Differential Migratory Timing of Western Populations of Wilson’s Warbler (cardellina Pusilla) Revealed by Mitochondrial Dna and Stable Isotopes

—Molecular markers and stable isotopes have provided important insights into the migratory connectivity of small landbirds. Research integrating these two methods has primarily focused on linking breeding and wintering sites, rather than focusing on timing of migratory movement of different breeding populations. We used mitochondrial DNA and isotopic markers to infer the timing of various breeding populations of migrating Wilson’s Warblers (Cardellina pusilla) moving through a migratory stopover site, demonstrating the value of multiple sources of information in estimating the origin of migrants. Using mixed-stock analysis, we found that early spring migrants sampled in southwestern Arizona were dominated by warblers migrating to the West Coast of the contiguous United States, whereas later migrants included a large pulse of birds migrating to Alaska and western Canadian provinces. Stable hydrogen isotope data from individual birds showed the same timing pattern as genetic data. Had we used stable isotopes alone, we would not have been able to infer whether birds later in the migration season were most likely migrating to Alaska or the Interior West, given the large overlap in isotope values between those regions. The lack of mitochondrial group 2, common in the Interior West, in late-season migrants strongly suggests that these birds were migrating to breeding areas in Alaska or other northern regions. Studies that reveal the timing of migration of different breeding populations through stopover sites lay the foundation for more in-depth examination of seasonal interactions between migration and the stationary phases of the annual cycle. Received 18 June 2013, accepted 13 September 2013.