Hindlimbs primarily used for the anchoring and locomotion of avians
The anatomy of
bird legs and feet
is diverse, encompassing many accommodations to perform a wide variety of functions.
[1]
Most birds are classified as
digitigrade
animals, meaning they walk on their
toes
rather than the entire foot.
[3]
[4]
Some of the lower bones of the foot (the
distals
and most of the
metatarsal
) are fused to form the
tarsometatarsus
? a third segment of the leg, specific to birds.
[5]
[6]
The upper bones of the foot (
proximals
), in turn, are fused with the
tibia
to form the
tibiotarsus
, as over time the
centralia
disappeared.
[7]
[6]
[4]
[8]
The
fibula
also reduced.
[5]
The legs are attached to a 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.
[8]
[9]
Hindlimbs
[
edit
]
Birds are generally
digitigrade
animals (
toe
-walkers),
[7]
[10]
which affects the structure of their leg skeleton. They use only their
hindlimbs
to walk (
bipedalism
).
[2]
Their
forelimbs
evolved to become
wings
. Most bones of the avian foot (excluding toes) are fused together or with other bones, having changed their function over time.
Tarsometatarsus
[
edit
]
Some lower bones of the foot are fused to form the
tarsometatarsus
? a third segment of the leg specific to birds.
[8]
It consists of merged
distals
and
metatarsals
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]
Tibiotarsus
[
edit
]
The foot's upper bones (
proximals
) are fused with the
tibia
to form the
tibiotarsus
, while the
centralia
are absent.
[5]
[6]
The anterior (frontal) side of the dorsal end of the tibiotarsus (at the
knee
) contains a protruding enlargement called the
cnemial crest
.
[2]
Patella
[
edit
]
At the knee above the
cnemial crest
is the
patella
(kneecap).
[4]
Some species do not have patellas, sometimes only a cnemial crest. In
grebes
both a normal patella and an extension of the cnemial crest are found.
[2]
Fibula
[
edit
]
The
fibula
is reduced and adheres extensively to the tibia, usually reaching two-thirds of its length.
[2]
[7]
[8]
Only
penguins
have full-length fibulae.
[4]
Knee and ankle ? confusions
[
edit
]
The bird
knee
joint between the
femur
and
tibia
(or rather
tibiotarsus
) points forwards, but is hidden within the
feathers
. The backward-pointing "
heel
" (
ankle
) that is easily visible is a joint between the
tibiotarsus
and
tarsometatarsus
.
[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 the
family
Burhinidae
is a misnomer because their heels are large.
[2]
[8]
The chicks in the orders
Coraciiformes
and
Piciformes
have ankles covered by a patch of tough skins with tubercles known as the
heel-pad
. They use the heel-pad to shuffle inside the nest cavities or holes.
[11]
[12]
Toes and unfused metatarsals
[
edit
]
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 2
phalanges
.
[2]
Some birds, like the
sanderling
, have only the forward-facing toes; these are called tridactyl feet while the
ostrich
have only two toes (didactyl feet).
[2]
[4]
The first digit, called the
hallux
, is
homologous
to the human
big toe
.
[7]
[10]
The
claws
are located on the extreme phalanx of each toe.
[4]
They consist of a horny
keratinous
podotheca
, or sheath,
[2]
and are not part of the skeleton.
The bird foot also contains one or two metatarsals not fused in the
tarsometatarsus
.
[8]
Pelvic girdle and synsacrum
[
edit
]
The legs are attached to a very strong, lightweight assembly consisting of the
pelvic girdle
extensively fused with the uniform spinal bone called the
synsacrum
,
[7]
[10]
which is specific to birds. The synsacrum is built from the
lumbar
fused with the
sacral
, some of the first sections of the
caudal
, and sometimes the last one or two sections of the
thoracic
vertebrae
, depending on species (birds have altogether between 10 and 22 vertebrae).
[9]
Except for those of
ostriches
and
rheas
,
pubic bones
do not connect to each other, easing
egg
-laying.
[8]
Rigidity and reduction of mass
[
edit
]
Fusions of individual bones into strong, rigid structures are characteristic.
[1]
[7]
[10]
Most major bird bones are extensively
pneumatized
. They contain many air pockets connected to the pulmonary
air sacs
of the
respiratory 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 in
diving birds
.
[14]
For example, in the
long-tailed duck
, the leg and wing bones are not pneumatic, in contrast with some of the other bones, while
loons
and
puffins
have even more massive skeletons with no aired bones.
[15]
[16]
The
flightless
ostrich
and
emu
have pneumatic
femurs
, 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]
Plantigrade locomotion
[
edit
]
Most birds, except
loons
and
grebes
, are
digitigrade
, not
plantigrade
.
[2]
Also,
chicks
in the
nest
can use the entire
foot
(toes and
tarsometatarsus
) with the heel on the ground.
[4]
Loons tend to walk this way because their legs and
pelvis
are highly 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.
[10]
This position, however, is highly suitable for swimming because their feet are located at the rear like the
propeller
on a
motorboat
.
[2]
Grebes
and many other waterfowl have shorter
femur
and a more or less narrow pelvis, too, which gives the impression that their legs are attached to the rear as in
loons
.
[2]
Functions
[
edit
]
Because avian
forelimbs
are
wings
, many forelimb functions are performed by the
bill
and
hindlimbs
.
[10]
It has been proposed that the hindlimbs are important in
flight
as accelerators when taking-off.
[18]
[19]
Some leg and foot functions, including conventional ones and those specific to birds, are:
- Locomotion
- Perching (as on a branch) or clinging
[3]
- Carrying (like
ospreys
holding
fish
)
[3]
- Flight
-related
- Serving probably as the primary take-off accelerator. In the
common vampire bat
, by contrast, the required force is generated by the
wing
.
[18]
[19]
- Absorbing the shock of landing on a perch and on the water, becoming "
water skis
"
[3]
- Feeding and related
- Catching and killing prey in
raptors
(
hawks
,
owls
)
[3]
- Holding (used like hands in
parrots
) and pulling apart food (with help from the
bill
)
[3]
- Scratching the ground in search of food
[2]
- Reproduction and related
- Preening and cleaning.
[10]
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.
[2]
- Heat loss regulation (
herons
,
gulls
,
giant petrels
,
storks
,
New World vultures
,
ducks
,
geese
)
[1]
[2]
Toe arrangements
[
edit
]
Typical toe arrangements in
birds
are:
- Anisodactyl
: three toes in front (2, 3, 4), and one in back (1); in nearly all
songbirds
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 of
convergence
, 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 in
trogons
.
[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]
Common in
Coraciiformes
, including
kingfishers
and
hornbills
.
[7]
- Pamprodactyl
: two inner toes in front (2, 3), the two outer (1, 4) can rotate freely forward and backward. In
mousebirds
and some
swifts
. Some
swifts
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]
Claws
[
edit
]
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 (including
nightjars
,
herons
,
frigatebirds
, owls and
pratincoles
) have comb-like serrations on the claw of the middle toe that may aid in scratch
preening
.
[23]
Webbing and lobation
[
edit
]
Palmations and lobes enable swimming or help walking on loose ground such as
mud
.
[3]
The webbed or palmated feet of birds can be categorized into several types:
- Palmate
: only the anterior digits (2?4) are joined by webbing. Found in
ducks
,
geese
and
swans
,
gulls
and
terns
, and other aquatic birds (
auks
,
flamingos
,
fulmars
,
jaegers
,
loons
,
petrels
,
shearwaters
and
skimmers
).
[20]
[21]
Diving ducks
also have a lobed hind toe (1), and gulls, terns and allies have a reduced hind toe.
[24]
- Totipalmate
: all four digits (1?4) are joined by webbing. Found in
gannets
and
boobies
,
pelicans
,
cormorants
,
anhingas
and
frigatebirds
. Some gannets have brightly colored feet used in display.
[3]
[21]
- Semipalmate
: a small web between the anterior digits (2?4). Found in some
plovers
(
Eurasian dotterels
) and
sandpipers
(
semipalmated sandpipers
,
stilt sandpipers
,
upland sandpipers
,
greater yellowlegs
and
willet
),
avocet
,
herons
(only two toes), all
grouse
, and some domesticated breeds of
chicken
. Plovers and
lapwings
have a vestigial hind toe (1), and sandpipers and their allies have a reduced and raised hind toe barely touching the ground. The
sanderling
is the only sandpiper having 3 toes (tridactyl foot).
[3]
- Lobate
: the anterior digits (2?4) are edged with lobes of skin. Lobes expand or contract when a bird swims. In
grebes
,
coots
,
phalaropes
,
finfoots
and some palmate-footed
ducks
on the hallux (1). Grebes have more webbing between the toes than coots and phalaropes.
[20]
[4]
[21]
The palmate foot is most common.
Thermal regulation
[
edit
]
Some birds like
gulls
,
herons
,
ducks
or
geese
can regulate their temperature through their feet.
[1]
[2]
The
arteries
and
veins
intertwine in the legs, so heat can be transferred from arteries back to veins before reaching the feet. Such a mechanism is called
countercurrent 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 via
evaporation
(
storks
,
New World vultures
).
[1]
See also
[
edit
]
References
[
edit
]
Wikimedia Commons has media related to
Bird feet
.
- ^
a
b
c
d
e
f
g
h
i
j
k
l
m
Gill, Frank B. (2001).
Ornithology
(2md ed.). New York: W.H. Freeman and Company.
ISBN
978-0-7167-2415-5
.
- ^
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
t
u
v
w
Kochan, Jack B. (1994).
Feet & Legs
. Birds. Mechanicsburg: Stackpole Books.
ISBN
978-0-8117-2515-6
.
- ^
a
b
c
d
e
f
g
h
i
j
k
l
m
n
Kochan (1994)
;
Proctor & Lynch (1993)
;
Elphick et al (2001)
- ^
a
b
c
d
e
f
g
h
i
j
k
l
Kowalska-Dyrcz, Alina (1990). "Entry: noga [leg]". In Busse, Przemysław (ed.).
Ptaki
[
Birds
]. Mały słownik zoologiczny [Small zoological dictionary] (in Polish). Vol. I (1st ed.). Warsaw: Wiedza Powszechna. pp. 383?385.
ISBN
978-83-214-0563-6
.
- ^
a
b
c
Proctor & Lynch (1993)
;
Kowalska-Dyrcz (1990)
;
Dobrowolski et al (1981)
- ^
a
b
c
d
Romer, Alfred Sherwood; Parsons, Thomas S. (1977).
The Vertebrate Body
. Philadelphia, PA: Holt-Saunders International. pp. 205?208.
ISBN
978-0-03-910284-5
.
- ^
a
b
c
d
e
f
g
h
i
j
k
l
m
n
Proctor, 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.
ISBN
978-0-300-07619-6
.
- ^
a
b
c
d
e
f
g
h
Dobrowolski, Kazimierz A.; Klimaszewski, S?dzimir M.; Szel?giewicz, Henryk (1981). "Chapters: Gromada: Ptaki - Aves: Układ kostny; Pas miednicowy i ko?czyna 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.
ISBN
978-83-02-00608-1
.
- ^
a
b
Kowalska-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.
ISBN
978-83-214-0563-6
.
- ^
a
b
c
d
e
f
g
h
i
j
k
Elphick, John B.; Dunning, Jack B. Jr.; Sibley, David Allen (2001).
National Audubon Society: The Sibley Guide to Bird Life & Behavior
. New York: Alfred A. Knopf.
ISBN
978-0-679-45123-5
.
- ^
Munn, Philip W. (1 January 1894). "On the Birds of the Calcutta District".
Ibis
.
36
(1): 39?77.
doi
:
10.1111/j.1474-919x.1894.tb01250.x
.
ISSN
1474-919X
.
- ^
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.
JSTOR
41559544
.
- ^
a
b
Wedel, Mathew J. (2003).
"Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs"
(PDF)
.
Paleobiology
.
29
(2): 243?255.
doi
:
10.1666/0094-8373(2003)029<0243:vpasat>2.0.co;2
.
- ^
Schorger, A. W. (September 1947).
"The deep diving of the loon and old-squaw and its mechanism"
(PDF)
.
The Wilson Bulletin
.
59
(3): 151?159.
- ^
Fastovsky, David E.; Weishampel, David B. (2005).
The Evolution and Extinction of the Dinosaurs
(2nd ed.). Cambridge, UK: Cambridge University Press.
ISBN
978-0-521-81172-9
.
- ^
Gier, H. T. (1952).
"The air sacs of the loon"
(PDF)
.
The Auk
.
69
(1): 40?49.
doi
:
10.2307/4081291
.
JSTOR
4081291
.
- ^
Bezuidenhout, A.J.; Groenewald, H.B.; Soley, J.T. (1999).
"An anatomical study of the respiratory air sacs in ostriches"
(PDF)
.
Onderstepoort Journal of Veterinary Research
.
66
(4): 317?325.
PMID
10689704
.
- ^
a
b
Earls, Kathleen D. (Feb 2000).
"Kinematics and mechanics of ground take-off in the starling Sturnis vulgaris and the quail
Coturnix coturnix
"
(PDF)
.
The Journal of Experimental Biology
.
203
(Pt 4): 725?739.
doi
:
10.1242/jeb.203.4.725
.
PMID
10648214
.
- ^
a
b
Whitfield, John (10 March 2000).
"Off to a flying jump-start : Nature News"
.
Nature
. Nature Publishing Group.
doi
:
10.1038/news000316-1
. Retrieved
17 January
2014
.
- ^
a
b
c
d
e
f
g
Gill (2001)
;
Kochan (1994)
;
Proctor & Lynch (1993)
;
Elphick et al (2001)
- ^
a
b
c
d
Kalbe, Lothar (1983). "Besondere Formen fur 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
.
- ^
Stettenheim, Peter R. (August 2000). "The Integumentary Morphology of Modern Birds?An Overview".
American Zoologist
.
40
(4): 461?477.
CiteSeerX
10.1.1.559.1172
.
doi
:
10.1668/0003-1569(2000)040[0461:timomb]2.0.co;2
.
ISSN
0003-1569
.
- ^
Kochan (1994)
;
Elphick et al (2001)