When the vertebrates fly, for some reason their genome reduces in size. Bats among mammals, birds and the extinct reptilian pterosaurs all have their genome size reduced.
The most diverse fliers are the birds, and this investigation has conclusively proved that 4 characteristics are heavily influenced in this way. The size of flight muscles and the heart are traits that are more affected than wing shape or size, but all are heavily involved with the reduction of gene variation in a metabolic context.
Read the full, original story: Flying and genome size: its true about the reduction