Age-dependent response to anthropogenic habitat during migration of an endangered raptor.

Decisions made by migrating animals can impact individual fitness and population dynamics.^1,2 For avian migrants, these decisions can be affected by environmental^3,4,5,6,7 and anthropogenic^8,9,10,11,12 factors and by internal^{13,14,15,16,17,18} states. However, recent reviews have pointed to multiple gaps in our understanding of these decisions.^19,20,21,22 We studied the decisions made by migrating endangered Steppe Eagles Aquila nipalensis by tracking individuals for up to 7 years with GPS transmitters. We used weather reanalysis models and high-resolution remote sensing to obtain environmental and anthropogenic information.^23,24 Using complementary statistical methods, we differentiated between two behavioral states, migratory flights and stopovers, and studied how different factors shape the birds' movements and the transition between these states.^25,26 Most prominently, we detected effects of experience on the birds' response to anthropogenic habitats, with juvenile eagles drawn to them, adults avoiding them, and sub-adults showing no preference. Experience also affected the choice of tailwind and flight direction during migration, with juvenile individuals choosing stronger winds and more direct routes than more experienced eagles. During stopover, experienced eagles flew greater distances than less experienced eagles, and during both stopover and migratory periods, stronger tailwinds increased the distance birds moved. Finally, winds blowing toward the migratory direction increased the probability that a bird would initiate migration after a stopover, while opposite winds had the opposite effect. Our results advance our understanding of the ontogeny of bird migration and the effects of environmental and anthropogenic factors on migratory decision-making, with implications for the conservation of migratory species.