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Noa, Leslie

Wildlife and Fisheries Biology Program

2005

M.S.

Understanding the effects of habitat selection for a declining species is integral to developing effective management strategies; this is particularly important in the case of the endangered black-capped vireo (Vireo atricapilla). I analyzed differences in abundance, age structure, and nesting success of black-capped vireos in 2 early successional habitat types found on Fort Hood, an 87,890-ha army base in central Texas. These habitats were: 1) large areas of continuous shrubby vegetation, referred to as shrubland habitat, and 2) small patches of shrubby vegetation centered on one or several large trees, known locally as donut habitat. Black-capped vireos prefer shrubland habitat, and we know that this represents high quality habitat. The objectives of my study were to determine whether there were differences in abundance, age structure, and nesting success in these 2 habitat types and to determine whether donut habitat is high- or low-quality habitat. I used a Wilcoxon Rank Sum test to examine differences in abundance, and I used Fisher's Exact test to examine differences in age structure. The data indicated that there were differences in abundance and age structure of black-capped vireos between shrubland and donut habitat. Donut habitat had a lower abundance of vireos (2 times less than shrubland per point count) and a higher percentage of second-year males. Second-year males are first-time breeders and are presumed to be subordinate to older, experienced breeding males. The younger age structure of male vireos in donut habitat indicates that it is lower quality habitat. Analyses of nesting success using an information- theoretic approach indicated that habitat, nest height, and year were all important variables. In both shrub land and donut habitat daily nest survival increased as nest height increased. Differences in daily survival rates for the 2 years of the study may be explained by environmental fluctuations, which are known to affect food availability, or may be explained by predator/prey cycles. Wildlife biologists on Fort Hood should continue to monitor areas of donut habitat to determine if they represent potential population sinks and continue to manage of vireo habitat through the use of prescribed burning and mechanical disturbance. This study provided clear evidence that habitat is a limiting factor for black-capped vireos at Fort Hood and throughout the range of this federally endangered species.