Abstract
External factors such as geography and weather strongly affect bird migration influencing daily travel schedules and flight speeds. For strictly thermal-soaring migrants, weather explains most seasonal and regional differences in speed. Flight generalists, which alternate between soaring and flapping flight, are expected to be less dependent on weather, and daily travel schedules are likely to be strongly influenced by geography and internal factors such as sex. We GPS-tracked the migration of 70 lesser kestrels (Falco naumanni) to estimate the relative importance of external factors (wind, geography), internal factors (sex) and season, and the extent to which they explain variation in travel speed, distance, and duration. Our results show that geography and tailwind are important factors in explaining variation in daily travel schedules and speeds. We found that wind explained most of the seasonal differences in travel speed. In both seasons, lesser kestrels sprinted across ecological barriers and frequently migrated during the day and night. Conversely, they travelled at a slower pace and mainly during the day over non-barriers. Our results highlighted that external factors far outweighed internal factors and season in explaining variation in migratory behaviour of a flight generalist, despite its ability to switch between flight modes.
Highlights
External factors such as geography and weather strongly affect bird migration influencing daily travel schedules and flight speeds
That flexibility is governed by an interplay between (1) external factors such as weather conditions and geography that influences connectivity and creates so-called ecological b arriers[3,4,5]; and (2) internal factors such as motion capacity, orientation ability, and the individual’s age, sex, and/or reproductive state that shape the internal motivation to m ove[3]
Considering internal factors, age and experience are important factors mediating the response to weather conditions, whereas sex typically has a small effect on travel speed of soaring migrants[18,19]
Summary
External factors such as geography and weather strongly affect bird migration influencing daily travel schedules and flight speeds. That flexibility is governed by an interplay between (1) external factors such as weather conditions and geography that influences connectivity and creates so-called ecological b arriers[3,4,5]; and (2) internal factors such as motion capacity (dependent on, for example, wing morphology), orientation ability, and the individual’s age, sex, and/or reproductive state that shape the internal motivation to m ove[3]. Studies that take into account the interplay of external and internal factors in shaping migratory behaviour (commonly measured via metrics such as ground speed, travel distance, duration of stopovers, and route straightness8–10) present a bias towards large soaring birds Studies on these species have demonstrated that variation in weather (e.g., winds, thermals, and orographic updrafts) is often the prevailing factor explaining migration patterns, such as seasonal and regional differences in hourly and daily s peeds[8,11]. We might expect internal factors and the underlying geography to have a dominant role in shaping their migratory behaviour[20]
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