The condensed genomes of birds may also be an adaptation to powered flight. Birds have the smallest genomes of any amniote, the group of animals, including reptiles and mammals, that lay their eggs on land. A typical mammal has a genome ranging from 1 billion to 8 billion base pairs, whereas in birds, it hovers at around 1 billion, a result of fewer repeat elements and a large number of so-called deletion events, in which DNA has been expunged over evolutionary time. A more compressed genome may allow a bird to regulate its genes more rapidly to meet the requirements of flight.
