Flexibility in Migratory Behaviour of a Flight Generalist: Migrate Day and Night, Sprinting Across Ecological Barriers 

Abstract Understanding what factors drive variation in movement patterns is a key challenge in current migration research. Environmental drivers such as landscape heterogeneity and weather strongly affect birds’ migration influencing daily travel schedules and flight speed. For strictly thermal-soaring migrants, typically large birds, weather explains most seasonal and regional differences in speed. In contrast, smaller-sized flight generalists, which alternate between soaring and flapping flight, may be less dependent on weather and thus more likely to be strongly influenced by landscape in daily travel schedules and internal drivers, such as sex. We GPS-tracked the migration performance of 70 lesser kestrels (Falco naumanni), to estimate the relative importance of environmental (wind and landscape), internal (e.g. sex) and seasonal drivers and to what extend do they explain variation in migratory performance, namely speed, distance, and travelling time. We found that tailwind strength explained most of the seasonal difference in migratory parameters. In both seasons lesser kestrels sprinted across ecological barriers and frequently extended migration into the night, while travelling at a slower pace and mainly during the day when not flying over barriers. Our results highlighted that environmental factors far outweighed internal and other seasonal drivers in explaining variation in migration performance of a flight generalist, despite the ability to switch between flight-modes.