Barrier crossings and winds shape daily travel schedules and speeds of a flight generalist

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.

Global positioning system (GPS) and platform transmitter terminal (PTT) tags reveal fine-scale migratory movements of small birds: A review highlights further opportunities for hypothesis-driven research

“…For example, GPS data from diurnal purple martins ( Progne subis ) were used to evaluate hypotheses about the function of nocturnal flights; results supported the hypothesis that nighttime flights help birds conserve time and energy when crossing barriers and/or in areas where foraging opportunities are poor (Lavallée et al 2021). Other studies evaluated the relationship between environmental factors (e.g., wind speed/direction, temperature, and topography) and aspects of flight behavior like flight speed, altitude, and flight path (Senner et al 2018, Lopez-Ricaurte et al 2021).…”

Understanding the ecological and evolutionary function of stopover in migrating birds

“…However, some birds may switch their flight mode from soaring to flapping to continue migration into the night and/or across the sea, where thermals and updrafts become increasingly less reliable (Meyer, Spaar & Bruderer, 2000; Becciu et al ., 2020; Santos et al ., 2020). These (mostly exceptions, but see Lopez‐Ricaurte et al ., 2021) are often found in species with a low gliding performance, for which flapping is not too costly, and when adopting a time‐ rather than energy‐minimising migration strategy (Hedenström, 1993; Spaar, 1997). The general function of landing when thermal and updraft conditions deteriorate is to minimise transport costs (Hedenström, 1993).…”

Itinerant lifestyle and congregation of lesser kestrels in West Africa

“…This study aims to describe the non-breeding movements of lesser kestrels from the Spanish breeding population in West Africa, and to test for potential differences in the mobility and timing of intra-African movements between sexes. Because of the high movement ability of these flight generalist species (Lopez-Ricaurte et al, 2021; Bildstein, 2017), we expect lesser kestrels to show itinerant movements within West Africa, moving progressively further south over the season to follow the seasonal shifts in insect abundance (c.f., other locust-eating steppe birds, Berthold 2002, 2004; Trierweiler et al, 2013; Schlaich et al, 2016). Finally, we characterise land use at their staging sites using the GlobCover land use map (2009) (resolution 300m) in a similar way to previous studies (Trierweiler et al, 2013; Schlaich, 2019).…”