Host-egg removal by brown-headed cowbirds: A test of the host incubation limit hypothesis

Abstract

The incubation limit hypothesis (Davies and Brooke 1988) states that female brood parasites remove host eggs to reduce the parasitized clutch volume to a size that the host can incubate effectively, thereby ensuring that the parasite's egg will hatch. We tested this hypothesis by adding freshly laid Brown-headed Cowbird (Molothrus ater) eggs to Yellow Warbler (Dendroica petechia) clutches and comparing hatching success in clutches from which one or no host eggs, were removed (Treatments 1 and 2, respectively). Hatching success of cowbird eggs differed significantly between Treatments 1 and 2 in one of the three years of study. Warbler hatching success tended to be higher in control and in treatment 1 clutches than in Treatment 2 clutches. Climatic conditions varied between years but did not influence cowbird hatching success. Cowbird eggs tended to hatch before warbler eggs whether or not a host egg was removed. Neither incubation period, hatching spread, nor hatching order was significantly influenced by egg removal. These results indicate that removal of a host egg is not necessary for successful hatching of cowbird eggs in Yellow Warbler clutches in all years, although it many increase hatching success in some years.