Long-distance migratory birds have relatively smaller brains than short-distance migrants or residents. Here, we test whether reduction in brain size with migration distance can be generalized across the different brain regions suggested to play key roles in orientation during migration. Based on 152 bird species, belonging to 61 avian families from six continents, we show that the sizes of both the telencephalon and the whole brain decrease, and the relative size of the optic lobe increases, while cerebellum size does not change with increasing migration distance. Body mass, whole brain size, optic lobe size and wing aspect ratio together account for a remarkable 46% of interspecific variation in average migration distance across bird species. These results indicate that visual acuity might be a primary neural adaptation to the ecological challenge of migration.

Brain region sizes, body mass, wing morphology and migration distance in 152 species of birdsThis table contain data on 152 species of birds, from 61 avian orders and from six continents. Data covers: migration distance (km), body mass (g), brain mass (g), size of cerebellum (g), size of telencephalon (g), size of optic lobe (g), wing aspect ratio (dimensionless) and wing area (m2).Supplementary Dataset.xls