Black-capped Vireos and White-eyed Vireos: How two birds adapt to challenges of living in Central Texas

Thesis (Master's)--University of Washington, 2016-12Black-capped vireos (Vireo atricapilla) and white-eyed vireos (Vireo griseus) are closely related songbirds, which occupy the same landscape in Central Texas. While white-eyed vireos are common and widespread, black-capped vireos are federally endangered with a restricted distribution. Both species are vulnerable to brown-headed cowbird (Molothrus ater) parasitism, and brood parasitism has been implicated in the decline of black-capped vireos. Here we use a comparative approach to investigate the divergent species responses to elucidate causes of the black-capped vireos imperiled status. From 2011 to 2015 we studied black-capped and white-eyed vireos across a range of six study sites, which represented a spectrum of vegetation types and levels of cowbird control regimes. We monitored 404 black-capped vireo territories and 813 nests and 141 white-eyed vireo territories and 268 nests. We collected vegetation measurements in breeding adult vireo territories to test for differences in vegetation use. We performed 34 nest observations to examine nest building behaviors as they relate to cowbird parasitism. We performed n = 229 nest vegetation surveys to examine differences in nest placement and potential influences on cowbird parasitism. We tracked 122 juvenile black-capped and 25 juvenile white-eyed vireos during the pre-migration period to estimate survival, dispersal, and vegetation use. We documented higher levels of brood parasitism in black-capped vireos than white-eyed vireos and identified vireo’s singing rate within 15m of the nest during the building stage as being associated with parasitism risk. We found white-eyed vireos exhibited a wider range of vegetation use than black-capped vireos, including areas with higher levels of canopy cover, which is associated with lower levels of cowbird parasitism. We found that compared to the white-eyed vireo, black-capped vireos seem to be especially prone to brood parasitism from brown-headed cowbirds, and cowbird control will likely remain an important factor in black-capped vireo conservation