EL NINO-SOUTHERN OSCILLATION INFLUENCES ANNUAL SURVIVAL OF A MIGRATORY SONGBIRD AT A REGIONAL SCALE

Global climate cycles have been shown to influence demographic rates of birds at local scales, but few analyses have examined these effects at larger, regional scales. We examined the relationship of broad-scale climate indices to apparent survival of a subspecies of Swainson's Thrush (Catharus ustulatus) across a large portion of the subspecies' breeding range along the Pacific slope of North America. We developed 69 a priori Cormack-Jolly-Seber models to examine effects of El Nino-Southern Oscillation (ENSO), North Atlantic Oscillation, Pacific Decadal Oscillation, latitude, region, and residency status on survival. The most parsimonious model included an ENSO effect, a regional effect, and a residency effect on survival. The ENSO had a positive effect on survival probability, and the effect was consistent across the entire portion of the breeding range examined. Additional analyses of a posteriori models provided strong support for an effect of dry-season precipitation along the spring migration route in western Mexico on annual survival. Our results suggest that survival of this Neotropical migrant is strongly influenced by ENSO-related weather changes during one or more periods of its annual cycle. Because many western Neotropical migrants migrate through and winter in the same general geographic area as Swainson's Thrushes, it is possible that other such species are similarly influenced by ENSO. If, as some climate models predict, annual variation in ENSO increases, Swainson's Thrush may suffer greater variation in annual survival. Directly associating climate with key demographic parameters provides a powerful approach to predicting a species' response to climate change. Received 6 February 2012, accepted 4 July 2012.