Migration distance, ecological barriers and en-route variation in the migratory behaviour of terrestrial bird populations

AimSeasonal migration is a common strategy used by terrestrial birds that breed in temperate regions of the Northern Hemisphere. Evidence suggests that long-distance migrants have limited flexibility in migratory behaviour during the onset of migration but greater flexibility during the migration journey. Here, we examine how two geographical factors are correlated with en-route flexibility. Due to stronger time-selection pressures, we expected species with longer migration distances to have greater en-route variation, that increases when large ecological barriers to migration are present. LocationWestern Hemisphere. MethodsWe estimated annual population-level migration trajectories for 55 terrestrial bird species over 7 years (2009-15) based on occurrence information from eBird. We calculated migration speed and annual variation in migration speed, timing and location. We examined these metrics as a function of migration distance for species in the eastern (n=41) and western (n=14) migration flyways. The eastern flyway contains two large ecological barriers to migration, the Gulf of Mexico and the Atlantic Ocean. ResultsSpring migration speeds exceeded autumn migration speeds in both flyways. Migration speeds were faster in the eastern flyway during both seasons. Annual variation in migration speed increased as migration distances increased in both flyways during both seasons. Annual variation in migration timing and location increased as migration distance increased in the eastern flyway during both seasons. In the western flyway, annual variation in migration timing and location increased as migration distance increased, but only during the autumn migration. Main conclusionsIncreasing time-selection pressures related to migration distance and ecological barriers were associated with greater en-route variation in migratory behaviour. Variation in migration speed was most consistently associated with variation in migratory behaviour, whereas variation in migration timing and location differed by season and was strongest when large ecological barriers were present. Our results suggest that the capacity to respond to environmental variation during migration increases as migration distances increase.