Allometric scaling of the tectofugal pathway in birds.

Recent studies have shown that the relative sizes of visual regions in the avian brain are correlated with behavioral differences among species. Despite the fact that the tectofugal pathway is the primary source of visual input to the avian brain, detailed interspecific comparisons of the relative size of nuclei within the pathway, the optic tectum, nucleus rotundus and entopallium, are wanting. Here, we examine the allometric scaling relationships of each of these brain regions relative to the brain as a whole using conventional and phylogenetically based statistics across 113 species. Our results show that the relative size of tectofugal regions of the avian brain varies significantly among avian orders. More specifically, waterfowl (Anseriformes), parrots (Psittaciformes) and owls (Strigiformes) have significantly smaller tectofugal brain regions than other birds. At the opposite end of the spectrum, we found little evidence for the significant enlargement of any tectofugal region among the orders that we sampled. The lack of such hypertrophy likely reflects the heterogeneous organization of the optic tectum, nucleus rotundus and entopallium. We therefore speculate that if neural adaptations do exist in the avian tectofugal pathway that are correlated with behavior, they occur at a more refined level than simple volumetrics.