Flight energetics in migratory birds: integrating wind-tunnel measurements and migration ecology

Birds migrate across the globe on their wings, a behavioural adaptation that allows birds to exploit seasonal environments. Their migrations are ultimately influenced by their ability to fly, a costly but high energy-efficient mode of locomotion that enables birds to travel far distances in a short amount of time. This thesis investigates bird migration from a flight energetic perspective, focusing on small passerines, to understand how birds complete their long-distance journeys.To address this, I have combined two complementary approaches: controlled windtunnel experiments and individual-based tracking across full migratory journeys. In the wind tunnel, I quantified flight energetics in thrush nightingales Luscinia luscinia and barn swallows Hirundo rustica. Power–speed relationships were species-specific and followed the canonical U-shape in both cases. Crucially, energy conversion efficiency varied with speed, peaking at intermediate speeds aligned with ecologically relevant speed for sustained flight during migration. Importantly, conversion efficiency also differed between both species, suggesting a specialization-flexibility trade-off consistent with each species’ flight ecology.In parallel, individual tracking of thrush nightingales revealed their migration patterns from which we could infer their migration strategies under the scope of optimal migration theory. Thrush nightingales do not stick to single strategy throughout the migratory journey, but switch strategies depending on the environment. Ecological barriers shaped the behaviour seasonally: the Sahara crossing in autumn suggest a risk minimization strategy; Arabian Peninsula crossing in spring was followed by intensive daytime fuelling that sustained consecutive nocturnal migratory flights, consistent with a time minimization strategy. This was facilitated by an increasing daylength gradient as spring migration progressed.Integrating both approaches allowed to explore bird migration energetics in detail. Pairing wind tunnel flight cost measurements with precise migratory flight durations from tracking yielded total time and energy required for the journey. In spring, time and energy budgets matched theoretical ratios; in autumn, both nearly doubled, indicating season-specific constraints on performance. Different daylength gradient on each season likely modulated the observed patterns, underscoring the importance of available fuelling time in the migration speed. Together, these studies highlight the synergy of complementary approaches in advancing our understanding of bird migration ecology.