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Rates of parasitism, but not allocation of egg resources, vary among and within hosts of a generalist avian brood parasite


Brown-headed cowbirds (Molothrus ater) deposit their eggs into the nests of other birds, which then raise the cowbird chick. Female cowbirds thus have limited options for impacting their offspring’s development via maternal effects compared to most other passerines. Cowbirds can impact their offspring’s phenotype by choosing among potential host nests, and by adjusting egg resources based on host characteristics. To examine whether cowbirds exhibit either or both of these strategies, we investigated rates of cowbird parasitism and egg investment (egg size, yolk-to-albumen ratio, and yolk testosterone and androstenedione) among and within host species in a shrubland bird community. We found that the probability of being parasitized by cowbirds, controlling for host status as a cowbird egg accepter or rejecter and ordinal date, varied significantly among host species, indicating an apparent preference for some hosts. Parasitism rates did not differ with host size, however, and despite variation in cowbird egg size among host species, this variation was not related to host size or cowbird preference. Among host species with eggs that are larger than those of the cowbird, cowbirds were significantly more likely to parasitize nests with relatively smaller eggs, whereas parasitism rates did not vary with relative egg size in host species with smaller eggs. There was no evidence for variation in cowbird egg components among or within host species. Our data indicate that cowbirds discriminate among host nests, but do not appear to adjust the composition of their eggs based on inter- or intraspecific host variation.

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We would like to thank field and lab technicians (Emilie Ospina, Michael Olsta, Jason Newton, Caitlin Elkins, Eric Peterson, Victoria Lima, Ohad Paris, Andy Ondrejecht, Lauren Novak, Price Dickson, Shannon Darcy, and Morgan Helfrich) for their assistance in nest searching and preparing eggs for hormone analyses. Thanks to Tara Stewart for help in the field and feedback on the manuscript. Thanks to Steve Buck for access to the University of Illinois’ Vermillion River Observatory, Champaign County Forest Preserve District, Urbana Parks District and Derek Liebert, Vermilion County Conservation District, Forest Preserve District of Kane County, Forest Preserve District of DuPage County, and Illinois Department of Natural Resources for use of their properties. Funding was provided by the USFWS and IDNR Federal Aid in Wildlife Restoration grant numbers W-161-R and W-181-R to TJB, the Forest Preserve District of Kane County, and the Illinois Natural History Survey as part of a postdoctoral fellowship to LM. Eggs were collected under USFWS permit# MB31458B-0 to TJB.

Author contribution statement

LM, SJC, and TJB conceived and designed the study. LM and SJC collected the data, RTP ran the hormone samples, and SJC and TJB performed the statistical analyses. LM, SJC, and TJB wrote the manuscript with editorial advice and feedback from RTP.

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Correspondence to Loren Merrill.

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Communicated by Christopher Whelan.

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Merrill, L., Chiavacci, S.J., Paitz, R.T. et al. Rates of parasitism, but not allocation of egg resources, vary among and within hosts of a generalist avian brood parasite. Oecologia 184, 399–410 (2017).

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  • Androgens
  • Cowbird
  • Maternal effects
  • Parental investment
  • Yolk