Hummingbirds have the most highly-modified genomes of any vertebrate.
Of all vertebrates, hummingbirds have the most reduced genome size, the
greatest A+T bias, and the greatest differentiation of the genome into
isochores (4 separate G+C-rich isochores compared with the mammalian 3).
Since these changes are all arguably related to the metabolic demands of flight (hummingbirds have metabolic rates 50X higher than comparable sized mammals, with correpondingly increased rates of oxidation of Guanine in DNA), these genomic modifications argue for caution in the interpretation of DNA sequence data from bats, particularly those with constant high metabolic rates and no torpor (e.g. the majority of megabats, phyllostomoid and rhinolophoid microbats).
See Flying DNA.