Brood Parasitism of the Abert's Towhee: Timing, Frequency, and Effects

Abstract

The effects of brood parasitism by the dwarf race of the Brown-headed Cowbird (Molothrus ater obscurus) on the nesting success of the Abert's Towhee (Pipilo aberti) in the lower Colorado River valley were studied. The frequency of cowbird parasitism varied significantly with time of season. The laying season of cowbirds paralleled that of migratory songbirds, but began a month later than that of the sedentary Abert's Towhee. After an influx of breeding cowbirds, 44% of towhee nests were parasitized. Egg ejection by adult cowbirds caused towhee clutches to decline significantly from a laying average of 2.9 eggs to 1.7 eggs at hatching. The probability of nesting success declined from 0.278, before cowbirds arrived on the breeding grounds, to 0.046 for all parasitized nests. The lack of a significant difference between daily mortality rates of nonparasitized nests before and after cowbirds started breeding indicated that the reduced success of late towhee nests was primarily due to cowbird parasitism. The success of cowbird eggs in towhee nests was also low, suggesting that towhees may not be favorable hosts.