A Survey of Population Size and Parasitism of Gray Vireos in the Los Piños MountainsJaclyn N. Andberg

Niles H. Brinton

Introduction Little is known about the population densities and

effects of brood parasitism on the Gray Vireo in New Mexico

The Gray Vireo is A Species of Conservation Concern (USFWS 2002) Listed as Threatened in the State of New Mexico (New

Mexico Game and Fish 2008) Frequently parasitized by the Brown-Headed Cowbird

Increased knowledge of this species will Aid in improved wildlife management of the Sevilleta

National Wildlife Refuge (SNWR) Improve our understanding of biotic interactions between

SNWR and surrounding rangeland

Gray Vireo (Vireo vicinior) Small, insectivorous passerine with

gray plumage Inhabits low-elevation piñon (Pinus

spp.)-juniper (Juniperus spp.) woodlands in the southwestern United States

Cup-shaped nests suspended in fork branches

Egg dates: 20th April -2nd Aug (but usually late May)

Clutch size: 2 – 4 eggs Incubation period: 12-14 days Nestling period: 13-14 days Parasitized nests are often abandoned

Brown-headed Cowbird (Molothrus ater) Stocky, brood parasite from the

family Icteridae (blackbirds) Forages for ground seeds and

grains, grasshoppers and beetles in open grasslands

“nests” in trees from surrounding habitats

Energy spent on egg production rather than parental care

Can lay up to 3 dozen eggs in one breeding season

Lay eggs in the nests of over 220 bird species

Clutch size: 1-7 eggs Incubation period: 10-12 days Nestling period: 8-13 days

Species Distribution

Objectives

1) Determine population density of the Gray Vireo by surveying suitable sample habitats located within the SNWR

2) Monitor nests in order to determine a) nest success of parasitized versus non-parasitized nests b) productivity of breeding Gray Vireo pairs

3) Determine the effects of brood parasitism on reproductive success by comparing sites based on proximity to heavily utilized rangeland

Methods Two study sites based on

habitat suitability and proximity to actively grazed rangeland

Sampling of two sites alternates daily

Territories are identified and nests are located

Sepultura Flats

Northern Piños Mountains

Methods

300 m² grid points at each study site

60 points surveyed per site

~546 Ha. Sites (1,350 acres)

Surveying begins around 7:00 am

Methods

Song playback is used at each point

Gray Vireos are detected by sight and sound

Individuals are recorded and observed for nest building/tending behavior in order to locate nests

MethodsIf nests are not immediately located, territories are returned to for further observation

Extend-A-Vue mirrors are used to observe out-of-reach nests

Nests are monitored every 3-4 days

Census Results 1.50 Vireos / 40 Ha

Previous research found density range of 0.2 - 0.9 Vireos / 40 Ha

Found 41 individuals

12 mated pairs

7 nests (3 parasitized, 4 depredated)

Northern Piños Mountains

1.83 Vireos / 40 Ha

25 individuals 6 unknown individuals 9 known pairs

4 nests

N

Northern Piños Nest Data

No nests produced vireo fledglings

2 out of 4 nests were parasitized, then abandoned

2 non-parasitized nests were depredated

Sepultura Flats

1.17 Vireos / 40 Ha

16 individuals 10 unknown individuals 3 known pairs

3 nests

map

Sepultura Nest Data

1 nest was parasitized (with an unhatched cowbird egg)

This nest was successful

2 out of 3 nests were depredated, but not parasitized

Discussion

1.50 Vireos / 40 Ha There were 5 mated pairs

for which no nest was ever located

Cowbird parasitism was observed at both sites

Not enough nests were found to perform statistical analyses

Discussion Every nest that was not

predated upon was either parasitized or abandoned

Parasitism commonly leads to nest abandonment

The only successful nest was parasitized

Nests were all found in trees very close to arroyos

Drought

Food source availability Reproductive habits Productivity was very low

during this breeding season (0.29 young / territory)

Literature: 0.7 – 3.0 young / territory

Future Research Expand sample areas

and produce multi-year data set

Determine if nest-arroyo proximity is significant

Use cameras to determine nest predators

Cowbird trapping

Drought effects Were vireos pairing

normally? Were vireos nesting

normally? Is productivity higher in

years with more precipitation?

Literature Cited Martin, Thomas E., and Geupel, Geoffrey R. Nest-Monitoring Plots: Methods for

Locating Nests and Monitoring Success. Journal of Field Ornithology: 64(4). 1993. pp 507-519.

Mayfield, Harald. F. Suggestions for Calculating Nest Success. The Wilson Bulletin: 87(4). 1975. pp. 456-466.

Pierce, Leland J., Gray Vireo (Vireo vicinior) Recovery Plan. New Mexico Department of Game and Fish. May 3, 2007.

Rothstein, S. I., Verner, J., and Steven, E. Radio-Tracking Confirms a Unique Diurnal Pattern of Spatial Occurrence in the Parasitic Brown-headed Cowbird. Ecology: 65(1). 1984. pp. 77-78.

Stake, Mike M., and Garber, Gail. Gray Vireo Monitoring in Northwestern and Southeastern New Mexico. Proceedings of the New Mexico Department of Game and Fish Gray Vireo Symposium, April 2008. pp 11-13.

Acknowledgements Kathy Granillo Scott Collins Jennifer Johnson Fellow REU students Sevilleta Interns