Using Stable Isotope Analysis to Infer Breeding Latitude and Migratory Timing of Juvenile Pacific-Slope Flycatchers (Empidonax Difficilis)

Effective conservation of migratory species is hindered by a lack of knowledge of population links between breeding, wintering and stopover habitats. The Pacific-slope flycatcher (Empidonax difficilis) is one of the many Neotropical migratory songbirds whose populations are steadily declining throughout western North America. This research contributed to the assessment of connectivity in this species by inferring relative breeding origins and habitat selection of juvenile birds that migrate along the Pacific Flyway to the South San Francisco Bay Area in the fall. Feather data collected from July 20 to October 12, 2014 were analyzed for three stable isotopes (δ2H, δ13C, δ15N). Findings revealed that populations migrated sequentially from western regions throughout expected breeding latitudes, with early season migrants most likely coming from the more southern, warmer, and dryer regions of northern California, Oregon, and southern Washington while late season migrants had probable origins in the more northern, cooler, and wetter regions of northern Washington and southwestern British Columbia. This study provided new information on the annual cycle and migratory timing of Pacific-slope flycatchers, and lays the foundation for future assessments of migratory connectivity of this species