Variations in flight strategy of small migratory birds crossing major ecological barriers

The recent development of tracking technologies has revealed remarkable diel flight altitude changes over the Sahara Desert in small migratory bird species. However, the drivers and the traits that explain these crossing strategies remain poorly understood in part because so few species and barriers have been studied. Using a unique dataset from 67 recovered multi-sensor loggers, deployed across 17 species, we investigated when, where, and how birds cross two major marine barriers (Bay of Biscay, Mediterranean Sea) and a desert barrier (Sahara). Then, we relied on a comparative approach to examine the influence of wing morphology and plumage colour on these strategies. Our findings reveal important differences across barrier types. On average, birds fly at 1,600 m over the desert during nighttime, ascending to 2,800 m for prolonged daytime flights, while species crossing marine barriers fly significantly lower (750 m on average), in some cases flying just above the water surface during prolonged daytime flights. Wing morphology and plumage colour influence barrier-crossing strategies: flight altitude increases with wing area during both sea and desert crossings, and darker birds ascend to higher elevations during daytime Sahara crossings, likely to access cooler air and reduce solar heating. These findings refine hypotheses on barrier-crossing strategies and suggest broader ecological and evolutionary implications for migratory birds facing extreme environments.