Influence of fuel load and weather on timing of completing nocturnal flights in Reed warblers, Acrocephalus scirpaceus, during season migrations

Abstract

Time of completing nocturnal migratory flights in passerines is essential for their safety during landing and stopover site selection. I studied landing time, fuel stores and weather at landing in Reed warblers migrating through the Courish Spit on the Baltic Sea in autumn and spring. I used data on 697 Reed warblers tape-lured into a habitat atypical of this species in 1999–2002. In both seasons, more than one-half of all birds (59% in autumn, 56% in spring) were captured in the last two hours before sunrise, while the rest in the beginning and middle of the night. I tested the hypothesis that early landing could be caused by take-offs in the beginning of the night with small fuel stores, not sufficient for the flight throughout the night. It was confirmed only for the young birds that migrated in the second half of the autumn season. Comparing wind assistance, cloud cover and precipitation in the days of landings in the beginning-middle of the night versus late night, I found no impact of these potentially important weather parameters on the temporal distribution of early and late landings. It is assumed that early landings of some Reed warblers could be caused by short-term nocturnal flights not directly related to the migration rush to the target.