Evolutionary Biology

, Volume 38, Issue 2, pp 214–224 | Cite as

Heightened Exposure to Parasites Favors the Evolution of Immunity in Brood Parasitic Cowbirds

  • D. Caldwell HahnEmail author
  • William K. Reisen
Research Article


Immunologists and evolutionary biologists are interested in how the immune system evolves to fit an ecological niche. We studied the relationship between exposure to parasites and strength of immunity by investigating the response of two species of New World cowbirds (genus Molothrus, Icteridae), obligate brood parasites with contrasting life history strategies, to experimental arboviral infection. The South American shiny cowbird (M. bonariensis) is an extreme host-generalist that lays its eggs in the nests of >225 different avian species. The Central American bronzed cowbird (M. aeneus) is a relative host-specialist that lays its eggs preferentially in the nests of approximately 12 orioles in a single sister genus. West Nile virus provided a strong challenge and delineated immune differences between these species. The extreme host-generalist shiny cowbird, like the North American host-generalist, the brown-headed cowbird, showed significantly lower viremia to three arboviruses than related icterid species that were not brood parasites. The bronzed cowbird showed intermediate viremia. These findings support the interpretation that repeated exposure to a high diversity of parasites favors the evolution of enhanced immunity in brood parasitic cowbirds and makes them useful models for future studies of innate immunity.


Ecoimmunology Brood parasitism Cowbird West Nile virus Parasite-mediated selection Evolution of immunity 



We thank Katsi Ramos Alvarez and Marilyn Colon, Division Recursos Terrestre, Puerto Rico for making shiny cowbirds available and arranging transport, and Scott Summers, The Nature Conservancy of Texas, for making bronzed cowbirds available and arranging transport. This research was funded, in part, by Research Grants RO1-39483, RO1-AI47855, and AI55607 from the National Institutes of Allergy and Infectious Diseases, NIH, the Coachella Valley and Kern Mosquito and Vector Control Districts, special funds for the Mosquito Research Program allocated annually through the Division of Agriculture and Natural Resources, University of California, and by base funds from USGS-Patuxent Wildlife Research Center. We thank staff of the Center for Vectorborne Diseases for excellent technical support: V.M. Martinez, H.D. Lothrop, S.S. Wheeler and B.D. Carroll assisted with bird collections; V.M. Martinez assisted with bird maintenance and bleeding; and Y. Fang, M. Shafii, S. Garcia, R.E. Chiles and S. Ashtari assisted with laboratory diagnostics. We thank R.B. Payne, S.M. Lanyon, D. Mock, and two anonymous reviewers for helpful comments on the manuscript. Use of trade, product, or firm names does not imply endorsement by the U.S. Government.


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Copyright information

© Springer Science+Business Media, LLC (outside the USA) 2011

Authors and Affiliations

  1. 1.USGS-Patuxent Wildlife Research CenterLaurelUSA
  2. 2.Center for Vectorborne Diseases and Department of Pathology, Microbiology and Immunology, School of Veterinary MedicineUniversity of CaliforniaDavisUSA

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