UNDERSTANDING NEST SUCCESS AND BROOD PARASITISM IN THE ENDANGERED BLACK-CAPPED VIREO: COMPARISONS WITH TWO SYMPATRIC SONGBIRDS

Nest predation and brood parasitism likely cause declines in many songbird species, yet the rate of these declines varies among species. Simultaneously studying co-occurring species with overlapping territories provides an opportunity to identify differences among the species that may explain differential reproductive success. We monitored nests of three co-occurring songbird species to determine whether the federally endangered Black-capped Vireo (Vireo atricapilla) experienced lower reproductive success than Northern Cardinals (Cardinalis cardinalis) and White-eyed Vireos (V. griseus) on private lands with and without trapping of brood parasites. We assessed if habitat metrics and nesting phenology were associated with nest success and frequency of brood parasitism by Brown-headed Cowbirds (Molothrus ater) to determine if these characteristics explain poor reproductive success. We monitored 188 nests in shrubland and ecotones with woodlands in central Texas from 2006-2008. We found species-specific differences in nest success and brood parasitism, year effects, and differential impacts of cowbird trapping. Black-capped Vireos nesting in locations without cowbird trapping experienced the lowest daily survival rate of nests (0.86) and period nest survival (0.01), whereas estimates in locations with cowbird trapping were similar to the other species (0.93 for daily and 0.11 for period nest survival). Nest initiation date was important for predicting brood parasitism, showing high probability of brood parasitism in the middle of the breeding season and low probability during the beginning and end of the season. Habitat metrics were not good predictors of nest success or brood parasitism. Management for conservation of Black-capped Vireos focused on vegetation manipulation may not increase reproductive success in the absence of cowbird trapping in some areas of the breeding range, because daily survival rate of nests was associated with cowbird trapping but not habitat metrics.