information:classification, information:filtering, structural:expansion
Description

Tinamous are plump, compact birds with slender necks, small heads and, usually, short, decurved bills, though a few have long bills. Females are usually larger than the males. The smallest species, the dwarf tinamou, weighs about 43 g (1.5 oz) with a length of 14.5 cm (5.7 in). Females of the largest, the grey tinamou, weigh up to 2 kg (4.4 lb) with a length of up to 49 cm (19 in). Their feet have three forward-facing toes; a hind toe is either higher and retrogressed, or absent. The back of the tarsus is covered with scales, the color of which may aid in identification.

Tinamous have a pneumaticized skeleton with a sternal keel, 16–18 cervical vertebrae, and fused thoracic vertebrae. They have poor circulation, evidenced by a greenish tint to the skin. They also have relatively the smallest hearts and lungs of all birds, comprising only 1.6–3.1% of their body weight, whereas the equivalent in a domestic chicken is 12%. Despite their poor flying ability, the percentage of their body mass that is muscle is 28.6–40%, which is similar to that of hummingbirds. The preen gland is small and tufted. The male has a corkscrew shaped penis, similar to those of the other ratites and to the hemipenis of some reptiles. The female has a small phallic organ in the cloaca which becomes larger during the breeding season.