The detrimental effects of nest ectoparasites on breeding birds have been well documented, but interactions between birds' parasites and their environment are less well understood. The Saltmarsh Sparrow, Ammodramus caudacutus is a ground-nesting bird endemic to tidal wetlands in the eastern USA, where the majority of nests are lost to tidal flooding. This study examined evidence that Saltmarsh Sparrows face a trade-off between flood-risk and ectoparasite exposure in nest-site selection and nest initiation. We monitored nesting attempts during the breeding season, and collected 23 nests from which we extracted, described and analyzed the invertebrate communities. Many invertebrate taxa were present in nests, especially Acari, Amphipoda and Isopoda. Flooding-failure was by far the best predictor of nest parasite load (specifically Acari), superior to the investigated characteristics of habitat, nest construction, phenology and research methods. Nests that failed because of flooding had 15% fewer ectoparasites per gram of material. Nests made of smooth cordgrass, Spartina alterniflora had parasite loads 16% lower than those made of other materials, but this variable was confounded with flooding-failure. These results suggest that selecting flood-prone sites has benefits, by reducing exposure to ectoparasites, in addition to the often severe costs of flooding. This study improves understanding of bird nest-parasite interactions in a habitat in which they have been little studied.
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