Population Dynamics of the Endangered Black-capped Vireo (Vireo atricapilla)

Abstract

Landscape heterogeneity and fragmentation create local populations that differ in habitat availability and predator communities and present unique tradeoffs between reproductive success and survival. Varying demography across the landscape can result in a metapopulation structure and the recognition of source-sink dynamics could have far-reaching implications for wildlife management, particularly of endangered species. The black-capped vireo (Vireo atricapilla; vireo) is an endangered songbird that suffers from high rates of parasitism by the brown-headed cowbird (Molothrus ater) across its breeding range from northeastern Mexico to southern Oklahoma. The largest population located on the Fort Hood Military Reservation in central Texas is well-studied, but its role in landscape-level vireo population dynamics is largely unknown. From 2011 through 2014, we monitored seven vireo populations on Fort Hood and nearby private and state lands that varied in vegetation and nest parasitism rates. We calculated measures of fecundity and survival and assessed populations as sources or sinks using population matrices. Sites with cowbird control had greater overall nest success and the most successful source population had complete cowbird control in all study years. To investigate tradeoffs in habitat selection, we monitored breeding territories and tracked juvenile vireos using radio-telemetry. Vireos in general selected shrubland and forest habitats; canopy cover may present a tradeoff that improves juvenile survival but reduces breeding success. We recorded vocalizations at vireo nests and found little evidence for tradeoffs in social communication and mediation of parasitism or predation risk. Vireos did benefit, however, by optimizing temporal patterns in vocalizations. Finally, we documented some flexibility in mating strategy through an observation of polygyny in a territorial male vireo. Land managers of vireo breeding populations should consider habitat needs across the entire reproductive cycle: cowbird control will increase nest success and breeding sites that provide nearby or adjacent forested areas may improve survival of recently independent juveniles.