Climate warming reduces the energetic advantage of long-distance migration in a migratory bird

Climate warming is driving shifts toward shorter migration distances in migratory birds, yet the energetic mechanisms underlying this transition remain unclear. To understand how migratory birds balance energetic costs of movement with wintering benefits requires quantifying their energy expenditure across the annual cycle. Here, we present a novel approach to reconstruct annual energy budgets of free-flying birds from multi-sensor geolocator data on atmospheric pressure, ambient light, and activity. We apply this method to tracking data from Eurasian Wrynecks Jynx torquilla breeding in Central Europe. Tracking revealed striking individual differences in migration distance ranging from short-distance migrations of approx. 2000-2500 km to southern Europe and northern Africa to long-distance migrations of >5000 km to sub-Saharan Africa, with the former assumed to be a recently evolved strategy. Estimated mean daily energy expenditure across all individuals was 73.1 kJ/day, however, it was dependent on migration distance: long-distance migrants displayed 21-26% lower annual energy requirements than shorter-distance migrants despite 2-3 times higher costs of migration. The lower energy budgets of long-distance migrants resulted primarily from low thermoregulation costs during the ca. 5-month stationary non-breeding period. However, long-distance migrants faced extreme physiological demands during migration, with daily energy expenditure exceeding 9.5 times BMR approaching the upper limits of animal metabolic performance. We show that since 1950, climate warming has progressively reduced winter thermoregulation demands disproportionately benefiting and potentially promoting shorter-distance strategies. These results reveal shifting energetic trade-offs between migration and overwintering under climate change, potentially driving the evolution of migration.