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1 April 2012 Fine-Scale Population Structure and Asymmetrical Dispersal in an Obligate Salt-Marsh Passerine, the Saltmarsh Sparrow (Ammodramus caudacutus)
Jennifer Walsh, Adrienne I. Kovach, Kimberly J. Babbitt, Kathleen M. O'Brien
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Understanding the spatial scale of gene flow can yield valuable insight into the ecology of an organism and guide conservation strategies. Fine-scale genetic structure is uncommon in migratory passerines because of their high vagility and presumed high dispersal abilities. Aspects of the behavior and ecology of some migratory species, however, may promote structure on a finer scale in comparison to their mobility. We investigated population genetic structure in the Saltmarsh Sparrow (Ammodramus caudacutus), a migratory passerine that breeds along the northeastern coast of the United States, where it is restricted exclusively to a narrow strip of patchily distributed tidal marsh habitat. Using genotyping with 10 microsatellite loci, we detected weak but significant population structure among Saltmarsh Sparrows from nine marshes on the breeding grounds between Scarborough, Maine, and Oceanside, New York. Genetic variation among marshes was largely consistent with a pattern of isolation by distance, with some exceptions. One inland marsh was genetically divergent despite its proximity to other sampled marshes, which suggests that mechanisms besides geographic distance influence population genetic structure. Bayesian clustering, multivariate analyses, and assignment tests supported a population structure consisting of five groups. Estimates of migration rates indicated variation in gene flow among marshes, which suggests asymmetrical dispersal and possible source-sink population dynamics. The genetic structure that we found in Saltmarsh Sparrows may result from natal philopatry and breeding-site fidelity, combined with restricted dispersal due to obligate dependence on a patchy habitat. Our findings suggest that fine-scale population structure may be important in some migratory passerines.

© 2012 by The American Ornithologists' Union. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press's Rights and Permissions website,
Jennifer Walsh, Adrienne I. Kovach, Kimberly J. Babbitt, and Kathleen M. O'Brien "Fine-Scale Population Structure and Asymmetrical Dispersal in an Obligate Salt-Marsh Passerine, the Saltmarsh Sparrow (Ammodramus caudacutus)," The Auk 129(2), 247-258, (1 April 2012).
Received: 12 July 2011; Accepted: 1 February 2012; Published: 1 April 2012
Ammodramus caudacutus
genetic structure
migratory passerine
Saltmarsh Sparrow
source-sink dynamics
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