The paired hind limb. The main parts are the thigh, the lower-leg (equivalent to shin), the so-called tarsus (equivalent to foot, but in birds the bones for the toes are largely fused) and the toes. See picture under 'bird topography'. The joints between these components are respectively the knee, the inter-tarsal joint or ankle, and the joints of the toes. As most birds stand on their toes, the raised ankle is often mistaken for the knee (the real one being concealed under the plumage) although it bends in the wrong direction. The thigh, knee and upper part of the lower leg are thus normally (except some 'running' birds) completely hidden by the flank feathers. Birds depend upon their legs for locomotion on land and on water. Therefore the legs of a bird are (when its standing) situated just behind the bird's centre of gravity, balance being maintained by the toes. In swimming species though, the legs are set back further, making walking difficult, which is particularly true of the divers (family Gaviidae). Because of the different locomotive functions between species, the form and relative length of the legs are variable. E.g. birds that walk or run will have long, strong legs, while arboreal birds have relatively weak, small legs. The tarsus in most species is rounded in cross-section, but in many swimming species it is laterally compressed to reduce water friction. Some species possess spurs on the back- and inner surface of the tarsus; bony structures covered with a pointed horny sheath used in battles (e.g. cockfighting!). The tarsus and toes are usually uncovered by feathers, except for some species living in cold climates. These parts are covered with a thickened, hardened structure of the skin, the podotheca; this may be horny (corneous) in landbirds or softer and leathery in waterbirds. Usually the surface of the podotheca is made from little scales or plates, but sometimes it's undivided (or has only a few scales near the toes) which is called 'holothecal' or 'booted' and is often found in passerines (birds of the order Passeriformes). Unfeathered legs and feet are a potential source of heat loss, but at low external temperatures the blood flow can be reduced and also the temperature of the blood flowing into the leg can be lowered.
See also 'foot' or 'toe(s)'.

Alternative forms for leg : ankle, booted, legs, podotheca.