Isotopic Delineation of North American Migratory Wildlife Populations: Loggerhead Shrikes

The ability to link breeding and wintering sites of declining or threatened species is fundamental to their effective conservation, but previous conventional methods such as banding have proved largely unsuccessful at the population level. New approaches to tracking migratory birds in North America, using stable isotope measurements of feathers, can assist in determining where feathers were grown and, hence, the origins of migrants on wintering grounds or stopover sites. Moreover, this approach is applicable to the use of metabolically inactive tissues of migratory animals other than birds. Here, we demonstrate how stable isotopes can be used to investigate the origins of a wintering population of a threatened species. The Loggerhead Shrike (Lanius ludovicianus) is declining throughout most of its range, but various factors may operate on different subpopulations. We used the stable-carbon (δ13C) and hydrogen (δD) isotope values in the outer tail feather of Loggerhead Shrikes wintering in Texas (n = 70), Florida (n = 121), and Georgia (n = 24) in 1997–1998, and northern Mexico (n = 72) in 1999, in order to ascertain the minimum percentage of wintering birds that were northern migrants. For comparative purposes, we also measured stable isotopic compositions of feathers from nestling shrikes from Manitoba (n = 6) and from known resident shrikes from Texas (n = 16). As expected, shrikes from Manitoba were more depleted in deuterium than shrikes from Texas, providing a clear isotopic gradient with which to compare the wintering population structure. Of wintering shrikes in Florida, ∼10% were northern migrants, compared with 8% for the Mexican sample and only 4% of the Texas population. We detected no northern migrants in the Georgia population. Most U.S. wintering shrikes had δD values around −30‰, indicative of southern latitudes, but a broader range in δD values was found for shrikes in northern Mexico. Shrikes wintering in Mexico may be composed largely of individuals originating from the eastern Rockies and central regions of the breeding range. The considerable variation in δ13C values indicated a broad range of C3- to C4-based food webs used by shrikes. Our study indicates that the stable isotopic technique answers a number of long-standing and fundamental questions concerning the breeding or natal origins of wintering populations of Loggerhead Shrikes and thereby provides a template for isotopic approaches to ecological investigations of other migratory species.

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