Mechanisms of Short Incubation Period in Brood-Parasitic Cowbirds

Abstract

Field estimates of incubation periods of brood parasitic cowbirds (Molothrus spp.) indicate that they are unusually short given the sizes of their eggs. As a consequence, cowbirds usually hatch before host young, even though cowbird eggs are frequently larger. Field-estimated incubation periods, however, have an inherent uncertainty because of in- termittent attendance by incubating birds, and the mechanisms for early hatching are un- known. I report incubation periods of Shiny Cowbird (M. bonariensis) eggs under controlled conditions in the laboratory, and test the hypothesis that cowbirds exhibit accelerated rates of embryo development, by exploring two possible mechanisms for shortening the incubation period of cowbird eggs: (1) cowbird eggs may have a low energy content, which results in a short incubation period because yolk reserves are depleted earlier; or (2) cowbird embryos may have elevated growth rates, which would be reflected in their metabolic rates. Energy content of cowbird eggs was 23.8% lower than expected from egg mass. Mean incubation period in the laboratory was 11.7 days, which is 28.8% shorter than expected from egg mass, but close to the expected from energy content. Similarly, yolk-free, dry hatchling mass was lower than expected from egg mass, but not significantly different from the expected from egg energy content. These results support the hypothesis that cowbirds shorten incubation period by reducing the energy content of the egg, although it results in a slightly smaller hatchling. The hypothesis that cowbird embryos have elevated growth rates, in contrast, was not supported because embryos did not have higher metabolic rates than expected from their size. Although the evidence presented here is correlational, there is experimental evidence that reducing yolk reserves may act as a proximate cue for early hatching in other vertebrates. Received 7 June 1993, accepted 13 November 1993.