Weather and fuel reserves determine departure and flight decisions in passerines migrating across the Baltic Sea.

Departure decisions of how and when to leave a stopover site may be of critical importance for the migration performance of birds. We used an automated radiotelemetry system at Falsterbo peninsula, Sweden, to study stopover behaviour and route choice in free-flying passerines departing on flights across the Baltic Sea during autumn migration. In addition, we had an offshore receiver station (FINO 2) located about 50 km southeast from Falsterbo. Of 91 birds equipped with radiotransmitters, 19 passed FINO 2. The probability that a departing migrant passed near FINO 2 was primarily affected by winds and timing of departure. Probably, the migrants were subjected to drift by westerly winds, leading to southeasterly flight paths and an enhanced probability of passing FINO 2. Most birds passing the offshore station departed early in the night, which indicates that southward departures across the Baltic Sea usually take place during this time window. Wind condition was the dominant factor explaining the variation in flight duration between Falsterbo and FINO 2. After considering wind influence, we found additional effects of fat score and cloud cover. Birds with a higher fat score performed the flight faster than leaner individuals, as did birds that departed under clear skies compared to birds departing during overcast skies. These effects may reflect a difference in migratory motivation and airspeed between lean and fat birds together with difficulties in controlling orientation in overcast situations on oversea flights when celestial cues are unavailable. Thus, winds, clouds and fuel reserves were the primary factors determining departure and flight decisions in passerine migrants at Falsterbo in autumn. [ABSTRACT FROM AUTHOR]