Possible link between brain size and flight mode in birds: Does soaring ease the energetic cost of the brain?

Elucidating determinants of interspecies variation in brain size has been
a long-standing challenge in cognitive and evolutionary ecology. As the
brain is an energetically expensive organ, energetic tradeoffs among
organs are considered to play a key role in brain size evolution. This
study examined the tradeoff between the brain and locomotion in birds by
testing the relationship between brain size, flight modes with different
energetic costs (flapping and soaring), and migratory behavior, using
published data on the whole-brain mass of 2,242 species. According to
comparative analyses considering phylogeny and body mass, soarers, who can
gain kinetic energy from wind shear or thermals and consequently save
flight costs, have larger brains than flappers among migratory birds.
Meanwhile, the brain size difference was not consistent in residents, and
the size variation appeared much larger than that in migrants. In
addition, the brain size of migratory birds was smaller than that of
resident birds among flappers, whereas this property was not significant
in soarers. Although further research is needed to draw a definitive
conclusion, these findings provide further support for the energetic
tradeoff of the brain with flight and migratory movements in birds and
advance the idea that a locomotion mode with lower energetic cost could be
a driver of encephalization during the evolution of the brain.