An experimental investigation of strategies used by brown-headed cowbirds to optimize parental care

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McMaster, Donald Glen
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This study tested hypotheses concerning strategies used by Brown-headed Cowbirds (Molothrus ater) to optimize parental care during the host's laying, incubation, and hatching periods. To test if female cowbirds remove host eggs from parasitized clutches to reduce the volume of eggs to a level the host can incubate effectively, live cowbird eggs were placed in Yellow Warbler (Dendroica petechia) nests. Hatching success of cowbird eggs was significantly greater in clutches from which one host egg was removed than clutches from which no host egg was removed in one of the three years of study. Removal of a host egg, therefore, is not required for successful hatching of cowbird eggs in all years, but may increase hatching success in certain years. Cowbird eggs were artificially incubated both in warbler clutches and in isolation to test whether the short incubation period of cowbird eggs is due to (1) cowbird embryos developing more rapidly than host embryos, (2) cowbird eggs gaining more heat than host eggs dueto their larger size, or (3) cowbird embryos hatching in response to stimuli from host eggs. Cowbird eggs incubated in isolation took longer to hatch than either host eggs or naturally incubated cowbird eggs. Cowbird eggs incubated in warbler clutches, however, hatched at the same time as host eggs. These results suggest cowbird eggs receive more heat than smaller host eggs under natural conditions, and hatch slightly sooner in response to stimuli from host eggs. Finally, I added cowbird eggs and nestlings to warbler clutches to test whether the presence of a cowbird alters parental behaviour during the laying and hatching periods. Cowbird nestlings were brooded and fed more than warbler nestlings during the hatching period, but parental behaviour was unaltered during the laying period.