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The abundance, distribution and brood parasitism of upland-breeding warbling vireos in a fragmented forest landscape

University of British Columbia

In a two-year study of the Warbling Vireo Vireo gilvus, I explore patterns of distribution, habitat, and brood parasitism along an elevation gradient in the South Okanagan region, British Columbia using landscape analyses. The Warbling Vireo is threatened at low elevations in the Okanagan by intense parasitism pressure by Brown-headed Cowbirds Molothrus ater. Vireo persistence may depend upon neighbouring source populations in the montane forests overlooking the valley. In the first part of the study, I found brood parasitism to be significantly lower in higher elevation forests compared to the valley. Cowbirds are less and parasitise fewer species upland. Of sampled nests, only the Warbling Vireo is parasitised frequently, with approximately 30 percent of all nests suffering parasitism in the forest. Such low levels of cowbird activity are likely due to food limitation, since host species dominate the songbird community. Levels of upland parasitism were not linearly correlated with proximity to the valley, either due to sampling error or to local-scale ecological factors. Forest cover appears to affect parasitism levels, with most parasitism occurring on sites of moderate tree density. The second part of my study employs variables derived from digital forest inventory maps that are used to predict Warbling Vireo distribution and avian species richness across the Okanagan Forest at two spatial scales. The presence of vireos is positively correlated with early serai stage variables at both the local stand level and the landscape level. At the landscape scale, species richness is associated positively with habitat diversity, and negatively with mature forest area in a multiple regression model. Though sustained industrial forestry in the Okanagan Forest may enhance avian diversity and Warbling Vireo habitat alike, the frequency of vireo detections is significantly lower at higher elevation. This reduces the likelihood that montane forest is a source of Warbling Vireos for the valley, in spite of decreased parasitism pressure in upland habitat.

Thesis/Dissertation

application/pdf

2009-05-23

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http://hdl.handle.net/2429/8081

Zoology

University of British Columbia

UBCV

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