Brood Parasitism

Brood Parasitism by the Brown-headed Cowbird (Molothrus ater) is considered the most immediate threat to the Black-capped Vireo (U.S Fish and Wildlife Service, 1996). It is estimated that 90% of vireo nests in the Kerr Wildlife Management Area are invaded by the cowbird (U.S Fish and Wildlife Service, 1996). Cowbirds lay their eggs in other birds’ nests (including vireos’) leaving the host bird to raise their young. The cowbird eggs hatch several days before the vireo eggs; cowbird nestlings are larger than vireo nestlings, and vireo nestlings usually die soon after hatching because they are unable to compete for food and space. Thus, few parasitized nests actually produce vireo fledglings (U.S Fish and Wildlife Service, 1996).

Systematic management of cowbird populations may help the vireo. A study found that removal of the parasitic Brown-headed Cowbird from vireo range increased the numbers of vireo fledglings (U.S Fish and Wildlife Service, 1996). According to J. Grzybowski, "The actual fecundities of 1.2 – 1.25 female young per adult female needed to maintain long-term viability of vireo populations may occur in some populations, but in those studied across a broad portion of the vireo’s range in Oklahoma and Central Texas, these levels of fecundity have only been achieved when Cowbird removal was implemented" (Gryzbowski, 1995).

Loss and Fragmentation of Nesting Habitat

Overgrazing by both livestock and wildlife are detrimental to the Black-capped Vireo; the resulting reduction of browse decreases suitable nesting habitat. Fire-suppression has an impact as well; Black-capped Vireos occupy habitat of a certain successional stage, and lack of disturbance such as fire has resulted in growth of vegetation beyond the suitable successional stage (U.S Fish and Wildlife Service, 1996). Overgrazing contributes to this fire-suppression, as a lack of fine fuel results from excessive grazing.

Habitat fragmentation isolates vireo populations and exposes them to the dangers of the habitat edge: cowbirds and predators. Agricultural and range practices such as clearing and grazing cause this fragmentation, as does urbanization. Direct habitat loss due to urban and suburban development is a major threat in Travis, McLennan, Dallas, Bexar, and Kerr counties (Campbell, 1995); Austin is particularly cited for its encroachment on vireo habitat, as it continues its fast rate of urban expansion.

Predation