Timing of departures in nocturnally migrating songbirds
In a new paper entitled "Ecological factors influence timing of departures in nocturnally migrating songbirds at Falsterbo, Sweden" in Animal Behaviour, we (myself Sissel Sjöberg, Thomas Alerstam, Susanne Åkesson and Rachel Muheim) investigated the departure timing of four species of nocturnally migrating songbirds using the automated radiotelemetry system in Falsterbo.
We found that the variation in departure time in relation to sunset was large, indicating that departure timing is affected by several factors and not only a behaviour triggered by the setting sun. 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 (tailwind or weak winds) at sunset, they departed earlier.
Thus, it appears that the decision to take off for a long-distance flight is adapted both with regard to 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.