Ecological constraints on the evolution of avian brains

Abstract

Birds have brains that are comparable in size to those of mammals. However, variation in relative avian brain size is greater in birds. Thus, birds are ideal subjects for comparative studies on the ecological and behavioral influences on the evolution of the brain and its components. Previous studies of ecological or behavioral correlates in relative brain size were mainly based on gross comparisons between higher taxa or focussed on the relationships between the sizes of specific brain structures and the complexity of different tasks. Here we examine variation in dimensions of the braincase, relative overall brain size and size of its components, in reference to one general ecological and behavioral task: migration. We used data from three lineages of closely related species (14 Acrocephalines, 17 Sylvia and 49 parulid warblers). Within each group, species vary in their migratory tendencies. We found that species migrating long distances have lower skulls and smaller forebrains than resident species. We discuss four hypotheses that could explain smaller forebrain sizes, and suggest relevant taxa to use in comparative analyses to examine each of these hypotheses: Brain size is energetically constrained. Contrasts can be made not only between migrants and residents, but also between birds in habitats with high and low levels of available food. Brain size is developmentally constrained; birds with short growing periods should have smaller forebrains. Comparisons need to be made between birds living in habitats with long and short breeding seasons. Bill adaptations for foraging constrain braincase dimensions. Further analyses would need to be done on groups with high variation in bill dimensions and foraging modes. Birds with small brains have to migrate to compensate for low behavioral flexibility. Contrasts between members of families containing tropical residents and migrants need to be made. We also raise the question of whether only those parts of the brain are reduced that are most dispensable and whether brain size reduction limits foraging skills and social competence.