Morphological variation across an ecological boundary in a marsh obligate songbird
Ecological adaptation has been often implicated as a driver of species diversification. This may be particularly true in habitats that are structurally similar, but differ dramatically in species composition and abiotic factors, such as freshwater and saltwater marshes. In songbirds few taxa have successfully colonized saltmarsh ecosystems. Among songbirds that are saltmarsh endemics, many have closely related taxa endemic to freshwater marshes or grasslands. We explored phenotypic and genotypic variation in six subspecies of a marsh obligate songbird, the marsh wren (Cistothorus palustris) to test whether subspecific variation is consistent with breeding ecology. We sampled 20 populations of marsh wren in the eastern United States (one freshwater marsh endemic subspecies, five saltmarsh endemic subspecies). We measured body size and shape, quantified plumage color using digital photography, and took blood samples for genetic analysis. We tested whether differences in morphology and genetics form diagnosable groups, and whether those groups are consistent with breeding ecology.
Marsh wrens endemic to freshwater marshes are can be distinguished from saltmarsh endemics on the basis of both body size and shape and plumage color. However, five described subspecies that inhabit similar saltmarsh breeding habitats have considerable morphological overlap. Freshwater marsh endemics are heavier, have longer wings, and have redder plumage than saltmarsh endemics. This is consistent with patterns of “saltmarsh melanism”, seen in saltmarsh endemic populations of other birds, mammals, and one snake. Despite these morphological and ecological differences the freshwater marsh and saltmarsh endemic populations are not genetically distinct based on neutral markers. Coupled with the fact that freshwater populations co-mingle with saltmarsh populations during the non-breeding season these results suggest that selection related to the differences in breeding season ecology maintain phenotypic differences within the marsh wren species complex.