foragers, nearest-neighbour effect draft · draft 2 abstract we examined the influence of the...

Draft

Forager density effect on feeding rates in spring staging

semipalmated sandpipers using different foraging modes

Journal: Canadian Journal of Zoology

Manuscript ID cjz-2017-0238.R2

Manuscript Type: Article

Date Submitted by the Author: 01-Feb-2018

Complete List of Authors: Novcic, Ivana; Kean University College of Natural Applied and Health Sciences, Natural Sciences Beauchamp, Guy; Independent Researcher

Keyword: Semipalmated sandpiper, Calidris pusilla, foraging mode, density of foragers, nearest-neighbour effect

https://mc06.manuscriptcentral.com/cjz-pubs

Canadian Journal of Zoology

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Forager density effect on feeding rates in spring staging semipalmated sandpipers using different

foraging modes

Ivana Novcic1 and Guy Beauchamp

2

1. Department of Biological Sciences, Kean University, Union, NJ 07083, USA

2. Independent Researcher, Canada

Corresponding author: [email protected]

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Abstract

We examined the influence of the density of foragers on feeding rates of semipalmated

sandpipers (Calidris pusilla (L., 1766)) while using different foraging modes at a spring stopover

site in Delaware Bay, USA. Using dynamic estimates of inter-individual distances obtained at

short intervals of time, we explored how forager density affected feeding rates when sandpipers

used visual pecking or tactile probing. Pecking rate significantly increased with inter-individual

distances, while probe rate was not affected by density. Our study also showed that in fast

moving foragers, such as sandpipers, in which the number of nearby foragers and distance to the

nearest neighbour continuously change throughout the foraging bout, pecking rates are more

affected by nearest neighbour distance than by the number of foragers in their immediate

vicinity. In addition, our study implies that foragers using different foraging modes might be

differently affected by nearby competitors perhaps in response to prey disturbance by

neighbours.

Keywords: Semipalmated sandpiper, Calidris pusilla, foraging mode, nearest-neighbour effect,

density of foragers

Introduction

Group foraging is widespread across various animal taxa (Pitcher et al. 1982; Wilkinson and

Boughman 1998; Hamner and Hamner 2000; Sridhar et al. 2009), as foragers, while searching

for food within groups, experience reduced predation risk and foraging benefits (Clark and

Mangel 1986; Cresswell 1994; Krause and Ruxton 2002). Thus, animals may devote more time

to foraging instead of vigilance (Powolny et al. 2012), they may locate food patches faster by

observing other foragers (Beauchamp and Giraldeau 1997; Giraldeau and Beauchamp 1999), and

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they may capture larger prey (Creel and Creel 1995) or defend prey more easily (Carbone et al.

1997). However, increased number of foragers may lead to increased competition for resources,

thus reducing the benefits of group foraging (Krause and Ruxton 2002). Therefore, the extent to

which foragers gather together depends on interaction between benefits and costs of group

foraging under various environmental conditions (Beauchamp 2014).

Typically, time allocated to foraging activities increases with group size (Beauchamp

1998), as animals may allot less time to antipredator behaviour due to dilution effect, collective

detection of predators or collective defense (Foster and Treherne 1981; Roberts 1996;

Beauchamp 2015). An alternative explanation is that foragers, due to scramble competition,

increase their resource-gaining activity in order to increase their portion of shared resources

(Parker 2000). In this way, foragers that compete over limited resources can enhance food intake

by increasing their feeding effort, which can be any element of foraging behaviour that improves

encounter with prey (Shaw et al. 1995; Beauchamp 2012). Many studies addressing the group

size effect documented an increase in foraging effort with group size (Johnsson 2003;

Beauchamp 2007). Nevertheless, increased aggression among foragers, monitoring of group

members, and prey disturbance can negatively affect foraging rates in larger groups (Selman and

Goss-Custard 1988; Creswell 1997; Rutten et al. 2010). Thus, increased number of foragers may

have at times opposing effect on their feeding rates.

Foraging technique can also influence the relationship between feeding rates and group

size (Beauchamp 1998). In redshank (Tringa totanus (L., 1758)), the rate of prey captures

decreased with forager density if birds searched for prey visually, i.e. “pecked” at the substrate

surface, while such a relationship was not detected when birds searched for prey tactilely, i.e.

“probed” the substrate (Goss-Custard 1976; Selman and Goss-Custard 1988). The negative

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relationship in the case of the former happened due to disturbance of birds’ major prey, the

amphipod Corophium volutator (Pallas, 1766), which retreated to their burrows in the presence

of predators (Goss-Custard 1970; Minderman et al. 2006). Similarly, intake rates of

oystercatcher (Haematopus ostralegus L., 1758) were differently affected by the density of

foragers when birds used different foraging methods to open mussels (Mytilus edulis L., 1758)

(Stillman et al. 1996). For that reason, it is desirable to include details on feeding techniques of

foragers as an important variable in empirical studies and theoretical models examining the

group size effect (Beauchamp 1998).

Here, we investigated the influence of the density of foragers on feeding rates in

semipalmated sandpipers (Calidris pusilla (L., 1766)) that use different foraging techniques at a

spring stopover site in Delaware Bay, USA. Each spring, this area attracts large number of

shorebirds that capitalize on eggs of spawning horseshoe crabs (Limulus polyphemus (L., 1758))

(Myers 1986; Novcic et al. 2015). Although large mixed-species flocks forage along sandy

beaches where horseshoe crabs spawn (Myers 1986; Botton et al. 1994; Tsipoura and Burger

1999), a considerable number of birds also use intertidal marshes and mudflats where they feed

on various invertebrates (Burger et al. 1997; Tsipoura and Burger 1999; Novcic et al. 2016).

Semipalmated sandpipers collected prey on Delaware Bay mudflats through pecking at the mud,

which is recognized as a visual foraging method, and probing and skimming the substrate, which

are more tactile foraging techniques (Baker and Baker 1973; Mouritsen 1994; MacDonald et al.

2012).

We explored how pecking and probing were affected by the density of foragers while

controlling for these variables through the entire observation period. In species where foragers

move slowly during a foraging bout or are confined to a specific area, group size can be a good

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estimate of forager density (Vahl et al. 2005; Fernández-Juricic et al. 2007; Fortin and Fortin

2009). However, in shorebirds that are often found in very large flocks, the density of foragers,

and more specifically, the distance to nearest neighbours have stronger impact on feeding rates

(Goss-Custard 1980). The density of foragers can be measured as the number of individuals

occupying an area of fixed size. Some researchers assessed density and nearest neighbour

distances at the beginning of a focal observation (Beauchamp 2013) or both at the beginning and

the end of observations (Goss-Custard 1976). Others did not specify how density measurements

were obtained during sampling (Minderman et al. 2006; Sansom et al. 2008; Fernandez and Lank

2008). Some researchers also estimated density at very short intervals of time but aggregated the

data over the whole sample (Vahl et al. 2007). Shorebirds often move quickly during a foraging

bout, which would cause continuous changes in the spatial distribution of foragers with the

potential to make the above single-point, static estimates of density less reliable. Using dynamic

estimates of inter-individual distances obtained at short intervals of time, we tested the prediction

that the foraging effort of semipalmated sandpipers will be affected by the density of foragers

and vicinity of nearest neighbours while using a visual foraging mode, but not when birds

employ a more tactile foraging mode.

Worldwide, migrating shorebirds depend on a small number of strategic stopovers, such

as Delaware Bay, where they reach high abundance (International Wader Study Group 2003).

These staging areas are critically important for migrants, as they provide nutrients needed for

successful completion of migratory flight, thus influencing migration, reproductive success and

survival of individuals (Baker et al. 2004; Dujins et al. 2017). Information on how interactions

among shorebirds affect their foraging success in Delaware Bay is essential to expand our

understanding on how shorebirds use resources while at stopover in key staging areas.

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Materials and methods

Fieldwork

The study took place during spring migration in 2011, at three locations on the New Jersey side

of Delaware Bay – Thompson’s Beach (39°12’N 74°59’W), Bivalve (39°14’N 75°02’W), and

Matts Landing (39°14’N 75°00’W). Thompson’s Beach and Bivalve are tidal marshes dominated

by grasses (Spartina spp.) with exposed mudflats where shorebirds forage during low tides.

Matts Landing is an artificial impoundment in which the water is maintained at a low level

during spring. Shorebirds use the area for roosting during high tides, but also for foraging. In

May, when semipalmated sandpipers migrated throughout the bay, each study location was

typically visited twice a week; in Matts Landing, observations lasted 1.5 to 2 hours before the

highest tide, whereas in Thompson’s Beach and Bivalve the birds were observed 2 to 4 hours

around the lowest tide.

To sample foraging behaviour, one of us (IN) recorded focal individuals with a digital

camera (Panasonic HDC-TM60, optical zoom 35X) from an approximate distance of 15–60 m

during daylight. Subsequent focal individuals were chosen from different portions of foraging

flocks, to reduce the possibility of observing the same individual twice, and recorded for 60

seconds or until the birds were lost from sight or changed their behaviour, e.g. stopped foraging

and started preening themselves. Occasionally, we recorded a group of foraging individuals from

which we chose one to several focal individuals (no more than one focal individual per quadrant

of the field of view) following the same procedure.

To obtain information on diversity and seasonal change in densities of potential

invertebrate prey, we collected benthic core samples within foraging areas on all three study

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locations, once every two weeks, from mid-April to the beginning of June 2011. In Matts

Landing, we sampled along transects at two opposite sides of the impoundment, while in Bivalve

samples were collected along a transect line across the mudflat; subsequent samples were ~ 1–

1.5 m apart. In Thompson’s Beach, sampling was conducted from a couple of points close to the

edge of mudflats. Sampling was conducted with a plastic pipe 7.62 cm in diameter, which was

inserted in the substrate to a depth of 10 cm. Each sample was washed through a set of two

sieves (mesh sizes 1 mm and 0.5 mm), and all extracted invertebrates were stored in 95%

ethanol. We identified invertebrates mainly to order or family level, following Pollock (1998),

while badly damaged individuals remained unidentified.

Video analysis

To analyze videos we used programs HD Writer AE 2.0 (Panasonic) and Windows Live Movie

Maker (Microsoft); all videos were observed in ½ to ¼ slow motion. We made a distinction

between several foraging modes used by sandpipers: single peck – the bill touched or slightly

penetrated the substrate (up to one quarter of the bill length) in a single motion; multiple pecks –

a rapid series of two to several pecks, distinguished from subsequent events if the focal bird

noticeably raised its bill from the substrate; single probe – the bill penetrated into the substrate

(more than one quarter of the bill length) in a single motion; multiple probes – a rapid series of

two to several probes, distinguished from subsequent events if the focal bird pulled around ¾ or

its entire bill from the substrate (Baker and Baker 1973). Each occurrence of multiple pecks or

probes was counted as one foraging event. As foraging birds moved quickly and their position

within flocks continuously changed, for each focal individual we measured feeding rates – the

number of pecks and probes within 5 s intervals. In addition, every five seconds we recorded

forager density (the number of neighbours up to three body lengths from the focal individual), as

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well as the distance to the nearest neighbour (in bird length units: one bird ~ 10 cm). For all these

calculations, a time bin included 2.5 s before and after the video frame used to obtain the

measures of densities. The exceptions were the beginning of each video, when the unit of time

lasted 2.5 s and the feeding rates were associated with the density recording on the first video

frame, as well as the end of a video, when a time bin lasted 2.5 – 6.5 s depending on video

duration. In addition, we recorded species of the nearest neighbour and its behaviour (foraging,

preening or roosting) and characteristics of the substrate where birds foraged following Novcic

(2016). However, as most focal individuals foraged on soft mud, only those individuals were

included in analyses. Finally, all focal individuals that exhibited aggressive behaviour were

dropped from analyses as it would interfere with foraging.

We were unable to obtain intake rates, i.e. the number of captures per unit of time, as we

could not reliably detect all prey captures during most focal observations. For that reason, to

determine whether capture rates co-varied with feeding rates, i.e. pecking and probing rates, we

measured capture rates in 33 birds that were close enough to observe captures from the videos. In

this subsample of the data, capture rates positively correlated with pecking rates (rp = 0.94, p <

0.0001), and probing rates (rp = 0.96, p < 0.0001). We concluded that pecking and probing rates

could be used as proxies of intake rates when birds employed these foraging methods.

Nevertheless, we could not ascertain the type of prey ingested in most cases.

Statistical analyses

Even though multiple pecks lasted longer compared to pecks, we combined these variables into a

single “peck” response variable as both events were discrete and aimed at visually capturing prey

(Baker and Baker 1973). Similarly, multiple probes lasted longer than probes, but they were

combined into a single “probe” response variable as birds typically use these techniques to

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capture prey by touch (Baker and Baker 1973). A series of rapid movements, constituting a

multiple peck or a multiple probe, was counted as one pecking or probing event, respectively.

In addition to inter-individual measurements, we also considered several other

independent variables that could potentially have an impact on foraging: time of year (early vs.

late; the median passage date of semipalmated sandpipers through the bay was May 22 (Novcic

2016), which was taken as a dividing point); recording location; recording time of day;

temperature (the temperature was obtained from the NOAA website, the station was less than 10

km away from all three locations); the occurrence of wind and direct sunshine (there were no

rainy days during the study period); position of the focal bird (edge vs. centre of the flock);

distance to cover (a distance to the nearest wooded area).

The distribution of pecks and probes during a time bin was strongly skewed to the right.

Therefore, we modelled these two distributions with the negative binomial distribution. To

analyse pecking and probing, we used a mixed negative binomial regression model including the

identity of the sandpiper nested within the location as a random factor and the independent

variables described earlier as fixed factors. The time available within a bin for pecking or for

probing (following transformation with the natural logarithm) was used as offset. To calculate

the amount of time devoted to one foraging behaviour in a time bin, we needed to transform the

occurrence of each event into time units. Using a sample of 30 pecks and a sample of 30 probes,

we calculated the median time needed to perform one peck, one multiple peck, one probe, and

one multiple probe. Using these estimates, we converted each event (e.g. one peck) into time

units, which were then summed over the total number of such events during the time bin.

To examine spatial and temporal differences in prey densities, we distinguished between

four prey categories: annelids, spionids, nereids, and all prey taxa combined. Annelids were the

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most abundant benthic invertebrates, and besides the two most abundant families, spionids and

nereids, this prey category also included capitellids and tubificids. The “all prey taxa” category

included nemerteans, bivalves, amphipods, isopods, insect larvae, and unidentified individuals

(Novcic et al. 2015). As we examined feeding rates of sandpipers foraging on soft mud, soil

samples collected under water or close to vegetation were not included in analysis. Thus, 52

samples were analyzed, 30 from Matts Landing and 22 from Bivalve; at Thompson’s Beach,

samples were collected close to vegetation (i.e. around tussocks of Spartina spp.) and were

omitted from analysis. In addition, samples collected in Matts Landing contained only two

spionids, and thus, only spionids collected in Bivalve were analyzed. To examine the effect of

sampling location and date on abundance of annelids and all prey taxa, we used two-way

ANOVA followed by Tukey’s post hoc test, while abundances of nereids were analyzed using

negative binomial generalized linear model. Temporal differences in spionid abundances were

analyzed using one-way ANOVA followed by Tukey’s post hoc test. Details on statistical

analyses are provided in Supplementary material.

Results

The dataset included 192 focal observations (Bivalve: n = 66 (34%), Matts Landing: n = 36

(19%), Thompson’s Beach: n = 90 (47%)). Focal observations lasted a median time of 40 s

(ranging from 12 to 60 s).

Overall, sandpipers pecked 54.6 times per minute. The expected number of pecks per unit

time significantly increased by 8.7% with each unit increase in temperature and by 1.6% with

each unit increase in nearest neighbour distance (Table 1). Sandpipers pecked at a higher rate in

the morning than in the afternoon. No other variables reached statistical significance. When we

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replaced nearest neighbour distance by forager density, the effect of neighbours was no longer

significant (β [SEM]: 0.99 [0.017], p = 0.99).

Overall, sandpipers probed 28.6 times per minute. The expected number of probes per

unit time significantly decreased by 10% with each unit increase in temperature (Table 2).

Sandpipers also probed at a lower rate in the morning than in the afternoon and early rather than

late in the migration phenology. Probing rate also varied according to the site with slightly higher

probing rate at Thompson’s Beach than at the other two sites. No other variables reached

statistical significance. When we replaced nearest neighbour distance by forager density, the

effect of neighbours was also not significant (β [SEM]: 1.0006 [0.021], p = 0.98).

The mean density of annelids (individuals/m2 ± SE) in samples collected in soft sediment

in Matts Landing, for entire sampling period, was 942 ± 99, nereids 587 ± 85, and all prey 1312

± 193. In Bivalve, the mean density of annelids was 2984 ± 467, spionids 1640 ± 346, nereids

860 ± 168, and all prey 3340 ± 438. The density of sampled annelids was significantly lower in

Matts Landing than in Bivalve (p < 0.001), and significantly lower on 1 June than on 30 April (p

= 0.026) (Supplementary Table S1 and Fig. S1). Nereid density was not affected by the sampling

site (p = 0.093) or date (p = 0.056), (Supplementary Table S2 and Fig. S2), while spionid density

was significantly lower on 1 June than on other sampling dates (for all comparisons p < 0.05)

(Supplementary Table S3 and Fig. S3). We found significant effect of site, but not date on the

density of all prey taxa, with higher density in Bivalve (p < 0.001) (Supplementary Table S4 and

Fig. S4).

Discussion

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The density of foragers can have a positive effect on foraging rates (Fernández-Juricic et al.

2004a; Beauchamp 2007; Fernández-Juricic et al. 2007), a negative effect (Goss-Custard 1976;

Creswell 1997) or no effect at all (Goss-Custard 1976; Yates et al. 2000; Rolando et al. 2001). In

our study, the density of foragers expressed as the number of individuals in the immediate

vicinity of focal individuals, even though measured at a very short time scale, did not have a

significant effect on birds’ feeding rates regardless of the foraging method used. However,

increase in the distance to the nearest neighbour had a positive effect on feeding rates in visually

pecking sandpipers, but no effect when birds searched for food tactilely.

These findings have two important implications. First, the results point to the fact that

different measures of density within foraging groups can lead to different conclusions regarding

foraging strategies that individuals adopt in the presence of conspecifics. In fast moving foragers,

such as sandpipers, in which the number of nearby foragers and distance to the nearest neighbour

continuously change throughout the foraging bout, feeding rates are probably more affected by

the distance to the nearest neighbour than by the overall number of foragers in their immediate

vicinity. Due to limitation of the visual system, animals more easily receive information from

their nearby neighbours regarding prey type or predation risk (Fernández-Juricic et al. 2004b).

Thus, pecking rates in both starlings (Sturnus vulgaris L., 1758) and cowbirds (Molothrus ater

(Boddaert, 1783)) decreased with an increase in the distance to nearest neighbours (Fernández-

Juricic et al. 2004a; Fernández-Juricic et al. 2007).

The second implication of our study is that competitors in close proximity can differently

affect foragers using different foraging modes. Nearby foragers affect prey residing close to the

substrate surface more than prey that is burrowed deeper in substrate, thus causing differences in

intake rates in animals searching for prey visually and tactilely (Goss-Custard 1976; Selman and

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Goss-Custard 1988). Interference through prey depression was detected in redshank pecking for

amphipods, but not in redshank and bar-tailed godwit (Limosa lapponica (L., 1758)) probing for

annelids (Goss-Custard 1976; Selman and Goss-Custard 1988; Yates et al. 2000; Minderman et

al. 2006). This is because amphipods, C. volutator and Orchestia spp., which are commonly

preyed upon by shorebirds, in the presence of predators reposition themselves from mud surface

deeper into their burrows, thus becoming temporarily less available to birds that are feeding by

sight (Goss-Custard 1976; Minderman et al. 2006; MacDonald et al. 2014). The average

response distance of C. volutator, at which they retreat deeper into the sediment in the presence

of redshank, is 0.6 m (range 0.1 to 1 m) (Stillman et al. 2000). Semipalmated sandpipers fed on

amphipods at Matts Landing, and presumably other study sites (Novcic et al. 2016), so

interference through prey depression could be responsible for lower pecking rates when nearest

neighbours were close, as the average distance to nearest neighbours was ~ 0.2 m (distance

expressed in mean body lengths ± SE was 1.8 ± 0.07). However, we do not have information on

the species composition of sampled amphipods, and therefore, we cannot assert that amphipods

on our study locations responded to the presence of shorebirds in the same way as C. volutator

did. In addition, it should be noted that from observed videos it was clear that sandpipers also

visually located various worms whose behavioural response to the presence of predators is also

not known.

Several environmental variables affected feeding rates of semipalmated sandpipers in

Delaware Bay during spring migration. Study site affected probing rates, as birds probed more

often in Thompson’s Beach compared to other locations, and slightly more in Bivalve than in

Matts Landing, but had no effect on pecking rates. These differences in the rate of probing at the

study sites were most likely a consequence of prey density and composition at these sites. In the

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Bay of Fundy, feeding rates of sandpipers increased with the prey density (Beauchamp 2007).

However, in our study fewer probes per unit of time were observed in Bivalve, even though the

overall prey density in Bivalve was significantly higher than in Matts Landing. Probing at lower

rate under higher prey density might happen if birds spent more time handling prey and less time

searching when prey are abundant. The overall mean density of annelids was higher in Bivalve

compared to Matts Landing. In addition, the composition of annelid taxa was different on these

sites, in Bivalve spionids were the most abundant annelids, whereas nereids were more common

in Matts Landing. Differences in prey communities could also contribute to observed differences

in probing rates between the study sites. The nereid polychaete, Hediste diversicolor (Müller,

1776), is commonly preyed upon by redshank tactilely (Goss-Custard 1976), while spionid

polychaetes are usually located in the upper layers of the sediment, particularly Streblospio

benedicti Webster, 1879 (Nelson and Capone 1990), the most abundant spionid in samples from

Bivalve, and therefore, they may be more subject to pecking than probing. The density of

spionids significantly declined at the end of stopover period, and this may be one of the reasons

responsible for higher probing rates recorded in the late phase of migration through Delaware

Bay. Pecking rates were positively affected by increasing air temperatures, while probing rates

decreased with increasing temperatures. Changes in temperature may influence activity of

benthic invertebrates, as well as their vertical zonation (Evans 1976; Pienkowski 1983; Somero

2002), thus causing decrease in probing rates with increasing temperature due to retreat of

intertidal invertebrates deeper into the sediment (Dominguez 2002; Nebel and Thompson 2005),

or increase in pecking rates with increasing temperature due to increase in surface activity of

invertebrates at higher temperatures (Pienkowski 1983). Time of day also had contrasting effects

on sandpipers’ pecking and probing rates; rates of pecking were higher in the morning, while

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probing occurred more often in the afternoon. Nebel and Thompson (2005) recorded the highest

probing rates in western sandpiper (Calidris mauri (Cabanis, 1857)) around mid-day, when

sediment temperatures reached their maximum throughout the day. However, at our study sites,

temperatures were significantly higher in the morning compared to the afternoon (t = 4.04, df =

4587.4, p < 0.001), suggesting that more research is needed on daily fluctuations in density,

distribution and activity of benthic invertebrates in order to reach conclusive answers about

influence of these variables on feeding rates.

To exploit diverse resources foragers are able to use different foraging modes. Our study

shows that in group foragers these techniques can be differently affected by the density of

competitors and more importantly, that the density effect is mediated through the distance to the

nearest neighbour rather than the number of nearby competitors. For that reason, both the

foraging mode and neighbour distance should be taken into account in studies on the effect of

forager density on feeding rates.

Due to habitat loss, human disturbance, prey depletion and increasing predation,

populations of many shorebird species have been decreasing worldwide (International Wader

Study Group 2003). Recent research suggests that conditions birds encounter on migratory

habitats may be responsible for population declines. Insufficient food on staging sites may lead

to reduced feeding and fueling rates, with negative fitness consequences (Baker et al. 2004;

Morrison 2006). In addition, coastal stopover habitats are often affected by the tidal cycle,

forcing birds to forage on restricted areas along intertidal beaches and mudflats at the same time,

leading to competition through both depletion and interference (Recher and Recher 1969; Burger

et al. 2007). Therefore, information on how density-dependent interactions between shorebirds

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affect their feeding rates should be accounted for while managing their foraging habitats on key

stopover areas.

Acknowledgements

We thank Sebastian Kvist and Louis Sorkin for help with identification of invertebrates. We also

thank two anonymous reviewers for helpful comments on the manuscript. This study was

supported by a CUNY Doctoral Students Research grant # 6.

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Table 1. Summary of the main effects of a number of independent variables on the expected

number of pecks per unit time in spring staging semipalmated sandpipers (Calidris pusilla). For

categorical variables, the table provides least squares means (SEM). For quantitative variables,

we provide beta values (SEM).

Independent variable Least squares means or betas (SEM) p-value

Location Bivalve: 0.92 (0.12)

Matts Landing: 0.85 (0.21)

Thompson: 0.85 (0.12)

0.94

Migration phenology Early: 0.99 (0.12)

Late: 0.76 (0.15)

0.35

Time of day AM: 1.2 (0.092)

PM: 0.66 (0.11)

0.001

Occurrence of wind Yes: 0.92 (0.10)

No: 0.82 (0.11)

0.56

Position in the flock Edge: 0.84 (0.087)

Centre: 0.90 (0.10)

0.65

Closest species Semipalmated sandpiper: 0.87 (0.072)

Other species: 0.87 (0.082)

0.98

Temperature (°C) β = 1.087 (0.031) 0.005

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Distance to cover (m) β = 1.0009 (0.0009) 0.33

Distance to nearest

neighbour (in body length

units)

β = 1.016 (0.067) 0.041

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Table 2. Summary of the main effects of a number of independent variables on the expected

number of probes per unit time in spring staging semipalmated sandpipers (Calidris pusilla). For

categorical variables, the table provides least squares means (SEM). For quantitative variables,

we provide beta values (SEM).

Independent variable Least squares means or betas (SEM) p-value

Location Bivalve: 0.35 (0.053)

Matts Landing: 0.38 (0.067)

Thompson: 0.60 (0.088)

0.032

Migration phenology Early: 0.25 (0.042)

Late: 0.74 (0.13)

0.0005

Time of day AM: 0.35 (0.044)

PM: 0.54 (0.066)

0.008

Occurrence of wind Yes: 0.42 (0.049)

No: 0.44 (0.054)

0.72

Position in the flock Edge: 0.47 (0.045)

Centre: 0.40 (0.047)

0.26

Closest species Semipalmated sandpiper: 0.44 (0.032)

Other species: 0.42 (0.049)

0.50

Temperature (°C) β = 0.90 (0.024) 0.0002

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Distance to cover (m) β = 1.0013 (0.0007) 0.057

Distance to nearest

neighbour (in body length

units)

β = 1.0065 (0.083) 0.48

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foragers, nearest-neighbour effect draft · draft 2 abstract we examined the influence of the...

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