The ecological–evolutionary interplay: density-dependent sexual selection in a migratory songbird

Authors

  • Thomas B. Ryder,

    1. Smithsonian Conservation Biology Institute, Migratory Bird Center, National Zoological Park, PO Box 37012-MRC 5503, Washington, DC 20013-7012
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  • Robert C. Fleischer,

    1. Smithsonian Conservation Biology Institute, Center for Conservation and Evolutionary Genetics, National Zoological Park, PO Box 37012-MRC 5503, Washington, DC 20013-7012
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  • W. Greg Shriver,

    1. Department of Entomology and Wildlife Ecology, University of Delaware, Newark, Delaware 19716-2160
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  • Peter P. Marra

    1. Smithsonian Conservation Biology Institute, Migratory Bird Center, National Zoological Park, PO Box 37012-MRC 5503, Washington, DC 20013-7012
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  • Ecology and Evolution 2012; 2(5): 976–987

Thomas B. Ryder, Smithsonian Conservation Biology Institute, Migratory Bird center, National Zoological Park, PO Box 37012-MRC 5503, Washington, DC 20013-7012. E-mail: rydert@si.edu

Funded by the Pedigree and Maryland Ornithological Society.

Abstract

Little is understood about how environmental heterogeneity influences the spatial dynamics of sexual selection. Within human-dominated systems, habitat modification creates environmental heterogeneity that could influence the adaptive value of individual phenotypes. Here, we used the gray catbird to examine if the ecological conditions experienced in the suburban matrix (SM) and embedded suburban parks (SP) influence reproductive strategies and the strength of sexual selection. Our results show that these habitats varied in a key ecological factor, breeding density. Moreover, this ecological factor was closely tied to reproductive strategies such that local breeding density predicted the probability that a nest would contain extra-pair offspring. Partitioning reproductive variance showed that while within-pair success was more important in both habitats, extra-pair success increased the opportunity for sexual selection by 39% at higher breeding densities. Body size was a strong predictor of relative reproductive success and was under directional selection in both habitats. Importantly, our results show that the strength of sexual selection did not differ among habitats at the landscape scale but rather that fine-scale variation in an ecological factor, breeding density, influenced sexual selection on male phenotypes. Here, we document density-dependent sexual selection in a migratory bird and hypothesize that coarse-scale environmental heterogeneity, in this case generated by anthropogenic habitat modification, changed the fine-scale ecological conditions that drove the spatial dynamics of sexual selection.

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