Within-season flexibility in migration strategies in a migratory songbird

Long-distance migratory songbirds alternate between flights and stopovers to complete their journeys. While migration is typically divided into spring and autumn, detailed insights into their migratory behaviour across and within seasons remain limited. Here we use multisensor data loggers in a long-distance migratory songbird, the thrush nightingale, to explore the structure of flight and stopover periods, the variation in fuelling intensity and the fuel load requirements across and within seasons. We evaluated the migratory behaviour data in light with migration strategies predictions derived from optimal migration theory. All 12 tracked individuals followed similar migratory routes, with both autumn and spring migrations segmented into distinctive flight and stopovers periods. Diurnal activity at stopovers, a proxy for fuelling intensity, varied with latitude, with almost zero activity when stopping during daytime across ecological barriers, reflecting limited fuelling opportunities. While crossing the Sahara Desert in autumn, the birds did not minimise pure time or energy strategies, indicating that survival is prioritised. During the last spring migratory period, from the Horn of Africa until the breeding grounds, the birds performed up to 22 consecutive nocturnal flights. After crossing the Arabian Peninsula, they switched strategy and adopted a sprint migration approach, refuelling intensively during daytime to sustain back-to-back nocturnal flights. The behavioural patterns observed demonstrate that spring and autumn migrations are not governed by a single strategy (time or energy minimiser). Instead, birds flexibly adjust their flight and stopover behaviour depending on the specific demands along the route, balancing energy, timing and survival risk. This study used multisensor data loggers to provide detailed insights into how a migratory songbird species structures its long-distance annual migrations, enhancing our understanding of the complexity of its migratory behaviour.