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Developing an adaptive management approach to cowbird control on the Santa Clara River, California

Sophie S. Parker, Linnea S. Hall, Mary J. Whitfield, Laura Riege, Kathryn R. Selm, René Corado

This paper presents results from a field-based study that can be used to benefit songbirds in riverside habitats like those found along the Santa Clara River in Ventura County, California. Songbirds such as the Least Bell’s Vireo suffer population declines in part because Brown-headed cowbirds lay their eggs in vireo nests, a behavior known as brood parasitism. Cowbird trapping can increase vireo populations, but it is costly, and can capture and harm species other than cowbirds. Therefore, wildlife managers have a compelling interest in evaluating multiple means of cowbird control. The authors of this study tested how vireo nest parasitism rates were impacted by shortening the cowbird trapping period or replacing traps with target-netting. They found that a shorter trapping period still resulted in large numbers of cowbird and other species captures with no parasitism of vireo nests. While the target-netting method was not effective for capturing cowbirds, “contingency” traps used for less than a week were effective at removing cowbirds without capturing large numbers of other species, and were less costly that two full months of trapping. Based on these findings, the authors recommend that an adaptive cowbird management program to benefit songbird populations should consider using regular point counts and nest monitoring to provide data on cowbird impacts, in lieu of continued intensive trapping without monitoring. Restoration of native vegetation is also crucial and effective at limiting parasitism.

Read the accompanying Cool Green Science blog post.