How Do Brown-headed Cowbirds (Molothrus ater) Cause Nest Failures in Song Sparrows (Melospiza melodia) ? A Removal Experiment
Smith, J.N.M.; Taitt, M.J.; Zanette, L.
The Auk 120(3): 2-83
The mechanisms by which brown-headed cowbirds (Molothrus ater) cause nest failure in song sparrows (Melospiza melodia) were investigated. Brown-headed cowbirds, a brood parasite, commonly use song sparrows as hosts. Reduction of cowbird numbers greatly reduced nest failure frequency in song sparrows during the last 80 days of the sparrow's breeding season. Nest failures decreased strongly at the egg stage and weakly at the nestling stage. Daily nest-failure rates were independent of parasitism by cowbirds. The effect of reducing cowbird numbers on nest desertion following egg removal and mortality of young song sparrows supported 2 hypotheses of nest failure: that egg removal by female cowbirds lowers clutch size below a threshold where the host deserts and that cowbirds cause host nests to fail by destroying entire clutches or broods. The findings indicate that cowbird removal programs are likely to benefit commonly used and endangered hosts.