Managing brown-headed cowbirds to sustain abundance of black-capped vireos

Authors: K. N. Smith, A. J. Campomizzi, M. L. Morrison, and R. N. Wilkins

Brood parasites can appreciably decrease fecundity of susceptible songbird hosts, which can often cause a decrease in host abundance. Wildlife managers use brown-headed cowbird (Molothrus ater) management to reduce parasitism frequency and benefit the conservation of the endangered black-capped vireo (Vireo atricapilla); however, intensity of management needed to increase vireo abundance is not well-understood. We used sensitivity analyses of population models for black-capped vireos to assess effects on abundance of particular changes in parasitism frequency. Our models suggest that the parasitism frequency vireos can tolerate while maintaining abundance in a particular location is ≤30%. If parasitism frequency is high, trapping cowbirds during ≥1 of every 3 years may be sufficient for reducing parasitism enough to maintain abundance at managed locations. Cowbird management programs may need to be intensive in initial years to increase abundance of vireos being managed if the initial abundance is low. Rotating locations of traps each year among managed locations may be effective for maintaining vireo abundance while decreasing overall trapping effort and making more efficient use of management funds. Increasing and restoring habitat concurrent with cowbird management would likely further increase the likelihood of establishing and maintaining vireo abundance in managed locations.

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