Birds are the group of amniotes with the smallest genomes. Whereas mammal and reptilian genomes range between 1.0 and 8.2 giga base pairs (Gb), bird genomes have sizes between 0.91 Gb (black-chinned hummingbird, Archilochus alexandri) and 1.3 Gb (common ostrich, Struthio camelus). Just as happens to any other living being, bird genomes’ reflect the action of natural selection upon these animals. Their genomes are the basis of their morphology and behaviour.