We test the hypothesis that the relative sizes of the different parts of the brain (brain stem, optic lobes, cerebellum and cerebral hemispheres), measured after body size effects have been removed, are associated with differences in behaviour and ecology across bird species.

The results demonstrate that behavioural and ecological correlates of relative brain size are not independent of each other. When the effects of variation in other categories are accounted for, the strongest single effect is due to relatively large brain sizes being associated with altricial development. It is unlikely that this effect is due to the confounding influence of taxonomic associations.

Overall, the results do not provide support for the idea that differences in measures of environmental complexity select for differences in relative brain size.