High-latitude night shortening forces birds to accept increasingly diurnal migration to maintain flight duration

Abstract

The ecological pressures that maintain the behavioral preferences of avian migrants, such as the timing and duration of nocturnal flights, remain elusive yet are critical to understand the evolution of the migratory program. In this study, we use an atypical light condition – extremely short to non-existent nights at high latitudes – to study responses to a forced tradeoff between nocturnality and migratory flight duration. Through this lens, we aim to elucidate the relative importance of the pressures shaping the dynamics of migratory flights. We use next-generation radar (NEXRAD) data from seven stations across Alaska to characterize the timing of peak migratory activity relative to dusk, the duration of elevated migratory activity, and the fraction of migratory activity falling within the night. We find that as night lengths decrease, the timing of peak migration clusters tightly around solar midnight, resulting in peak activity occurring slightly closer to dusk. Meanwhile, the duration of elevated migratory activity, while becoming less variable, is largely maintained, resulting in a major shift towards diurnal migration as night lengths decrease below the average duration of nightly migratory activity. These results demonstrate that the stabilizing pressure on the times during which migrants fly is strong and generally overrides the pressure of maintaining nocturnal migration, contextualizing geographic variation in migratory dynamics across Alaska, and elucidating the structure of decisions that determine migratory behavior.