Ecological factors influence timing of departures in nocturnally migrating songbirds at Falsterbo, Sweden

Most songbirds depart from stopover sites after sunset and migrate during the night. Several recent studies have reported larger variation in departure time than previously thought; yet, it is still unclear which factors govern departure timing. We investigated the departure timing of four species of nocturnally migrating songbirds using an automated radiotelemetry system at Falsterbo peninsula in southwest Sweden. We made a comprehensive analysis to test a range of factors that have been hypothesized to affect departure timing of nocturnal migrants, such as night duration, season, sun elevation and the birds' intrinsic and environmental conditions. We hypothesized that birds in good condition (large fuel reserves) and under advantageous flight conditions would depart sooner after sunset, in the expected migratory direction. Our analyses showed that the birds departed sooner after sunset during spring than autumn, and different species departed at different times in relation to sunset. In addition, birds departed earlier when nights were shorter, suggesting that night duration is an important factor that may drive much of the observed timing differences between seasons and species. Lean birds delayed their departures compared to fat individuals. When birds experienced favourable wind conditions (tail wind or weak winds) at sunset, they departed earlier. Thus, it appears that the decision to take off for a long-distance flight depends on both body condition and wind conditions. Timing of departure was not correlated with sun elevation, which would have been expected if availability of specific orientation cues (sun, skylight polarization pattern, stars) acted as triggers for departures. These results stress high flexibility and adaptive responses to a complex of ecological factors as the determinants for timing of nocturnal flights in songbirds.