Seasonally specific changes in migration phenology across 50 years in the Black-throated Blue Warbler

ABSTRACT The availability of detailed information that encompasses the geographic range of a species, spans a long-term temporal range, and yields individual information (e.g., age and sex), is a principle challenge in ecology. To this end, the North American Bird Banding Laboratory maintains a unique and underutilized dataset that can be used to address core questions of phenological change in migratory birds. We used records from 1966 to 2015 to quantify how the timing of migration has shifted in a long-distance migrant, the Black-throated Blue Warbler (Setophaga caerulescens). Additionally, we examined age and sex differences in the timing of migratory movements. We observed that early spring migrants passed through sites ∼1.1 days earlier per decade and the peak of spring migration also occurred earlier over the 50 yr of this study. Additionally, phenological change was more rapid with increasing latitude during peak spring migratory periods. During fall, the peak of migration stayed consistent across the 50 yr studied, but the migratory season showed protraction overall. During spring, males consistently migrated earlier than females and adults migrated earlier than young individuals. During fall, there was no difference in timing between males and females, but young birds migrated earlier than adults. Additionally, migration proceeded faster in spring compared with the fall. This study reveals differential strategies in migrant timing, across seasons, age groups, and by sex, and shows that en route adjustments across latitude may account for changes in migrant timing. This basic information about such a fundamental ecological process is crucial to our understanding of migration and we must utilize these unique data to appreciate critical shifts at relevant scales of migration.