Two Brown-headed Cowbird young in a Yellow Warbler nest.
My first introduction to field biology occurred in one of the most amazing ecosystems in the world—the Mono Basin in eastern California. Thanks to a landmark legal battle in the 1980's and 90's, this once threatened place has been a spectacular success in ecological restoration. Water that was once diverted to the city of Los Angeles now flows through creeks that brim with recolonized willows and cottonwoods and make their way to the unique and iconic shores of Mono Lake. While working for PRBO Conservation Science in the restored riparian zones, I became fascinated with one of the most unique bird species in North America: the Brown-headed Cowbird (Molothrus ater). These brood parasites reproduce solely by laying their eggs in the nests of other birds, ultimately limiting reproduction of their hosts. I became particularly interested in reproductive success from the perspective of female cowbirds, specifically, how micro-habitat around host nests could limit the success of female cowbirds.
These topics ultimately led to my masters research on the impacts of songbird nesting habitat and cowbird sex on host parasite hatching synchrony. Since hatching earlier than hosts is one way in which cowbirds out-compete their nest mates, I wanted to determine if nest concealment and distance from cowbird perches limit the ability of female cowbirds to find nests quickly, and thus their ability to time their eggs so that they hatch earlier than host eggs. I also measured cowbird nestling development in relation to host species, nest micro-habitat, hatching order, and nestling sex. This work revealed that the best predictor of cowbird development and hatching synchrony was nestling sex, with male cowbirds hatching earlier and developing faster. The results demonstrated the importance of taking sex into account when conducting developmental studies.
These topics ultimately led to my masters research on the impacts of songbird nesting habitat and cowbird sex on host parasite hatching synchrony. Since hatching earlier than hosts is one way in which cowbirds out-compete their nest mates, I wanted to determine if nest concealment and distance from cowbird perches limit the ability of female cowbirds to find nests quickly, and thus their ability to time their eggs so that they hatch earlier than host eggs. I also measured cowbird nestling development in relation to host species, nest micro-habitat, hatching order, and nestling sex. This work revealed that the best predictor of cowbird development and hatching synchrony was nestling sex, with male cowbirds hatching earlier and developing faster. The results demonstrated the importance of taking sex into account when conducting developmental studies.
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Relevant Publications
Croston, R., C.M. Tonra, S.K. Heath, and M.E. Hauber. 2012. Flange color differences in brood parasitic Brown-headed Cowbirds from nests of two host species. Wilson Journal of Ornithology 124: 139-145. (reprint) Tonra, C.M., M.D. Johnson, M.E. Hauber, and S.K. Heath. 2009. Does nesting habitat influence hatching synchrony between brood parasitic brown-headed cowbirds (Molothrus ater) and two hosts? Ecography 32: 497-503. (reprint) Tonra, C.M., M.E. Hauber, S.K. Heath, and M.D. Johnson. 2008. Ecological correlates and sex differences in early development of a generalist brood parasite. Auk 125: 205-213. (reprint) |
Collaborators
• Matthew D. Johnson, Humboldt State University (M.S. advisor) • Mark E. Hauber, Hunter College • Sacha K. Heath, UC Davis • Rebecca Croston, Hunter College |