Journal
JOURNAL OF AVIAN BIOLOGY
Volume 43, Issue 6, Pages 486-490Publisher
WILEY
DOI: 10.1111/j.1600-048X.2012.05705.x
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Funding
- Western Illinois Univ. URC Grant
- Western Illinois Univ. Summer Stipend
- American Ornithologists' Union
- Animal Behavior Society
- Chapman Fund of the American Museum of Natural History
- Cooper Ornithological Society
- Sigma Xi Honor Society
- Los Angeles Audubon Society
- Graduate Division
- Academic Senate at the Univ. of California
- Santa Barbara
- National Science Foundation (NSF) Doctoral Dissertation Improvement Grant program [IOB-0608263]
- NSF Long-Term Ecological Research Program at Konza Prairie Biological Station
- Direct For Biological Sciences
- Division Of Environmental Biology [823341] Funding Source: National Science Foundation
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The brown-headed cowbird Molothrus ater is a brood-parasite that lays eggs in nests of a wide range of host species, including the closely-related red-winged blackbird Agelaius phoeniceus and the dickcissel Spiza americana. Although cowbird eggs have accelerated development and hatch sooner than similar-sized host eggs, this development takes place within a thickened eggshell that could impede gas flux to the developing embryo. We tested the hypothesis that the accelerated development of the cowbird embryo relative to hosts is enabled by an increase in eggshell porosity that allows increased fluxes of respiratory gases to and from the developing embryo. We found cowbird eggshell thickness was significantly greater than the eggshells of these two common hosts. Although the number of pores per egg was similar among all three species, the total pore area per egg in cowbirds was significantly greater than that of either host, despite having a smaller eggshell surface area than the red-winged blackbird. Cowbird egg pore area was 1.9xlarger than that of the red-winged blackbird. Cowbird eggshells had a significantly greater gas flux than those of the red-winged blackbird and the dickcissel. When conductance was normalized to published values of egg mass, cowbird eggs had a higher mass-specific conductance than red-winged blackbird or dickcissel eggs. These results are consistent with the hypothesis that the rapid development of brown-headed cowbird embryos is facilitated by increased eggshell porosity, and that changes in eggshell porosity represent an adaptation that enables cowbird eggs to hatch earlier than equivalently-sized host eggs.
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