Host-Egg Removal by Brown-Headed Cowbirds: A Test of the Host Incubation Limit Hypothesis

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 eggs 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 I 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 may increase hatching success in some years.