The largest order of birds and the dominant avian group on Earth today. The passeriform birds are true perching birds, with four toes, three directed forward and one backward. Their rapid evolution and adaptation to virtually all terrestrial environments resulted in a large number of families, 126, and of species, some 6,250, compared with only about 4,200 species of all other birds.

Preceded by Coraciiformes and Piciformes as the dominant land birds of the early Paleogene Period, passerines first appeared in the fossil record of the late Oligocene Epoch (some 34–23 million years ago) of France. By the early Miocene Epoch (some 11.6 to 5.3 million years ago), however, passerines outnumbered all other birds combined.  Most modern species of birds arose during the early Pleistocene Epoch (about 2,600,000 to 11,700 years ago), a period of cooling temperatures, shifts in habitats, and advancing glaciers.

The evolutionary success of passerine birds begs for explanation. The combination of a flexible body plan and superior neural capacities enabled passerines to explore and adapt to novel environments. Added to those traits, it has been suggested that the complex nest-building behaviours of passerine birds released them from the obligatory cavity-nesting behaviours of their predecessors and permitted the move into new habitats and ecological zones.

M.Heimerdinger Clench, O.L. Austin, Jr, F. Gill in passeriform. (2011).  Encyclopædia Britannica Ultimate Reference Suite. Chicago: Encyclopædia Britannica.