Short Incubation Periods of Brown-Headed Cowbirds: How Do Cowbird Eggs Hatch before Yellow Warbler Eggs?

Abstract

Brown-headed Cowbird (Molothrus ater) eggs in natural nests hatch either before or at the same time as Yellow Warbler (Dendroica petechia) eggs despite being almost twice the size. Cowbird eggs may hatch before Yellow Warbler eggs because they (1) prolong the incubation of host eggs, (2) have more rapid embryonic development, or (3) hatch in response to stimuli from host eggs. We tested these hypotheses by comparing the incubation periods of cowbird eggs incubated in natural nests, incubated artificially in isolation from other eggs, and incubated artificially in clutches of Yellow Warbler eggs. Cowbird eggs incubated artificially in isolation from other eggs took significantly longer to hatch than either artificially incubated warbler eggs or naturally incubated cowbird eggs. The ratio of incubation period to egg volume was less for cowbirds than warblers, which indicates that cowbird embryos develop more rapidly relative to their size. The presence of a cowbird egg in natural nests significantly prolonged the incubation period of warbler eggs. Therefore, cowbird eggs appear to disrupt the incubation of smaller host eggs. Finally, the incubation periods of cowbird eggs incubated artificially in contact with warbler eggs were shorter, but not significantly so, than cowbird eggs incubated in isolation from other eggs. These results suggest that in addition to rapid embryonic development, cowbirds hatch before many hosts by disrupting incubation of smaller eggs and, possibly hatching early in response to stimuli from host eggs.