A removal experiment was conducted to measure how much and by what mechanisms brood parasitic Brown-headed Cowbirds (Molothrus ater) cause nest failures in a commonly used host, the Song Sparrow (Melospiza melodia). When numbers of female cowbirds were reduced experimentally, nest failures fell from 65.0% (n = 663 nests) to 49.9% (n = 331). Cowbird reduction reduced the frequency of nest failure to one-third of control levels in Song Sparrows during the last 80 days of the sparrow's breeding season, the period when most parasitic laying took place. Cowbird reduction decreased nest failures strongly at the egg stage, and weakly at the nestling stage. Daily nest-failure rates were independent of whether or not a nest was parasitized by cowbirds. Two hypotheses were tested to explain how cowbirds cause host nests to fail: first, egg removal by female cowbirds lowers clutch size below a threshold where the host deserts; second, cowbirds cause host nests to fail by destroying entire clutches or broods. In support of the first hypothesis, desertion following parasitism and egg removal was less frequent when cowbird numbers were reduced (8.9% of n = 158 nests) than for unmanipulated controls (16.5% of n = 424 nests). In support of the second hypothesis, there were fewer cases where young were killed in the nest, or found dead near it, after cowbird numbers were reduced (2.5% of 158 nests) than in controls (4.7% of 424 control nests). In contrast, proportions of nests that failed after the disappearance of all eggs, young, or both, and after unparasitized clutches were deserted, increased when cowbird numbers were reduced. Although our study supports both hypotheses, cowbird-induced desertion had a greater effect on nest failure rates than did cowbird predation. Our study suggests that cowbird removal programs are likely to benefit commonly used and endangered hosts by reducing rates of nest failure.