Long-term changes in the seasonal timing of landbird migration on the Pacific Flyway

The seasonal phenology of latitudinal movements is one of the key life-history traits of migratory birds. We used quantile regression to examine long-term changes in the timing of spring and autumn migration in 5 species of migratory passerine birds captured at a banding station in northern California, USA, over a 22 yr period from 1987 to 2008. Our 5 study species included 3 short-distance migrants, Pacific-slope Flycatcher (Empidonax difficilis), Orange-crowned Warbler (Oreothlypis celata), and Wilson's Warbler (Cardellina pusilla); and 2 long-distance migrants, Swainson's Thrush (Catharus ustulatus) and Yellow Warbler (Setophaga petechia). Median timing of migration advanced in spring for 2 of the 5 species (−2.5 days decade−1) but was delayed during autumn migration for 3 of the species (+2.9 days decade−1). The duration of the migration period also became compressed in some species but more protracted in others. We tested whether annual variation in migration timing was related to 3 indices of regional climatic conditions: the Pacific–North American index, multivariate El Niño–Southern Oscillation index, and North Atlantic Oscillation index. Climate indices explained relatively little of the variation in migration timing (r2 < 0.35), but advances in spring migration and delays in autumn migration were associated with warm, wet conditions during positive phases of the climate indices. The strongest effects of climatic conditions on migration timing were found among short-distance migrants, with the largest changes in timing due to variation in the El Niño–Southern Oscillation and Pacific–North American indices. Linkages between the timing of movements and climatic conditions indicate that passerine birds on the Pacific coast exhibit phenotypic plasticity in their migration timing, but future studies based on experimental methods are needed to test alternative ecological mechanisms.