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1 July 2011 Stable Isotopes (δD) Reveal East—West Differences in Scheduling of Molt and Migration in Northern Rough-Winged Swallows(Stelgidopteryx serripennis)
Sievert Rohwer, Keith A. Hobson, Sylvia Yang
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We used stable-isotope (δD) measurements of primary feathers to demonstrate that Northern Rough-winged Swallows (Stelgidopteryx serripennis) that breed in western North America migrate to the monsoon region of northwest Mexico for their annual postbreeding molt. Both adults and juveniles replace all their primaries in late summer. As expected, in samples of adults and fledglings collected at a northern breeding locality in eastern Washington, primary δD values of adults were of southern origin, whereas those of fledglings, whose feathers were grown in the nest, were consistent with their northern origin. Similarly, some young birds collected as they were replacing primaries in northwest Mexico had a southern signature in newly replaced primaries but a northern signature in their yet unreplaced juvenile primaries, which indicates that they were molt migrants from the north. Feather OD values increased from P1 to P9 in breeding adults collected in upstate New York. In the eastern United States, both adults and juveniles replace their primaries as they migrate southward in the fall. These isotopic results are consistent with the observation that Northern Rough-winged Swallows are halfway through their primary molt when they reach the Gulf Coast, where they pause in coastal marshes to finish the primary molt before crossing the Gulf of Mexico. The striking difference in molt and migration schedules between eastern and western populations of Northern Rough-winged Swallows suggests further investigations of their species status be conducted in the region where these populations make contact.

© 2011 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,
Sievert Rohwer, Keith A. Hobson, and Sylvia Yang "Stable Isotopes (δD) Reveal East—West Differences in Scheduling of Molt and Migration in Northern Rough-Winged Swallows(Stelgidopteryx serripennis)," The Auk 128(3), 522-530, (1 July 2011).
Received: 1 December 2010; Accepted: 2 March 2011; Published: 1 July 2011
monsoon molting
Northern Rough-winged Swallow
Stelgidopteryx serripennis
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