Evolution of brain size and encephalization in birds

Vertebrates show dramatic interspecific variation in the size of their brains. Large and complex brains evolved independently in birds and mammals. Despite many fundamental differences in their anatomical organization, recent evidence indicates that extensive regions of the avian cerebrum are homologous to pallial components of the mammalian brain. Birds have brains that are comparable in their relative size to the brains of mammals, and much larger than the brains of non avian reptiles. However, the selection pressures that have driven evolution of avian brains remain poorly understood. Comparative analyses identified various traits such as body size, metabolic rate, maternal investment, parental care, prolonged development, mating system and migratory behaviour as important correlates of relative brain size. By contrast, there seems to be a little evidence that sociality favour the evolution of large brains in birds. It has also been shown that particular behaviours such as song learning or food-hoarding correlate with size of those brain regions that participate in these behaviours. Future studies should not only look for novel determinants of brain size but also include known correlates and utilize path analysis modelling to describe the direct, indirect and spurious dependencies among these..