A Survey of Population Size and Parasitism of Gray Vireos in the Los Piños Mountains Jaclyn 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: 20 th April -2 nd Aug (but usually late May)  Clutch size: 2 – 4 eggs  Incubation period: days  Nestling period: 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: 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

Methods If 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 Vireos / 40 Ha Found 41 individuals 12 mated pairs 7 nests (3 parasitized, 4 depredated)

Northern Pi ñ os Mountains 1.83 Vireos / 40 Ha 25 individuals  6 unknown individuals  9 known pairs 4 nests N

Northern Pi ñ os 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) pp Mayfield, Harald. F. Suggestions for Calculating Nest Success. The Wilson Bulletin: 87(4) pp Pierce, Leland J., Gray Vireo (Vireo vicinior) Recovery Plan. New Mexico Department of Game and Fish. May 3, 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) pp 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 pp

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