Sixty percent of birds perch on one foot plan
Six in every ten bird species are passerines, members of the order Passeriformes, with more than 140 families and about 6,500 species. Their name comes from the Latin for sparrow, and their shared trademark is a foot with three toes forward and one long toe behind, ideal for gripping a perch.
The back toe joins the leg at about the same height as the front three, and a tendon at the rear of the leg tightens automatically when the bird crouches. That curls the toes around a branch and stiffens them, so a passerine can sleep perched without toppling. Most species carry 12 tail feathers, though the superb lyrebird has 16 and some spinetails as few as six, and trunk climbers such as treecreepers brace themselves on stiff tails.
The order has three branches. New Zealand wrens, tiny birds now found only there, split off first; the suboscines, which do not truly sing, live mostly in the Americas; and the oscines are the songbirds, masters of the syrinx muscles, although crows sound unmusical to us and lyrebirds imitate other sounds. Size runs from the short-tailed pygmy tyrant, 6.5 centimetres and 4.2 grams, to thick-billed and common ravens over 1.5 kilograms, while long-tailed lyrebirds and birds-of-paradise are longer overall.
Chicks hatch blind, naked and helpless. Unlike most other birds, passerines usually lay coloured eggs, and the vinous-throated parrotbill produces both white and blue ones to foil cuckoos. Clutch size ranges from a single egg in Australian lyrebirds and scrub-robins to as many as a dozen in northern hole-nesting tits. Some groups, including viduas, cuckoo-finches and cowbirds, skip nesting altogether and leave their eggs to other species.
The first passerines probably arose in the Southern Hemisphere about 50 million years ago and diversified as the southern continents drifted apart, the New Zealand wrens stranded on Zealandia, the suboscines in South America and the songbirds in Australia. One songbird branch, the Passerida, spread into Eurasia and Africa roughly 40 million years ago. Their fragile bones fossilise poorly, so much of this history comes from DNA.
Source: Passerine
