Bird feet and legs
Most birds are digitigrade animals, which means that they walk on their toes, not the entire foot. Some of their lower bones of the foot (distals and most of metatarsals) are fused to form tarsometatarsus – a third segment of the leg, specific to birds. The upper bones of the foot (proximals) in turn are fused with the tibia to form tibiotarsus, as over time the centralia disappeared. The fibula is also reduced.
The legs are attached to a very strong assembly consisting of the pelvic girdle extensively fused with the uniform spinal bone (also specific to birds) called the synsacrum, built from some of the fused bones.
- 1 Skeleton
- 2 Functions of bird legs and feet
- 3 Toe arrangements
- 4 Webbing and lobation
- 5 Heat loss regulation
- 6 See also
- 7 References
Birds are generally digitigrade animals (toe - walkers) as reflected in the structure of their leg skeleton. They use only hindlimbs to walk, which is called bipedalism. As mentioned earlier, most bones of the avian foot (except toes) are fused together or with other bones, having over time changed function.
Some lower bones of the foot are fused to form tarsometatarsus – a third segment of the leg specific for birds. It consists of merged distals and metatarsals II, III and IV. Metatarsus I remains separated as a base of the first digit. Tarsometatarsus is the extended foot area giving the leg an extra lever length.
The upper bones of the foot (proximals) are fused with the tibia to form tibiotarsus, while the centralia disappeared. The anterior (frontal) side of the dorsal end of the tibiotarsus (at the knee) contains a protruding enlargement called the cnemial crest.
At the knee above the cnemial crest, the patella (kneecap) is located. Some species don't have patellas; sometimes it is only a small extension of the cnemial crest. In grebes there exist both a normal patella and an extension of the cnemial crest.
Knee and ankle - confusions
The bird knee joint between the femur and tibia (or rather tibiotarsus) points obviously forwards, but is hidden in feathers. The backward-pointing "heel" (ankle ) we can see is a joint between the tibiotarsus and tarsometatarsus. The joint inside tarsus occurs also in some reptiles. It is worth noting here that the name "thick knee" of the birds in the family Burhinidae is incorrect, because the heel of these birds is large. So ornithologists confused knee and ankle here.
Toes and unfused metatarsals
Most birds have 4 toes (digits), typically 3 facing forward and 1 pointing backward. In a typical perching bird they consist respectively of 3, 4, 5 and 2 phalanges. Some birds have 3 forward-facing toes (tridactyl, for example sanderling) and the ostrich 2 toes (didactyl). The first digit called hallux is homologous to the human big toe and usually projects to the rear.
Pelvic girdle and synsacrum
The legs are attached to a very strong (but lightweight) assembly consisting of the pelvic girdle extensively fused with uniform spinal bone called synsacrum, which is specific to birds. Synsacrum is built from fused lumbar, sacral, some of first caudal and sometimes 1–2 last thoracic vertebrae, depending on species - altogether 10–22 vertebrae. Except for ostriches and rheas, pubic bones in the pelvic girdle do not connect each other, making egg-laying easier.
Rigidity and reduction of mass
Most major bird bones are also extensively pneumatized. They contain many air pockets connected to the pulmonary air sacs of the respiratory system. Their spongelike interior makes them very strong relatively to their mass. The number of pneumatic bones however depends on species. It is worth noting that the pneumaticity is slight or absent in diving birds. For example, in the long-tailed duck, the bones of the leg (and wing) are not pneumatic in contrast with some of the other bones, while loons and puffins have even more massive skeletons without any aired bones. Surprisingly, the flightless ostrich and emu have pneumatic femurs and so far this is the only known aired bone in these birds except cervical vertebrae of the ostrich.
Plantigrade locomotion in birds
Loons tend to walk this way because their legs and pelvis are deeply specialized for swimming. They have a narrow pelvis, which moves the attachment point of the femur to the rear, and their tibiotarsus is much longer than the femur. This shifts the feet (toes) behind the center of mass of the loon body. They walk usually by pushing themselves on their breasts; larger loons cannot take off from land. This position however is highly suitable for swimming, because their feet are located at the rear like the propeller on a motorboat.
Functions of bird legs and feet
Because avian forelimbs are converted to wings, many of their functions are performed by the bill and hindlimbs (feet and legs). It has been proposed that the hindlimbs are also very important in flight as accelerators when taking-off. Some leg and foot functions, including conventional ones and those specific to birds are:
- perching on a branch or clinging
- carrying (like osprey holding fish)
- flight - related
- feeding - related
- catching and killing prey in raptors (hawk, owl)
- holding (parrots - used like hands) and pulling apart food (along with the bill)
- scratching the ground in search of food
- reproduction - related
- cradling  and turning eggs during incubation. Birds lacking a brood patch incubate the eggs with their feet - grasping one or even two of them (gannets, boobies) or keeping them on the top surfaces of their feet (penguins under a pouch of belly skin, murres).
- courtship (sage grouse), aerial courtship (bald eagle)
- building nests (in addition to the bill)
- preening and cleaning. Sometimes birds use a special claw, for example barn owls have a so-called "feather comb", some herons and nightjars use the claw for cleaning the head.
- heat loss regulation (herons, gulls, giant petrel, storks, New World vultures, ducks, geese)
Typical toe arrangements in birds are:
- zygodactyl - two toes in front (2, 3) and two in back (1, 4) - the outermost front toe (4) is reversed
- Many perching birds - most woodpeckers and their allies, osprey, owls, cuckoos, most parrots, mousebirds, some swifts, cuckoo roller.
- Woodpeckers when climbing can rotate outer rear digit (4) to the side in an ectropodactyl arrangement. Black-backed woodpecker, Eurasian three-toed woodpecker and American three-toed woodpecker have three toes - the inner rear (1) is missing, outer rear (4) points always backward (never rotates).
- Owls, osprey and turacos can rotate outer toe (4) back and forth.
- The zygodactyl arrangement is a case of convergence, because it evolved in birds in different ways nine times.
- heterodactyl - two toes in front (3, 4) and two in back (2, 1) - the inner front toe (2) is reversed.
- syndactyl - three toes in front (2, 3, 4), one in back (1), two - outer and middle (2, 3) are joined for much of their length.
- pamprodactyl - two inner toes in front (2, 3), two outer (1, 4) can rotate freely forward and backward.
- Mousebirds, some swifts.
- Some swifts move all four digits forward to use them as hooks to hang.
Webbing and lobation
- palmate - only the anterior digits (2-4) joined by the webbing.
- Ducks, geese and swans, gulls and terns, and other aquatic birds (auks, flamingos, fulmars, jaegers, loons, petrels, shearwaters, skimmers).
- Diving ducks additionally have lobed hind toe (1), gulls, terns and allies have reduced hind toe.
- Gannets and boobies, pelicans, cormorants, anhingas, frigatebirds.
- Some gannets have brightly colored feet used in display.
- Some plovers (Eurasian dotterel) and sandpipers (semipalmated sandpiper, stilt sandpiper, upland sandpiper, greater yellowlegs, willet), avocet, herons (only two toes), all grouse, some domestic breeds of chicken.
- Plovers and lapwings have a vestigial hind toe (1), sandpipers and their allies a reduced and raised hind toe almost not touching the ground. Sanderling is the only sandpipper having 3 toes (tridactyl foot).
- Lobes expand or contract when bird swims.
- Grebes, coots, phalaropes, finfoots, some palmate-footed ducks on the hallux (1).
- Grebes have more webbing between the toes in comparison to coots and phalaropes.
Most common is the palmate foot.
Heat loss regulation
The arteries (carrying blood from the heart) and veins (carrying blood toward the heart) intertwine in the legs so heat can be transferred back from arteries to veins before reaching feet. Such a mechanism is called countercurrent exchange. Gulls can also open a shunt between these vessels, turning back the blood stream above the foot, and constrict the vessels in the foot. This reduces heat loss by more than 90 percent. In gulls the temperature of the base of the leg is 32°C (89°F), of the foot may be close to 0°C (32°F).
However, for cooling, this heat-exchange network can be bypassed and blood flow through the foot significantly increased (giant petrel). Some birds additionally excrete onto their feet, increasing heat loss thanks to evaporation (storks, New World vultures).
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