Allometric comparison of brain and main brain subdivisions in birds.

The brain weights and volumes of the main brain subdivisions: telencephalon, diencephalon, mesencephalic tegmentum, optic tectum, cerebellum and myelencephalon were measured in 8 species of Phasianoidea (Galliformes) and in 20 species of birds belonging to 13 orders. The sizes of these structures were compared using allometry formulae calculated for each structure from the 8 Phasianoidea. Encephalization and the relative size of the main brain subdivisions are expressed by size indices. Higher encephalization indices are obtained in species which are either highly telencephalized as in Psittaciformes or with a large cerebellum as in Sphenisciformes. The brainstem components: diencephalon, mesencephalic tegmentum and myelencephalon appear to be more conservative in their size variation than the telencephalon, cerebellum and optic tectum. The size of the cerebellum appears to be related to flight abilities. The size of the optic tectum is related to the importance of vision in foraging within the Charadriiformes. In tactile-feeding Charadriiformes and filter-feeding Anseriformes, the optic tectum is relatively small.