind each clump where the ground is not visible from the top of the perch. This distance increases with increasing distance of the clump from the perch until at some point it equals the average distance between grass clumps and no ground is

8

\\\

h

\ \\\

Figure 1. Geometric considerations of perches and vegetation,

9visible« Thus, ground area visible to a perched bird can be visualized as concentric rings of decreasing width around a percho Though the width of each ring decreases with distance, the size increases so that the area of each ring does not necessarily decrease„ Ring area as a function of distance from the perch, depends on the average clump distance and height but, in general, increases to a point and then decreases = Some examples are shown in Figure 2o When grass clumps are tall and closely spaced, very little ground is visible0

By increasing perch height, a bird can hunt more area because the distance behind each grass clump that is not visible decreases»But the geometric properties are such that increasing increments of perch heights result in successively small decreases in x For . the area within a given radius around a perch, area that. can be hunted increases in the manner shown in Figure J> and becomes asymptotic at the maximum area within the specified radiuso

This development of effects of perch height is based on simplified but robust assumptions«, Grass clumps or other vegetation obviously are not opaque and of even height, and do not occur in continuous concentric rings at regular distances around perches= The following analysis also assumes that prey are flato However, considerations of the real properties of vegetation and prey have little effect oh the qualitative aspects of the model which realistically indicates the unavailability of some prey in vegetation*

This analysis of perch geometry and area of the ground visible from perches leads to the following prediction*

VISI

BLE

AR

EA

DISTANCE FROM PERCH

Figure 2. Relative areas visible to a perched bird showing effects of grass height and density. — For curves A and B, g = 50 (tall grass) and d = 8 and 12* respectively. For curves C and D, g = 5 (short grass) and d = 8 and 12* respectively. Areas were calculated on the basis of a perch height (h) equal to 900. All numbers in cm.

HO

VISI

BLE

AR

EA

11

HEIGHT

Figure 3* Approximate increase in visible area with increasinghunting height. — Dotted line represents maximum area visible within a specified radius around a perch (see text).

12Prediction 1: A greater proportion of attacks should occur at

greater distances from tall perches than from short ones because more area is visible at greater distances,.

Test of Prediction Is Kestrels and shrikes made greater proportions of attacks at greater distances from higher perches (Table 1)„ Distances traveled for prey were significantly shorter for shrikesthan for kestrels from perches of equal heights (for perches

If prey are taken primarily from the ground, which appears to be a valid assumption for kestrels and shrikes, rate of prey encountered per search time (N/tg) should increase with height in approximately the same manner as area that can be hunted (Figo 3)° But handling time

13Table l o Effects of perch height on the distances traveled to prey

for kestrels and shrikes»

Kestrels ShrikesPerch % attacks at: attacks at:

Height (m) n 0-20 m 21-40 m >40 m n 0-10 m 11-20 m >20 m

also increases with height because the time to attack and return to a perch increases* As handling time increases, search time decreases; thus, encounter rate for the total time hunting (N/t) increases with perch height to a maximum and then decreases, as shown in Figure 4*The exact shape of the curve depends on the relative values of N/tg and th/tSo Increasing prey density increases encounter rate per search time (N/tg) but does not affect handling time; therefore, the perch height where encounter rate is maximized decreases as prey density increases*

Increases in perch height also increase foraging costs* Cost to attack prey (a) increases with height because the cost to return to the perch increases, though the cost of the drop from the perch to the ground is probably negligible because it is gravity assisted* It seems reasonable that cost of an attack is directly proportional to height* The increased height causes the energy gained per attack (E/A) in Equation (1) to decrease as shown in Figure 5° If mean energy content of prey were increased, the line in Figure 5 would shift'Upwards*

If attack rate were proportional to encounter rate arid net energy gain per attack decreased with height as outlined above, an optimal hunting height, where the net rate on energy gained is maximized, could be found by multiplying the equations of the curves in Figures 4 and 5* The result of such a multiplication is shown in Figure 6*. The preceding analysis suggests that optimal hunting height increases as mean prey size increases or as density decreases*

In some cases, resting metabolic rate might have a significant effect on optimal hunting height* RMR varies with environmental

PREY

EN

COUN

TER

RA

TE15

HEIGHT

Figure 4. Approximate prey encounter rate with increasing perch height.

NET

ENER

GY

GAIN

PE

R A

TTA

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16

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Figure 5* Effects of increasing height on net energy gain per attack. — Line B is for larger prey.

RATE

OF

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ERGY

G

AIN

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Figure 6. Rate of net energy gain as a function of hunting height. — This curve is obtained by multiplying net energy gain per attack (Fig. 5) and attack rate, which is assumed to be proportional to encounter rate (Fig. 4). See text for further explanation.

18conditions, especially temperature and windo At times of high winds, RMR could increase due to heat loss or an increase in the effort required to remain on a percho Because wind speed increases with height, BMB should be greater on higher percheso Birds could reduce this cost by perching lower or in a more protected place, otherwise RMR is a fixed cost for any given time or place

19increase in cost is probably small in comparison to costs of making attacks because only a horizontal flight is required*

Predictions and TestsThe previous discussion of the factors influencing costs and

benefits of foraging from perches suggests that patch selection should be based on perch height and vegetation density* From this analysis I make the following predictions*

Prediction 2s Areas of tall, dense vegetation should be avoided because little ground is visible regardless of perch height and the probability of prey escaping in the vegetation is high*

Prediction 3s Because larger birds generally take larger prey than smaller ones, their optimal hunting height should be higher and they should select higher perches* Different-sized birds are not strictly comparable, however, because energy to gain height is not the sane * Nevertheless , female kestrels, which weigh about 110 g, would be expected to perch the highest, male kestrels (about 100 g) slightly lower, and shrikes (about 50 g) considerably lower than either sex of kestrel*

Prediction 4: Optimal hunting height should increase as mean preysize increases or as prey density decreases*

Prediction 5: If wind velocity is sufficient to increase energetic costs due to heat loss or effort to remain on perches, optimal hunting height should decrease with increasing wind speed because wind speed is lower near the ground*

20Testing these predictions in the field was complicated by

several factors,. Perches in nature rarely present birds with continuous choices of height« In the study area, fenceposts (lo5 to 2 m) and utility poles and wires (8 to 10 m) were the most common and often the only perches available„ Some perches were apparently not suitable for reasons other than height,. Neither kestrels nor shrikes were ever seen perched on electric wires of utility poles; telephone wires were always usedo Both species also showed a definite preference for perches that provided greater stability; wooden fenceposts were preferred to metal ones, utility poles or wires near poles were preferred to wires midway between poles,. Because these respective perches were usually close in proximity and of similar height, however, these preferences had little influence on perch height selection,,

Test of Prediction 2: Kestrels and shrikes clearly avoided hunting in areas of tall, dense vegetation,. During the months when grasshoppers were abundant and were the primary food, kestrels and shrikes were observed hunting only in areas of short grass even though grasshoppers appeared to be more abundant in areas with tall grasso Avoidance of areas of tall, dense vegetation was best demonstrated by several observations of kestrels foraging sequentially along utility wires that crossed an area of tall, dense grass (Sporobolus wrightii) bordered by areas of short, sparse grasso Upon reaching the area of tall grass after foraging in the area of short grass, kestrels made flights much longer than the usual distance between hunting perches across the tall grass and resumed foraging in the area of short grass on the other side,.

21Hunting in areas of tall, dense grass might be profitable if

higher prey availability or greater prey size compensated for the low visibility0 Observations of a male kestrel hunting in a small clearing in tall, dense grass in February, when insect prey were scarce, suggested that such compensation may sometimes occur» After making a number of aborted attacks near the edges of the tall grass, a cotton rat (Sigmodon sp0) was capturedo Cotton rats are among the largest prey items that I recorded in the diets of kestrels in southeastern Arizona and were very abundant in the tall grass areas that winter«,

This observation also provided a possible example of hunting height being affected by a decreased probability of success with an increased distance from prey0 It seems reasonable that cotton rats were exposed to capture only at the edges of the tall grass for short periods of time» In order for an attack to be successful, the kestrel would have to perch a short distance away to reduce the time to reach the preyo Even though utility wires were available nearby, the kestrel hunted only from perches barely higher than the surrounding grass (t 105 m)o Such a reduction in hunting height is profitable only if encounter rate is high or prey size is large=

Test of Prediction J>% Mean perch height was highest for female kestrels (7°5 m), intermediate for male kestrels (6=3 m), and lowest for shrikes (5=0 m)» Because perches were normally either fenceposts or utility lines, perch differences are perhaps best shown by the percentage of times the birds perched higher than ?06 m (25 ft0)0 Kestrels perched at heights of 7=6 m or higher significantly more often (53̂ 9 n = 602) than did shrikes (3^, n = 217; = 23=9, p < o001),

22but there was no significant difference between male ( , n = 11?) and

pfemale kestrels (52^, n = 485; ̂ = 069, p > o90)= These patterns areas expected for reasons of optimal hunting height, but I have other data which suggest that perch selection also was affected by aggressive interactionso

The most striking characteristic of perched kestrels was that they chose the top of the tallest available perches* Ninety-three percent (n = 688) of kestrels observed were on the tallest perches available within a distance of 25 m* Shrikes also perched frequently on the highest perches but they did so a smaller percentage of the time (85#, n = 359) Selection of the tallest perches was most clearly demonstrated from observations of birds on manipulated perches. Both kestrels and shrikes always chose the highest pole in a set (n = 14 and 40, respectively) even though the highest poles were of different absolute heights in different sets.

In many areas differences between the highest and lowest perches were substantial, and perches of intermediate height were not available, but even in areas where intermediate perches were available, the highest perches were chosen. Where telephone wires ranged from heights of 6 to 9 m, the highest were chosen. Likewise, in leafless trees where an almost continuous range of perch heights was available, kestrels and shrikes nearly always perched within 1 m from the top on the highest good-sized branch. Large leafy trees presented an interesting situation. To maximize area hunted, a bird should perch on the side of the tree because from the top the view of the ground below would be blocked by the foliage. Kestrels consistently perched on the

23sides of these trees rather than at the top0 Shrikes were not observed in these treeso

These observations suggest that optimal hunting height for kestrels was usually higher than available perches® This is also suggested by the heights of birds using an alternative hunting techniques hoveringo Hovering height was usually between 1101 and l4®3 m (see Chapter 3), higher than virtually all perches on the study area®

Test of Prediction 4: It is difficult to assess whether changes inprey size or density affected perch height becauses (1) optimal hunting height often appeared to be higher than available perches; (2) it was difficult to assess changes in prey sizes and densities; and (3) wind was a confounding variable = It is also possible that optimal hunting height is primarily determined by the largest prey if these account for a large proportion of the prey biomass® However, field observations indicated that from August through December diets of kestrels and shrikes consisted primarily of grasshoppers® Grasshopper populations showed a marked decrease during this time (Fig® 7)® Although the grasshopper population Consisted of individuals of many body sizes much of the year, most seen after August were large ( >2®5 cm) and from September through December no marked change in their size was apparent® Analysis of kestrel pellets showed that the diet contained more rodents toward the end of this period® This decrease in prey density for both kestrels and shrikes and the inclusion of more rodents in kestrel diets should cause an increase in optimal hunting height® Kestrels and shrikes perched more often on perches >8 m on days of wind speeds

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2000

1500

1000

500

15 AUG I SEP 15 SEP I OCT 15 OCT I NOV 15 NOV I DEC 15 DECDATE

Figure ?• Index of grasshopper abundance in months of August through December. — Points indicate census dates.

25(Table 2) => Differences are significant; for kestrels, = 8o3? P < =005; for shrikes, X = 19=0, p < o005o

Other evidence for changes in hunting height due to changes in prey density and size comes from data on hovering kestrels (Chapter 3) o On several occasions kestrels hovering lower than usual were observed apparently capturing small abundant prey iterns0 Also, when no attacks were made on prey, successive hovers tended to be at increased heights Suggesting that the birds' estimates of prey densities decreased and hunting height was adjusted accordingly„ This may also explain observations by Pinkowski (1977) that bluebirds (Sialia sialis) moved to a

2higher perch (n = 65) significantly more often (X = 8o3? p

26Table 2= Perch height related to time of year=

Time Period Times Perched8 m

Kestrels 1 Septo=17 Octo 39 4530 Octo~31 Deco 20 60

Shrikes 1 Septo-17 Octo 23 530 Octo-31 Deco 23 46

Table 3o The relation between perch height and wind speedo

Wind Speed (mph) Times Seen at Perch Height0=3 m 4-7 m >8 m

Kestrels 10 116 33 96

Shrikes 10 100 13 7

Movement Between Patches In addition to choosing patches, foraging animals must make

decisions about moving between patcheso In many cases movement between patches is very complex because of the multidimensional nature and the effects of patch boundaries (e0go, Pyke 1978)0 Probably for these reasons few predictions or tests concerning movement between patches have appeared, though Gharnov (1973) has discussed some theoretical aspects of this topic and suggested that prey distribution is an important factoro For kestrels and shrikes, hunting from utility lines or fences in fairly homogeneous grasslands, movements between patches are limited to one dimension; thus, aspects of between-patch movement are simplified» Choices concerning movements between patches are restricted to whether to return to the same perch after an attack, which direction to go to the next perch, and how far to move* Here I consider only the latter two choices; the decision whether to return or not is apparently complicated and will be discussed elsewhere» However, kestrels and shrikes usually did riot return to the same perch after an attempt for preye

Net energy intake can be increased by decreasing the time and energy spent traveling between patches (C of Equation 1)® For birds hunting from a line of continuous perches, I make the following prediction®

Prediction 6s Kestrels and shrikes should forage unidirectionally and should move only far enough between patches so that overlap with adjacent patches is minimal® Due to difficulties in calculating the area that can be seen from a perch and the difficulties measuring

28appropriate parameters in the field, I cannot predict actual distances between patches,. However, a qualitative prediction that can be made is that distance between perches should be greater from higher perches than low ones because more area is visible from each percho

Test of Prediction 6: Kestrels and shrikes nearly always foragedunidirectionaily along a line of continuous percheso Only occasionally did a bird return to a perch after a visit to a different one„ Distances between perches were significantly greater from tall perches than short ones when kestrels and shrikes left without attacking prey (Table 4; t = 3o5» P < =005; t = 6,1, p

29Table 40 Effects of perch height on distances traveled between

percheso — Only distances between continuous equal-height perches after giving-up times are includedo

Perch Height (m) x Distance (m)» Kestrels x Distance Cm), Shrikes

2-3 18*2 15=5>7 53=9 68 oO

Table % Effects of wind speed on distance traveled between perches*

x Distance (m) Between Perches for:Perch Kestrels Shrikes

Height (m) Wind 10 mph Wind 10 mph

2-3 23°2 13o4 13o9 13=2>7 67=5 35=1 38=9

30tendency was noted for shrikes on low perches to hunt into the wind at times of high wind speeds, and there was no difference in distance between these perches for times of high and low wind (t = 37* p >o30)o Sample size of shrikes for distance between perches on high perches was too small for analysis0

Allocation of Time in Patches Most studies of optimal allocation of time in patches concern

"giving-up timeso” Giving-up time is the period waited since the last capture before an animal leaves a patch* Although there is general agreement that giving-up times are derived from information from previous experience, the kind and quality of information animals use has not been determined* Charnov (1973) has proposed the marginal-value theorem, a deterministic model that relies only on the mean times waited in previous patches* This model has recently been criticized by Oaten (1977), who suggested that a stochastic model, where an animal uses the variance as well as the means, is necessary for optimal foraging* It also seems likely that information gathered while forage ing in a patch may affect giving-up time* For birds hunting from perches, one such source of information may be assessment of prey that are seen but not attacked*

-Qiving-up time could be measured for kestrels and shrikes when they left a patch without attacking prey* My limited data on kestrels and shrikes does not allow a determination of exactly how these birds use past experience to determine giving-up time* However, it seems that part of the information used should be the means of some number

of times waited in previous patches before prey were attacked,. Therefore, ’I make the following prediction*

Prediction 7: Giving-up time should correlate with some number ofprevious times waited for prey*

Test of Prediction 7° The mean of the last three times waited in a patch before prey was attacked was a better predictor of giving-up time than just the last time* For kestrels r = =59 (n =17, p < o005) and r = .14 (n = 32, p >.25), respectively; for shrikes r = .78 (n =32, p

32Table 6= The effects of perch height on giving-up times for kestrels

and shrikeSo

Perch Height (m) h* x Giving-up Time (s)

Kestrels 0-3 34 146 o 634 18606

Shrikes 0-3 72 67=7>3 13 l83o4

Includes only giving-up times of less than 600 s0

33

Figure 8

75

65

5 55

i45if)if)LU8 35 Z)if)

75

65

55

45

35

LxJ

crif)if)LUOUz>if)

SO ND JF MB

95 <I

85 iif)

75 LU

65 z>io55 5

50 lx

280 «I I4if)

LU

SO ND JF M TIME PERIOD (MONTHS)

250

220 f-

190 %io160 >

oo |x

Success rates and lengths of giving-up times for shrikes (A) and kestrels (B) as functions of season.

34is not unexpected because diets of both species varied during the year. If diets shifted to smaller more abundant prey items as a preferred prey decreased, giving-up. time would likely decrease even though overall prey quality decreased. This appeared to be the case for shrikes in. January and February; smaller prey were taken and giving-up time decreased.

Optimal DietOptimal diet theory assumes that animals evaluate prey and make

decisions whether or not to attack each item encountered. For birds hunting from perches, evaluation can be based on prey size (e), energy and time required to attack (a), or the birds' estimates of chances of success (fs). Time and energy to attack increase with and are affected primarily by distance to prey. As previously discussed, success rate might also decrease as distance to prey increases. Evaluation for chance of success seems especially likely because of the high cost of an unsuccessful attack. Any evaluation of prey and subsequent selectivity lowers the proportion of prey attacked (Pp). From my observations, I am able to examine aspects of prey selection based on distance to prey and chance of success.

I am certain that kestrels evaluate prey. Hunting birds often showed evidence of sighting prey with behaviors normally associated with attacks such as head-bobs, tail jerks, and plumage depression, and then did not attack. Shrikes showed similar behavior but less obviously.

If kestrels and shrikes do not evaluate prey on the basis of distance, the proportion of attacks made at any distance from a perch

35should be proportional to the visible area at that distance= I cannot evaluate whether this occurs because it requires a quantitative measure of ground area that is visible as a function of distance from the perch* Topographic irregularities and vegetation opacity affect areas that can be hunted and are difficult to measure in the field* Even if these problems are neglected, quantitative evaluation of even the simple model presented earlier is too complicated to be practical

36Table 7= Distances to prey at different times of yearo

From P e r c h e s m High From PerchesJ>8 m Hi^h Time x Distance x Distance

Period n to Prey (m) n to Prey Cm)

Kestrels

1 Septo- 17 0cto30 Octo—31 Deco

32

10

1106

I606

31

kl

25=2

50=0

Shrikes1 Septo™ 17 Octo30 Octo«-31 Deco

22

19

5=2

11 o2

3

32

15=023=0

37Success rate did not decrease significantly with increasing

pdistance- of attack for kestrels or shrikes (Table 8? X = 6=2, p > o10;2

X = o20, p o95» respectively)c The apparently lower success rate for. kestrels at very great distances is due almost entirely to attacks on birds (11 of l6)» Either the probability of success did not decrease with distance for the majority of prey or kestrels and shrikes evaluated their chance of success and attacked only more vulnerable prey at greater distances= Laboratory work by Sparrowe (1972) showed that attack responses by kestrels were affected by prey exposure time0 This suggests that evaluation of probable success is at least partly responsible for the constant success rate with distance0

Evidence that prey are evaluated on the chances of success is that success rates of kestrels decreased significantly with hunting time on a perch (Table 9| X^ = 15=59 p p >=10)o The last two time categories for shrikes were combined for analysis0 Distance to prey did not increase significantly with hunting time for kestrels or shrikes (t = =62, p > 025? t = =29, p> o40, respectively) o This pattern suggests that prey items are evaluated on the basis of chance of success, and the threshold for an attack diminishes with time= This threshold apparently is renewed when birds change patches= The exact manner by which the threshold is renewed cannot be determined from my data because success rate also decreased significantly (X^ = 12=59 p < =005) with time since the last capture for kestrels (Table 10; small samples precluded analysis for

38Table 80 Effects of distance to prey on success rates of kestrels and

shrikeso

0-20 Distance to Prey (m) 21—40 41-60 >6oattempts successful 66 24 10 8

Kestrels attempts unsuccessful 44 16 6 16% success 60 60 65 33

0-10 11-20 >20 'attempts successful 28 12 6

Shrikes attempts unsuccessful 21 8 3% success 57 60 67

Table 9° Effects of time on perch shrikes300

attempts successful 50 21 15 . 7Kestrels attempts unsuccessful 25 16 13 22

% success 67 57 54 24

0-4o 41-120 > 120attempts successful 19 18 8

Shrikes attempts unsuccessful 12 13 13% success 61 58 38

39Table 10= Effects of time since last prey capture on success rate of

kestrelso

Time Since Last Capture (s) 0-300 301-600 >600

attempts successful attempts unsuccessful % success

408

83

8753

181949

4oshrikeso Renewal may be complete (Fig* 9a) or only partial (Figo 9b) with each change of perch0

The diminishing threshold model of evaluation of capture success provides a simple mechanism for partial preferences in diets if assessment of prey types changes on a short time scale in a manner similar to the chance of successo Most theories of optimal diet predict that animals should not show partial preferences? i0e0? a prey type should either be included in the diet every time it is encountered or not at alio Pulliam (1974) has suggested that partial preferences would be expected if dietary constraints were important or if the predator’s assessment of prey densities changed during the time it searched for prey« In a later review (Pyke et alo 1977), dietary constraints are discussed at some length but short-term assessment is not mentioned* The diminishing threshold shown by kestrels and shrikes support the latter theory and suggest that dietary constraints may not be necessary to explain partial preferenceso

Kestrels also appear to evaluate escape strategies of prey0 Roest (1957) and Collapy (1973) have mentioned differences in attack behavior for different prey types* These were also noted in this study* For insects, kestrels usually glided down from a perch with few wing- beats; for attacks on rodents and lizards, flights from perches were usually direct with many wingbeats as if to minimize time to reach the prey; for birds, attack flights were fast and powered but kestrels dropped quickly from the perch and completed the attack from grasstop level* The latter method suggests that surprise is important when birds are attacked* . '

THRE

SHO

LD

OF

SE

LEC

TIV

ITY

a perch change▲ perch change after prey capture

TIME

Figure 9« Two possible mechanisms for threshold renewal. — In ’’A*' threshold renewal occurs with each perch change regardless of prey capture; in "B” the threshold is only partially renewed with each perch change and completely renewed only after a prey capture.

42The above discussion suggests that birds can control their

success rate by varying their threshold of selectivity«, One factor that influences this threshold is prey size„ If prey are small relative to the size of the predator, success rate must be high to forage profitably, especially if the cost to attack each prey is higho If prey are large, a lower success rstte may be toleratedo

Some data suggest that success rate may be affected by an aversion to the risk of starvation or falling below a positive energybalance rather than simply maximizing net energy: gaino Figure 8 showshow success rates of kestrels and shrikes covaried with the lengths of time waited in patches where no prey were attacked (giving-up times)o If lengths of giving-up times are inversely proportional to prey densities, as suggested by Charnov (1975)» these data indicate thatselectivity based on estimates of chances of success increases as prey density decreaseso Craig (1978) presents data for shrikes, showing asimilar relationship between prey density and success rate® Thispattern conflicts with optimal foraging theories predicting selectivity should decrease as prey density decreaseso An explanation for this pattern may be that when prey are scarce, birds minimize variance in food intake by attacking only prey that have a high probability of capture, even if such behavior may also lower the mean net energetic gaino In this way risk of starvation decreaseso As food becomes less plentiful and the probability of starvation increases, risk aversion increaseso This seems especially likely for selectivity based on chance of success because of the high cost of an unsuccessful attacke If prey reach a critically low level, this conservative strategy may

not be sufficient to provide the food requirements of the animalP At such times, birds may be forced to take more risks and attack prey with a low probability of capture success but a high energetic reward

Concurrent GoalsAs outlined in the introduction, all predictions were made

assuming the goal of maximizing net .energy, reward while foragingo This goal seems reasonable for many animals (see Schoener 1971; Charnov 1973; Pyke et alo 1977), and the agreement between predicted and observed foraging behaviors suggests it is appropriate for kestrels and shrikes in winter0 Some aspects of prey selection, however, appear to be influenced by risk avoidance = In some systems, other goals such as escaping predators, searching for mates, maximizing a specific component of the diet, or territoriality, may operate concurrently and influence foraging behavior0 I do not believe that any of these significantly influenced the aspects of foraging behavior discussed in this papero

Of the concurrent goals that might influence foraging behavior of kestrels and shrikes, territoriality appears to be most likely0 Both species are territorial in winter (Cade 1955, Mills 1975, Miller cited in Bent 1950)o One might argue that kestrels choose the tallest perches to "advertize" territories or to better survey territories for intruderso Unidirectional foraging may be a mechanism to patrol territory boundaries,. But some patterns are not consistent with goals of territorial defense* Shrikes do not always perch on the highest perches; kestrels perch on the sides of leafy trees where the area that can be hunted is maximized, not at the top where intruders are more easily located* Behaviors associated with boundary conflicts suggested that kestrel foraging behavior was little influenced by territoriality* Birds at territorial boundaries appeared to forage no

k5

differently than others, even when a neighbor was nearbyQ Very little time was spent in territorial interactions and rarely did birds fly long distances to pursue an intruder» In the boundary disputes I observed, an intruding bird was attacked only when it flew off a perch after prey= Neither bird involved appeared to notice the other until movement occurred,. Cade (1955) and Welty (1962) also have noted that movement of an intruder is often necessary to elicit an attack from a kestrel= This seems a reasonable method to defend a feeding territory at relatively low costo An intruder is no detriment as long as it takes no prey from the territoryo If an intruder is prevented from capturing prey, it will be advantageous for it to forage elsewhere0

Although kestrels and shrikes are occasionally preyed upon by other raptors, it is apparently rare0 During this study the only, attack on a kestrel that I witnessed was an unsuccessful one by a Cooper* s Hawk (Accipiter cooperi)o This attack occurred in an area of fairly dense oak woodland? no Cooper*s Hawks were seen in the open grasslandso Kestrels showed little concern for other raptors except to occasionally mob a Red-tailed Hawk (Buteo jamaicensus) or Prairie Falcon (Falco mexicanus)o One shrike showed some alarm when a Marsh Hawk (Circus cyaneus) passed near but, except for attacks by kestrels which appeared to be motivated by competition rather than predation, no attacks on shrikes were observed,.

Most data were taken at a time of year when searching for mates was evidently of little importance= Some kestrels remained paired in winter; these birds appeared to forage no differently than unpaired one So

46Although some particular component of the diet may be an espe

cially important requirement for some species, it seems unlikely that carnivorous animals would have to take certain prey types in order to obtain essential nutrients. Even if this were the case, the searching behaviors studied here would be little affected. At times, however, kestrels appeared to search for a specific prey type. In addition to a kestrel apparently hunting Sigmodon in tall grass, on at least two other occasions it appeared that rodents were being hunted specifically. In these cases, kestrels hunted small areas for long periods. It appeared that a rodent had been sighted there previously and the kestrel was waiting for it to reappear.

ConclusionsForaging patterns of kestrels and shrikes are consistent with

predictions of a tactical model for ground-hunting raptors developed from considerations of perch geometry and optimal foraging theory.These patterns show that kestrels and shrikes can measure distance and time and respond appropriately to quantities such as means and, perhaps, variances. These are not unexpected results. Perhaps more important than demonstrating that animals appear to be selected to optimize foraging behavior is the demonstration of the uses of optimal foraging theory as a tool to better understand animal behavior. Optimal foraging theory is certainly useful in understanding and examining decisionmaking processes that enable animals to solve problems posed by alternative prey types with variable temporal and spatial distributions. It also shows great promise in analyzing and understanding community

structure0 I am currently working on a paper which uses optimal foraging theory to analyze coexistence among predatory birdso

CHAPTER 3

PATTERNS OF HUNTING WHILE HOVERING

Many animals, especially predators, have a repertoire of foraging techniques. An animal would be expected to use the technique that best meets its foraging goal, which often may be to maximize net energy gain while foraging (Pyke et al. 1977)° Various techniques are often used to take different prey types but may also be used by one animal to hunt one prey type. When different techniques are used to hunt a single prey type, variations in prey behavior or environmental conditions are apparently responsible for the choice of technique. In this chapter I examine hovering, a hunting technique used by some birds, and some factors that affect its use.

Although hovering flight is possible for most birds and many species occasionally hover momentarily, only a few species hover habitually while foraging. Aside from hummingbirds, regular hovering is fairly restricted to predatory species that hunt over open ground or water and take prey from the surface. Physiological aspects of hovering flight of hummingbirds (Weis-Fogh 1972, 1973) and ecological aspects of hovering by Ospreys (Pandion haliatus) (Grubb 1977) have been studied, but ecological aspects of hovering of other species havebeen studied only casually (Roest 1957; Balgooyen 1976). Here, I/analyze patterns of hovering in the American Kestrel (Falco sparverius) and relate them to a model of optimal foraging.

48

49Methods

I collected data on foraging of American Kestrels in the grasslands of southeastern Arizona from 1975 to 1977= Most field work was conducted in fall and winter months= Data collected included hovering heights, hovering times, times on perches, attempts to capture prey and their success, and types of prey taken from perches and hoverso Hovering time was determined by recording the start and finish of a hover on a portable tape recorder and then timing it later» The recorder was found to be accurate to within less than d sec/min0 Wind speeds were measured at a height of 2 m with a Dwyer hand-held wind gauge and hovering heights were estimated visually by comparison with stationary objectso A Dietzgen "Duo-site Range Height Finder" was also used to measure hovering height but was found to be of little value due to large distances between the kestrels and the observero When kestrels were close enough for the instrument to be useful, results agreed well with my estimates*

Advantages of Hunting While Hovering Although a small percentage of the hovers observed were clearly

over prey that had been previously located from perches, hovers were primarily used to search for prey* This was clearly demonstrated by the systematic manner in which hovering occurred* Two common patterns of hovering were series of hovers en route from one line of utility poles to another and series of hovers parallel to a line of utility poles made after flying some distance perpendicular to these poles and then returning to the same line some distance away* In these series distances between hovers were fairly constant*

50Occasionally, it appeared that some hovers were made after prey

had been located from a previous hover= In these cases, there was essentially no horizontal distance between successive hovers; one or more subsequent hovers were made after rapid, nearly vertical drops as described by Roest (1957)° Seventy-three percent (11 of 15) of these series resulted in attacks whereas attempts for prey were made from only 29^ (126 of *+30) of all hovers recorded,.

Many birds that habitually hover also hunt from perches0 However, areas that can be hunted from perches are limited by the distributions and heights of those percheSo Advantages for hovering appear to be that the rate of prey capture can be increased by hunting areas where prey are more available and that optimal hunting height can be more closely approachedo In Chapter 2, I have demonstrated the existence of such optimal hunting height0 I would then predict the following,.

Prediction It Hovering should not occur close to perches of heights similar to hovering heightso

Test of Prediction 1: Of 402 hovers observed where distances toutility wires were determined, over 94% were made at distances greater than 40 m from those wires,. Hovers that were made within 40 m were low and momentary (

51telephone lines approximately 9 m higho Hovering height varied with wind speed, but most (71%) hovers occurred above 9 mo Some hovers were made above fences less than 2 m high that were also used as hunting perches by kestrels, but these hovers were nearly always above 9 m0

Costs of Hovering Hovering is energetically more costly than hunting from a

percho For hovering to be advantageous, it should be used only when the increased cost is exceeded by an increased energy rewardo For hummingbirds the magnitude of the increased cost of hovering over perching is reasonably treated as a constant, but for most birds the cost of hovering is affected by wind speedo Hovering is flying at zero ground speed, so that as wind speed increases a hovering bird must increase its air speed to maintain zero ground speedo (Note that unless a bird can fly backwards or sideways it must face the Wind to hover0) The power required to fly at various air speeds has been esti-) mated (Pennycuick 1969; Greenwalt 1975), and these estimations suggest that there exists for each bird, depending on the aerodynamics of its flight, a speed at which the rate of power output is minimal (Vmp; see Fig® 10)® Laboratory measurements by Tucker (1968) have confirmed the general shape of the predicted curve® To hover for the least cost, a bird should select a wind speed equivalent to Vmp, and thus one might expect hovering to be done most often when winds are of this velocity® But wind speed varies with altitude® The exact pattern of this relationship is complex and depends on several factors (see Lowry 1969) but, in general, wind speed increases with height above the ground

Powe

r Re

quire

d to

Fly

Vmp

Air Speed

Figure 10. The effect of air speed on the power required to fly. — (From Pennycuick 1969.) Vmp is the air speed at which flight cost is minimized.

53surfaceo Thus, under a fairly wide range of conditions, a bird should be able to find the optimal wind velocity by varying its altitude„ But the existence of an optimal hunting height (determined in large part by prey size and density) restricts the range of altitudes where hovering is profitableo Optimal wind speed and optimal hunting height will coincide only at certain times; hovering when they do not coincide increases foraging costs or decreases prey captures and thus lowers net energy intake

Prediction 3s Most hovering should be done at times when optimal wind speed for hovering and optimal hunting height coincide= Whereoptimal hunting height is nearly constant, most hovering would be ex-

jpected at times of an optimal wind speed measured at a fixed height with a decreasing amount of hovering at increasing deviations above or below this optimumo

Test of Prediction 2s Hovering kestrels captured food at a rate 7=6 to llo2 times greater than birds hunting from percheso The increased energetic costs of hovering cannot be calculated precisely but appear to be of the same magnitude as the above numbers0 Total weight of prey items taken from hovers and perches was calculated by assigning approximate weights to each prey item captured.(Table 11)0 Approximate rates for energetic gain were obtained by dividing the sum of these weights by total time spent either perch-hunting or hovering (energy per gram is approximately equal for insects and vertebrates; Cummins 1967)o The calculated rate of prey taken from hovers was 7o6 times that taken from perches«, In only 70% of attacks from perches and 4-7% of attacks from hovers was the outcome (capture or escape of prey) observedo If the same kinds and proportions of prey were taken in these attacks as in those of unknown outcome, the rate of weight capture from hovers would be 1102 times that from percheso However, most attacks where outcome was unknown were probably unsuccessful, or if successful, the prey taken was very small and immediately cottsumedo Therefore this value probably gives an approximate upper limit for the ratioo

55Table 11„ Comparison of prey types and rates of prey capture from

perches and hovers0

Prey TypeApproxo Wgto (g)

PerchesTotal

# Taken WgtoHovers

Total # Taken Wgt=

Sigmodon spo 60 4 240 2 120Baiomys taylori 8 1 8 0Perognathus flavus 8 0 = 1 8lizards1 k 5 20 02Scaphiopus hammondi 14 1 14 0 —Rana pipiens 2 6 12 0grasshoppers 1 43 45 5 5beetles o5 5 2=5 2 1Igeo arthropods 1 22 22 7 7stoo arthropods o2 6 1=2 13 2=6

Total weight 562=7 143=6Seconds watched Rate of weight capture

119416 6332

(g/s x 1000) 3o0 22=7^Small Sceloporus undulatus

56Mammals, especially Sigmodon, account for a large proportion of

the total weight of prey taken but a small percentage of total individual s= This may result in some inaccuracy in comparisons of rates of biomass due to sampling error= But even discounting the mammals taken, prey biomass was captured from hovers at a rate 2o6 to 3=7 times that captured from percheso

The cost of flying at Vmp for the American Kestrel has not been measured* However, data for three species of birds flown in wind tunnels indicated that metabolic rates while flying at Vmp were approximately 6 times resting metabolism (Bernstein, Thomas and Schmidt- Nielsen 1975)° Greenwalt (1975) suggested that wind tunnel measurements exaggerate flight costso Additionally, some hovers may not be as costly as flying at Vmp because under certain conditions updrafts provide some lift and a hovering bird does not have to expend as much energy to remain stationary* Extreme "hangs" where no wingbeats occur are the equivalent of static soaring and can occur only when the sinking speed of the bird matches the vertical component of the wind (updraft) and the flight speed of the bird matches the horizontal component* Although I have recorded hangs for kestrels that lasted as long as 17 s, most were short (

2o The energy required to reach hunting height was greater while hovering0 Mean hovering height was 12=5 m whereas mean perch height was 7°3 fflo

3o Total- energy spent flying between hunting locations was greater when hoveringo Hovering kestrels searched approximately 2=4 times as many areas before an attack was made than did perched birds (3=4 area changes per attack and 104 area Changes per attack, respectively) =,

The relative costs of these activities are not known and thus the costs of hovering cannot be compared quantitatively with those of perch-huntingo Nonetheless, the relative magnitude of the increased costs and benefits of hovering seem approximately equal as predictedo

Test of Prediction J>% Hovering effort, i0eo, time hovering/time observed, as well as the number of hovering birds increased with wind speed measured at 2 m to a peak at a wind speed range of 18-24 km/h and then decreased (Figo 11)= Virtually no hovering occurred at times of no windo The shape of this curve is as expected, but even when wind conditions were in the optimal wind speed range, hovering occurred less than 8% of the timeo Note that male kestrels generally hovered more than femaleso

More evidence that hovering is wind dependent is provided by other observations. In several cases, kestrels paused as if to hover during flights between lines of utility poles, but then continued on without hovering. On two occasions kestrels even left utility wires and flew parallel to them before returning as described earlier and made similar pauses without hovering. These aborted hovers took place

58

d^% time males hovered9 % time females hovered# % time all kestrels hovereda % of all kestrels watched

that hovered

12.0

10- - T 100

8 — - - 807.8OZccLU>I G - - LUs

6.6 67;|.6 .467

- -6 0

4 - - - - 4 0

2.9

2.0 --202 “" 19 V

.59.31

Km/h 0-8.1 9.7-16.1 17.7-24.2 25.8-32.2 >32.2MPH 0 -5 6 -1 0 11-15 16-20 > 20

WIND SPEED

Figure 11. Hovering effort as a function of wind speed. — The points for wind speeds greater than 32.2 km/h are based on observations of only 8 birds for 4l min on one day.Samples for males at wind speeds above 24.2 km/h were too small for analysis.

% BI

RDS

WAT

CHED

TH

AT

HO

VER

ED

59on days when wind speed was extremely variable« Apparently, wind conditions were not right when these birds began to hover0

Hovering HeightAs previously discussed, hovering height is likely determined

by both wind speed and optimal hunting height = The latter in turn is influenced by prey density and size* If these stay reasonably constant, hovering height will be determined primarily by wind speed arid the following prediction can be made0

Prediction 4: Because wind speed increases with height, hoveringheight and wind speed measured at a fixed height should be inversely related® The exact relationship depends on the relation of wind speed to height, which is influenced by many variables, but above 2 m it is often close to linear (Geiger 1966)

602o Ground topography can influence the wind speed-height rela

tionship = Kestrels hovering in series that crossed ravines stayed at the same absolute height even though the recorded height above ground changedo The birds were probably staying in the same air speed layero

3o Hangs are not restricted by the same wind speed-height rela- tionshipo As mentioned earlier, hangs can occur only when the vertical and horizontal components of the wind are matched by the sinking speed and flight speed of the bird, respectively* But a bird's sinking speed is in part determined by its air speed (Tucker and Parrottl970)0 For a given updraft there is essentially only one air speed at which the sinking speed will match* This wind speed could be found by varying altitude as for flapping hovers, but the height at which conditions are right for hangs depends on the magnitude of the updraft as well as the wind speed* If hangs were used exclusively, no correlation between hovering height and wind speed measured at 2 m would be expected* The extent of the deviations from the wind speed-hovering height curve caused by hangs is probably determined by the stability and predictability of the conditions necessary for hangs* If these conditions are stable and predictable, kestrels may seek them because costs would be significantly lower than flapping hovers; but, if not, hangs may occur only opportunistically during flapping hovers* In the latter case, although the cost of hovering would still decrease, the hovering height should be determined primarily by the wind speed-height relationship*

4* Tradeoffs between the optimal wind speed and optimal hunting height may result in significant deviations from optimal wind speed if

61optimal hunting heighir becomes an overriding factor= Such a situation might be expected when prey were very abundant. In this case, search time (and thus hovering time) might be so low that it would be more profitable to hover low at a greater cost than to expend the energy and time to fly up to optimal hovering height. On three occasions I observed kestrels making very low (5 m or less) series of hovers at times.when the wind speed-holering height regression would have predicted a hovering height of approximately 15 m. In all of these cases, hovers were very short in duration and nearly all resulted in attacks (8%, 4l of 50) suggesting that prey (apparently ants) were very abundant.

Another possible consequence of a tradeoff between optimal hunting height and wind speed was suggested by an unpredicted pattern of hovering. In series of hovers uninterrupted by prey attacks, se- quential hovers were significantly (X = 50.3, p

62speed should be equal to Vmp0 No one has measured Vmp for the American Kestrel but it can be estimated from body weight and wing span using empirical or theoretical equations. Unfortunately, these equations predict values ranging from 8,1 to 42=1 km/h (see Table 12), A flight speed of 39°9 km/h has been measured for a kestrel in the field (Tucker and Schmidt-Koenig 1971) but the purpose of the flight, and hence whether it was likely to be at Vmp, was unknown.

Because wind speed increases with height, the wind speeds I measured at 2 m were not those at which kestrels were hovering, C, Richard Tracy (personal communication 1978) has informed me that the actual wind speeds at which kestrels hovered can be robustly estimatedfrom my wind speed measurements by using the equation:

u* (z - d)u(z) = k In ( zQ ) (6)

where u(z) is the wind velocity as a function of height, z; u* is the shear velocity, which can be calculated from measured wind speeds at a known height; k is the Karmen constant, ,41; d is "zero plane displacement,” 063 h; zq is "roughness length,” ,13 h, and h is the average vegetation height. For the wind speed range measured at 2 m of 17o7 to 24,2 km/h, at which kestrels hovered most frequently, hovering heights calculated from the linear regression equation of the wind speed-height relationship range from 11,1 to 14,3 m. Wind speeds at these heights calculated from the above equation are 24,8 to 32,5 km/h. This range of values compares favorably with the range of the middle four values of Vmp from Table 2 which suggests that kestrels do indeed hover at wind speeds equivalent to Vmp,

63Table 12o Calculations of Vmp for the American Kestrelo^

Vmp (km/h) Equation Source

8*1=760 (y)^ pVs/ Pennycuick 1969, P= 530

33=8 14=6 mo2° Tucker 1973, p= 70742ol (835=3 x 10”6h + 15=73)m°169 Tucker 1974, p= 30631=9 from Table 22 Greenwalt 1975, P= 3726o 2 e*wB Greenwalt 1975, P° 4424c42 e

64If wind velocity were the only factor that affected hovering

height, an estimate of the actual wind speed at which kestrels hovered could be obtained by extrapolating the slope of the linear regression line in Figure 12 to a height of 2 m= However, because optimal bunging height should also affect hovering height, hovering birds should Compromise between the. height where wind speed equaled Vmp and the optimal hunting height0 Such a compromise would result in a shallower slope of the regression line than would be expected from considerations of wind speed alone, and extrapolation of the line to 2 m should give a wind speed value greater than Vmp0 Extrapolation of the regression line gives a wind speed of 4l08 km/h which, although it is quite close to the flight speed recorded by Tucker and Schmidt-Koenig and the highest calculated value for Vmp, is substantially higher than most of the calculated values for Vmp, as expectedo

Other factors may also affect the optimal wind speed at which to hoverq Hovering at a given wind speed may not be equivalent to flying at that air speed because wind speed is often quite variable while the bird is hovering whereas a bird in forward flight can maintain a relatively constant speedo If wind speed is variable while hovering, the best mean wind speed at which to hover is one that minimizes energy output over the range of wind speeds experienced^ From the shape of the power required-air speed curve shown by Pennycuick (1969; she also Fig* 10) it can be seen that deviations from Vmp at higher speeds are less costly than those at lower speeds because of a steeper slope below Vmp0 Thus, for a bird hovering in a variable wind.

65

65 f

X 30

• •» •» •»>Xw\

•> •J55* •* •»>•»»•» •».•»•> e>• • • emX»»

0Km/h 0 MPH 0

WIND SPEED AT 2M

Figure 12. Hovering height as a function of wind speed at 2 m. —Hovers over prey apparently seen prior are not included. In situations where wind speed changed during a hover, a median value was used. Line si is the linear regression line of best fit and line b is the best logarithmic fit.

66it might be advantageous to select an average wind speed slightly greater than Vmp0

Hangs could also affect the optimal wind speed at which to hover= Hangs are restricted only to a range of wind speeds where gliding is possible and not just to a wind speed equal to Vmp0 The extent of this effect is determined by the stability and predictability of the conditions when hangs are possible, as mentioned earlier. The direction of the effect is determined by the relative position of Vmp to the range of air speeds where gliding can occur0

Hovering TimeTimes considered here are for hovers from which no prey were

attacked, i0e,, giving-up times. For 185 such hovers, times ranged from 3 to 112 s with a mean of 18,3 s.

According to optimal foraging theory, giving-up time in a patch should be inversely proportional to prey density (Pyke et al, 1977)= Although few data on prey abundance were taken, it appeared that prey became more scarce as the season progressed from September to April,Mean duration of hovers for each month significantly increased (r = ,76, p

Test of Prediction 5? A significant positive correlation existed between hovering time and the time of the last hover from which prey was attacked (r = o 4 9 » n = 39? p < o 0 0 5 ) o Sample size was limited largely because many attacks ended sequences of hoveringo

Hovering time might also be affected by profitability; if so,I would make the following prediction*

Prediction 6: Hovers made when optimal wind speed and optimalhunting height nearly coincided should be longer than those hovers made when optimal wind speed and optimal hunting height differed considerably*

Test of Prediction 6s When wind speeds measured at 2 m were between 17=7 and 32*2 km/h, which was when kestrels hovered most and apparently when optimal wind speed and optimal hunting height most nearly coincided, hovers were significantly (t = 1*93? p

68considered to be worth more effort than others= Situations where prey

were seen but not attacked would provide one source of such informa

tion*

3o If wind conditions were right for hangs, hovering time might be lengthened because of lower cost*

Hovering as an Alternate Hunting Technique Results of this study show that hovering by American Kestrels

occurs in predictable patterns that, in general, support ideas of optimal foraging theory* Hovering appears to be an alternate foraging technique that complements hunting from perches* But high costs restrict hovering to times when prey availability and wind conditions are favorable* For hovering to be most profitable, optimal hunting height must approximate the height where hovering is least costly, which is determined by wind speed*

Hovering by other perch-hunting birds also appears to be influenced by wind conditions* Schnell (196?) observed that Rough-legged Hawks (Buteo lagopus) spent more time in the air at times of higher wind speed, and I have observed that this species hovers at greater heights than kestrels at comparable wind speeds* Rough-legs should hover at greater heights for two reasons: because they have heavierwing-loading than kestrels, they should hover at the least cost at higher wind speeds and thus at greater heights; and because they take larger prey on the average, optimal hunting height should be higher*I have also observed that the hovering of Say*s Phoebes (Sayornis saya) and Mountain Bluebirds (Sialia currucoides) is affected by winds and

69normally occurs at much lower heights than those of kestrels, as would be expected for these smaller birds*

Some species that hover habitually, such as White-tailed Kites (Elanus leucurus), Ospreys (Pandion haliatus), and some terns, rarely hunt from perches, although they may hunt from flight without hovering* Wind conditions should still affect hovering flight in these species, and selection should favor the development of aerodynamic properties that make hovering least costly at prevailing winds* For White-tailed Kites wing-loading has been reduced to a point where hovering is apparently profitable even without wind (Balgooyen 1976)*

As 1 mentioned earlier, many species are capable of hovering; some that normally do not hover may take advantage of this technique when wind conditions are favorable* I have seen Red-tailed Hawks (Buteo .jamaicensis), a species that normally hunts from perches, hovering at times of high winds* However, most were immature birds which probably have lighter wing-loading than adults*

The use of hovering ranges from a rarely-used alternate hunting technique for some species to one of the primary techniques for others* Habitual hovering is restricted by environmental conditions, morphology, and the suitability of other techniques* Species that frequently hover normally occur in areas without perches and many occur in areas of fairly predictable winds*

Hovering may be encouraged by competition with other species*I have other data that suggest use of perches may be restricted by a dominant species and hovering may enable a second species to coexist*A similar situation may explain why male kestrels hover more than

70females; females apparently force smaller males into areas of lower perch qualityo Thus, hovering would be more profitable under a wider range of conditionso Dominance may also explain my observations that hovering is more frequent among immature Red-tailed Hawks than adultsc

An animal’s foraging repertoire appears to be affected by prey distributions, environmental conditions, and competition0 Repertoire size is in part determined by the predictability of these factors and is limited by morphological restrictions,.

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