Host Resource Partitioning Among Sympatric Molothrine Generalist Brood Parasites

  • Jameson F. ChaceEmail author
  • Alexander Cruz
Part of the Fascinating Life Sciences book series (FLS)


We examine host partitioning among sympatric brood parasites in southeastern Arizona and South Florida. In Florida, range expansion of three cowbirds (Molothrus bonariensis shiny cowbird, M. ater brown-headed cowbird, and M. aeneus bronzed cowbird) has brought them together in a region where host species have never been parasitized before. Whereas in southeastern Arizona, bronzed cowbird and brown-headed cowbird have been in sympatry longer. That sympatry and the larger size separation between the smallest race of the brown-headed cowbird (M. a. obscurus) and bronzed cowbird are correlated with the larger cowbird primarily parastizing larger host species. Furthermore, morphometric analysis shows that brown-headed cowbirds are significantly smaller in sympatry than allopatry, suggesting that the size variation across their range is not only a function of clinal variation but driven, perhaps, by long-term interspecific competitive interactions for nests to parasitize (i.e., character displacement model). Interspecific competition in southeastern Arizona for nests to parasitize has led to some partitioning of the host resources. In South Florida, recent contact between three parasitic species suggests a different outcome. In South Florida, the shiny cowbird was recorded mainly from coastal areas, whereas brown-headed cowbirds were found in coastal and inland areas leading to a degree of habitat partitioning. In Florida, brown-headed cowbirds parasitized species ranging from 6 to 100 g, with the majority being smaller species (20 g or less). There are far fewer breeding records for shiny cowbirds, but the similarities in size and species use outside of Florida suggest host overlap with the brown-headed. The longer breeding season of the shiny cowbird suggests that, while there will be host overlap with the brown-headed, a temporal separation may occur. The larger bronzed cowbird parasitized the larger hosts in the community, e.g., red-winged blackbird and spot-breasted oriole.


Alloxenia Bronzed cowbird Brown-headed cowbird Host partitioning Range expansion Shiny cowbird 



Access to museum specimens was enabled by the American Museum of Natural History’s Frank M. Chapman Memorial Fund, Sigma Xi Grants-in-Aid of Research, and the Walker Van Riper Fund of the University of Colorado Museum. The Florida work was supported by a grant from the U.S. Fish and Wildlife Service. For assistance in Florida, we thank the staff of the J. N. Ding Darling National Wildlife Refuge and the Sanibel-Captiva Conservation Foundation. We also thank the Everglades Science Center (National Audubon) for logistical help and H. W. Kale II and W. Pranty for providing cowbird records. A special thanks to J. W. Prather for assistance in the field. M. Soler, S.G. Sealy, and V. Fiorini provided valuable comments on earlier versions of this manuscript.


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Copyright information

© Springer International Publishing AG, part of Springer Nature 2017

Authors and Affiliations

  1. 1.Department of Biology and Biomedical SciencesSalve Regina UniversityNewportUSA
  2. 2.Department of Ecology and Evolutionary BiologyUniversity of ColoradoBoulderUSA

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