Brown Headed Cowbird

Brown Headed Cowbird

The brown headed cowbird, Molothrus ater, is a nest parasitic species which is depressing other passerine bird populations, including the least Bell's vireo. The cowbird has expanded its range from the short grass prairie of the high plains into agricultural and suburban landscapes throughout most of North America. The species has occupied most of California since the 1900s (Rothstein, et al., 1980), where it persists as a year-round resident. The cowbird's preferred habitat is varied and may include prairie, savanna grassland with low or scattered trees, woodland edges, fields, pasture, orchard and residential areas. Cowbirds are, to a certain extent, dependent upon food provided by humans and livestock operations. They are opportunistic generalist feeders, foraging primarily on the ground for seeds, arthropods, and animal waste.

Cowbirds lay their eggs in the nests of other species. While the cowbird's mating strategy may vary depending upon sex ratios in the population (Friedmann, 1929), females begin laying eggs in late April and may continue laying until mid-July. They seem to prefer to parasitize nests in woodland/shrubland edges, often in human-modified landscapes.

The cowbird's home range includes breeding, feeding, and roosting sites. The size of the home range is highly variable. Rothstein (et al., 1984), reported a home range of around 400ha for birds that flew distances of 2 - 7km to feeding areas.

The cowbird population will thrive as a result of Plans Build-Out, as seen in figures 73 and 74. Habitat could almost quadruple regionally, expanding by about 780,000ha, mainly in new rural residential areas. One important consequence of this expansion will be a decline in bird species parasitized by the cowbird.