The anatomy ofbird legs and feet is diverse, encompassing many accommodations to perform a wide variety of functions.[1]
Most birds are classified asdigitigrade animals, meaning they walk on theirtoes rather than the entire foot.[3][4] Some of the lower bones of the foot (thedistals and most of themetatarsal) are fused to form thetarsometatarsus – a third segment of the leg, specific to birds.[5][6] The upper bones of the foot (proximals), in turn, are fused with thetibia to form thetibiotarsus, as over time thecentralia disappeared.[7][6][4][8] Thefibula also reduced.[5]
The legs are attached to a strong assembly consisting of thepelvic girdle extensively fused with the uniform spinal bone (also specific to birds) called thesynsacrum, built from some of the fused bones.[8][9]
Birds are generallydigitigrade animals (toe-walkers),[7][10] which affects the structure of their leg skeleton. They use only theirhindlimbs to walk (bipedalism).[2] Theirforelimbs evolved to becomewings. Most bones of the avian foot (excluding toes) are fused together or with other bones, having changed their function over time.
Some lower bones of the foot are fused to form thetarsometatarsus – a third segment of the leg specific to birds.[8] It consists of mergeddistals andmetatarsals II, III and IV.[6] Metatarsus I remains separated as a base of the first toe.[4] The tarsometatarsus is the extended foot area, which gives the leg extra lever length.[7]
The foot's upper bones (proximals) are fused with thetibia to form thetibiotarsus, while thecentralia are absent.[5][6] The anterior (frontal) side of the dorsal end of the tibiotarsus (at theknee) contains a protruding enlargement called thecnemial crest.[2]
At the knee above thecnemial crest is thepatella (kneecap).[4] Some species do not have patellas, sometimes only a cnemial crest. Ingrebes both a normal patella and an extension of the cnemial crest are found.[2]
Thefibula is reduced and adheres extensively to the tibia, usually reaching two-thirds of its length.[2][7][8] Onlypenguins have full-length fibulae.[4]
The birdknee joint between thefemur andtibia (or rathertibiotarsus) points forwards, but is hidden within thefeathers. The backward-pointing "heel" (ankle) that is easily visible is a joint between thetibiotarsus andtarsometatarsus.[3][4] The joint inside the tarsus occurs also in some reptiles. It is worth noting here that the name "thick knee" of the members of thefamilyBurhinidae is a misnomer because their heels are large.[2][8]
The chicks in the ordersCoraciiformes andPiciformes have ankles covered by a patch of tough skins with tubercles known as theheel-pad. They use the heel-pad to shuffle inside the nest cavities or holes.[11][12]
Theostrich is the only bird that has the didactyl foot.[2]
Most birds have four toes, typically three facing forward and one pointing backward.[7][10][8] In a typical perching bird, they consist respectively of 3, 4, 5 and 2phalanges.[2] Some birds, like thesanderling, have only the forward-facing toes; these are called tridactyl feet while theostrich have only two toes (didactyl feet).[2][4] The first digit, called thehallux, ishomologous to the humanbig toe.[7][10]
Theclaws are located on the extreme phalanx of each toe.[4] They consist of a hornykeratinouspodotheca, or sheath,[2] and are not part of the skeleton.
The bird foot also contains one or two metatarsals not fused in thetarsometatarsus.[8]
The legs are attached to a very strong, lightweight assembly consisting of thepelvic girdle extensively fused with the uniform spinal bone called thesynsacrum,[7][10] which is specific to birds. The synsacrum is built from thelumbar fused with thesacral, some of the first sections of thecaudal, and sometimes the last one or two sections of thethoracicvertebrae, depending on species (birds have altogether between 10 and 22 vertebrae).[9] Except for those ofostriches andrheas,pubic bones do not connect to each other, easingegg-laying.[8]
Fusions of individual bones into strong, rigid structures are characteristic.[1][7][10]
Most major bird bones are extensivelypneumatized. They contain many air pockets connected to the pulmonaryair sacs of therespiratory system.[13] Their spongy interior makes them strong relative to their mass.[2][7] The number of pneumatic bones depends on the species; pneumaticity is slight or absent indiving birds.[14] For example, in thelong-tailed duck, the leg and wing bones are not pneumatic, in contrast with some of the other bones, whileloons andpuffins have even more massive skeletons with no aired bones.[15][16] Theflightlessostrich andemu have pneumaticfemurs, and so far this is the only known pneumatic bone in these birds[17] except for the ostrich's cervical vertebrae.[13]
Fusions (leading to rigidity) and pneumatic bones (leading to reduced mass) are some of the many adaptations of birds for flight.[1][7]
Loons tend to walk this way because their legs andpelvis are highly specialized for swimming. They have a narrow pelvis, which moves the attachment point of thefemur to the rear, and theirtibiotarsus is much longer than the femur. This shifts the feet (toes) behind thecenter of mass of the loon body. They walk usually by pushing themselves on their breasts; larger loons cannot take off from land.[10] This position, however, is highly suitable for swimming because their feet are located at the rear like thepropeller on amotorboat.[2]
Grebes and many other waterfowl have shorterfemur and a more or less narrow pelvis, too, which gives the impression that their legs are attached to the rear as inloons.[2]
Because avianforelimbs arewings, many forelimb functions are performed by thebill andhindlimbs.[10] It has been proposed that the hindlimbs are important inflight as accelerators when taking-off.[18][19] Some leg and foot functions, including conventional ones and those specific to birds, are:
Cradling and turningeggs during incubation.[3] Birds lacking abrood 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).[1]
Preening and cleaning.[10] Sometimes birds use a specialclaw (for example,barn owls have a so-called "feather comb"). Someherons andnightjars use the claw for cleaning the head.[2]
Anisodactyl: three toes in front (2, 3, 4), and one in back (1); in nearly allsongbirds and most other perching birds.[4][20]
Zygodactyl: two toes in front (2, 3) and two in back (1, 4) – the outermost front toe (4) is reversed. The zygodactyl arrangement is a case ofconvergence, because it evolved in birds in different ways nine times.[1][10]
Heterodactyl: two toes in front (3, 4) and two in back (2, 1) – the inner front toe (2) is reversed; heterodactyl arrangement only exists introgons.[20]
Syndactyl: three toes in front (2, 3, 4), one in back (1); the inner and middle (2, 3) are joined for much of their length.[2][1] This is often found inPicocoraciae, thoughrollers,ground rollers, andPiciformes (who are zygodactyl) are exceptions.[7]
Pamprodactyl: two inner toes in front (2, 3), the two outer (1, 4) can rotate freely forward and backward. Inmousebirds and someswifts. Someswifts move all four digits forward to use them as hooks to hang.[20]
The most common arrangement is the anisodactyl foot, and second among perching birds is the zygodactyl arrangement.[3][7][21]
All birds have claws at the end of the toes. The claws are typically curved and the radius of curvature tends to be greater as the bird is larger although they tend to be straighter in large ground dwelling birds such as ratites.[22] Some species (includingnightjars,herons,frigatebirds, owls andpratincoles) have comb-like serrations on the claw of the middle toe that may aid in scratchpreening.[23]
Palmations and lobes enable swimming or help walking on loose ground such asmud.[3] The webbed or palmated feet of birds can be categorized into several types:
Lobate: the anterior digits (2–4) are edged with lobes of skin. Lobes expand or contract when a bird swims. Ingrebes,coots,phalaropes,finfoots and some palmate-footedducks on the hallux (1). Grebes have more webbing between the toes than coots and phalaropes.[20][4][21]
Thearteries andveins intertwine in the legs, so heat can be transferred from arteries back to veins before reaching the feet. Such a mechanism is calledcountercurrent exchange.Gulls can open a shunt between these vessels, turning back the bloodstream 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), while that of the foot may be close to 0 °C (32 °F).[1]
However, for cooling, this heat-exchange network can be bypassed and blood-flow through the foot significantly increased (giant petrels). Some birds also excrete onto their feet, increasing heat loss viaevaporation (storks,New World vultures).[1]
^abcdRomer, Alfred Sherwood; Parsons, Thomas S. (1977).The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 205–208.ISBN978-0-03-910284-5.
^abcdefghijklmnProctor, Noble S.; Lynch, Patrick J. (1993). "Chapters: 6. Topography of the foot, 11. The pelvic girdle, and 12. The bones of the leg and foot Family".Manual of Ornithology. Avian Structure & Function. New Haven and London:Yale University Press. pp. 70–75,140–141,142–144.ISBN978-0-300-07619-6.
^abcdefghDobrowolski, Kazimierz A.; Klimaszewski, Sędzimir M.; Szelęgiewicz, Henryk (1981). "Chapters: Gromada: Ptaki - Aves: Układ kostny; Pas miednicowy i kończyna tylna [Class: Birds: The skeletal system; The pelvic girdle and the hindlimb]".Zoologia [Zoology] (in Polish) (4th ed.). Warsaw: Wydawnictwo Szkolne i Pedagogiczne. pp. 462–464, 469.ISBN978-83-02-00608-1.
^abKowalska-Dyrcz, Alina (1990). "Entry: synsakrum [synsacrum]". In Busse, Przemysław (ed.).Ptaki [Birds]. Mały słownik zoologiczny [Small zoological dictionary] (in Polish). Vol. II (1st ed.). Warsaw: Wiedza Powszechna. p. 245.ISBN978-83-214-0563-6.
^Chasen, F. N. (1923). "On The Heel-Pad in certain Malaysian Birds".Journal of the Malayan Branch of the Royal Asiatic Society.1 (87):237–246.JSTOR41559544.
^abcdKalbe, Lothar (1983). "Besondere Formen für spezielle Aufgaben der Wassertiere [Special adaptations of aquatic animals to specific lifestyles]".Tierwelt am Wasser [Wildlife by the Water] (in German) (1st ed.). Leipzig-Jena-Berlin: Urania-Verlag. pp. 72–77.
^Pike, A. V. L.; Maitland, D. P. (2004). "Scaling of bird claws".Journal of Zoology.262:73–81.doi:10.1017/S0952836903004382.
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