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Timing of departures in nocturnally migrating songbirds

How long a bird stays at a stopover site is known to depend on a number of factors, including the bird’s body condition, fuel deposition rate and the prevailing weather conditions. But when during the night do they leave once they are ready to depart? Are departures only triggered by the setting sun or is departure timing affected by intrinsic and external factors as well? Do birds regulate departure timing to control the duration of the upcoming flight?
Migrating birds at Falsterbo, Sweden.
Migrating birds at Falsterbo, Sweden. Photo: Björn Malmhagen

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.

Sissel Sjöberg

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