カラスなどのように都市部の暮らしに適応した鳥は、脳のサイズが大きくなっているそうです。

Brains and the city: big-brained passerine birds succeed in urban environments

Urban regions are among the most human-altered environments on Earth and they are poised for rapid expansion following population growth and migration. Identifying the biological traits that determine which species are likely to succeed in urbanized habitats is important for predicting global trends in biodiversity. We provide the first evidence for the intuitive yet untested hypothesis that relative brain size is a key factor predisposing animals to successful establishment in cities. We apply phylogenetic mixed modelling in a Bayesian framework to show that passerine species that succeed in colonizing at least one of 12 European cities are more likely to belong to big-brained lineages than species avoiding these urban areas. These data support findings linking relative brain size with the ability to persist in novel and changing environments in vertebrate populations, and have important implications for our understanding of recent trends in biodiversity.（Biology Letters April 27, 2011）

これは要するに、それまで暮らしていた環境から、都市部の環境に適応するに伴って変化したということですね。生物が環境に適応するにあたって、脳のサイズが重要という一般的な仮説を補強する報告ということになります。

ちなみに、ツバメなどは小さいままですが、これはたまたま自分がそれまで生きていた環境と同じような状況を都市部に見つけることができたからだそうで。