Range Expansion and Diurnal Changes in Dispersion of the Brown-Headed Cowbird in the Sierra Nevada

Rothstein, Stephen; Stevens, Ernest; Verner, Jared

An extensive literature search and survey of specimens in major scientific collections in California showed that the Brown-headed Cowbird (Molothrus ater) was absent from virtually the entire Sierra Nevada of California prior to 1930. Present distribution, diurnal patterns of dispersal, seasonal occurrence, and host species were studied directly in the field from 1975 to 1978. The evidence of a recent range expansion of cowbirds into the Sierra is clearcut, the birds appearing to be ubiquitous over most or all of the mountains at this time. We did not investigate their presence in areas remote from human developments, however, so it is possible that the species has not invaded such areas, particularly if they support large, unbroken stretches of mature forest. Cowbirds apparently exploit horse corrals and other human developments for group foraging during midday, scattering by evening or early morning to dispersed breeding sites. Human influence may have been a necessary precursor to cowbird invasion of the Sierra, which is probably prime cowbird habitat in terms of host availability but marginal habitat in terms of food for adults. Twenty-two host species have thus far been discovered in the Sierra Nevada. Preliminary data suggest that smong these the rate and consequences of nest parasitism may be severe enough to threaten continued survival in the Sierra of some species, especially the Warbling Vireo (Vireo gilvus). We believe the situation is serious enough to warrant close monitoring of these species' populations in the future.

Range Expansion and Diurnal Changes in Dispersion of the Brown-Headed Cowbird in the Sierra Nevada

Series