Migrating by night, inactive by day: ecological barrier crossing in a migratory songbird

Long-distance migratory songbirds cross ecological barriers such as deserts and seas as part of their journeys. Yet, the behavioural adaptations used to perform these crossings remain poorly understood. Using accelerometer data from multisensor loggers carried by free-flying thrush nightingales (Luscinia luscinia), a nocturnal migratory passerine, we quantified fine-scale activity patterns during the entire migratory journey, including desert crossings during both spring and autumn migration. When migrating over ecological barriers, the birds performed consecutive nocturnal migratory flights, followed by daytime layovers when the birds remained inactive until the next sunset, when resuming migratory flights. The absence of diurnal locomotion while crossing desert environments suggests that thrush nightingales do not refuel while stopping within the barriers but instead rely on energy reserves accumulated prior to barrier entry. This multi-day behavioural inactivity highlights physiological and energetic constraints faced by small migratory birds when crossing ecological barriers and illustrates the careful allocation of energy reserves prior to the barrier crossing to endure several consecutive migratory flights without refuelling. Our results provide new detailed insights into how long-distance migrants cross ecological barriers, improving our understanding of behavioural adaptations in bird migration ecology.