1) Oklahoma Biological Survey and Department of Zoology, University of Oklahoma, 111 East Chesapeake St., Norman, Oklahoma 73019 USA, 2) Museum of Vertebrate Zoology, University of California, Berkeley, California 94720-3140 USA, 3) Department of Ecology and Evolutionary Biology and Center for Tropical Research, University of California, Los Angeles, California 90095-1496 USA Ecological Applications (2005), V15 (5), pp. 1487–1494 Combining Isotopic and Genetic Markers to Identify Breeding Origins of Migrant Birds J.F. Kelly (1), K.C. Ruegg (2) and T.B. Smith (3) A quantitative method for linking reproductive and nonreproductive phases of migratory life cycles is fundamental to understanding the biology of migratory organisms. Here we combine genetic (mtDNA) and biochemical (stable isotope) information to examine seasonal movements in the Swainson’s Thrush (Catharus ustulatus), a Neotropical migrant. We show that when these intrinsic markers are used in concert, they can predict the sitespecific origin of thrushes with 76–80% accuracy. Genetic and isotope data needed for these classifications can be obtained from migratory organisms at any phase of the life cycle.