Many birds stand on one leg when resting on the ground (Clark 1973, Stiefel 1979;
Necker 2010; see Liste). This behavior can best be observed in long-
Functional Anatomy
Keeping balance in a biped vertebrate
Bipedal locomotion is found in humans and in birds. In humans the body is oriented vertically, i.e. in line with the line of gravity, and the center of gravity is located near the insertion of the legs. In birds the body is oriented more horizontally and the center of gravity is located rostral to the insertion of the hindlimbs (Fig. 1). This imposes special demands on keeping balance.
Birds standing on one leg: mechanisms and meaning
The legs of birds are different from bipedal humans in that they walk on their toes and there is an ankle joint which looks like the human knee but with a reversed angle. There is an upper leg with a femur and a lower leg with a tibia and a fibula. Upper leg and lower leg are connected by a knee joint (Fig. 2). The distal end of the tibia includes parts of tarsal bones. Therefore the lower leg is called tibiotarsus. The remaining tarsal elements fuse with metatarsal bones to make up the tarsometatarsus or tarsus (Baumel & Witmer 1993). The tarsus looks like a lower leg and this impression is furthered by the fact that upper and lower legs are normally hidden in the plumage. Tibiotarsus and tarsometatarsus are connected by the intertarsal joint (Fig. 2).
To keep balance when standing on both legs, the knees are flexed, which puts the knee joint near the center of gravity (Fig. 1). The antitrochanter (Fig. 2), a structure unique to birds, forms a joint with the neck of the femur which serves to prevent abduction of the femur and to absorb stresses which would otherwise act on the head of the femur (Hertel and Campbell 2007). The feet are positioned under the center of gravity which results in a stable balance of the body.
When resting on both legs, the femur is oriented nearly horizontally. A further unintended upward movement of the femur is prevented mechanically by the antitrochanter and the ligaments of the hip joint. The hip joint is now in a fixed position. Because of the high position of the knee, the center of gravity may shift to below the knee. This means that the body is suspended at the knee joint, which results in a very stable position that does not need much muscle activity to keep balance.
Standing on one leg
When resting the supporting foot is set below the center of gravity (Figs. 3, 4). This position stresses the lateral ligaments of both knee and intertarsal joints (see arrows) but these ligaments are well developed in birds. In a recent study in flamingo cadavers (Chang & Ting 2017) it was found that standing on one leg as shown in Fig. 3 (adduction of about 20 degrees from the vertical) results in a very stable support of the body. A vertically aligned leg as is the case during bipedal standing is followed by an instable body. This means that the unipedal stance of birds should not need much muscular energy expenditure.
Long-
In long-
There are other specializations in the intertarsal joint of long-
According to Stolpe (1932) there are specializations in the hip joint of long-
Conclusions
Most birds stand or sleep on one leg without having specializations in their legs.
The leg is positioned in such a way that the body is well balanced without much additional
muscle activity. Most long-
A useful function of standing on one foot with hiding the non-
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Anderson MJ, Williams SA (2009) Why do flamingos stand on one leg? Zoo Biology 28.:1-
Anderson MJ, Laughlin CP (2014) Investigating laterality, social behavior, and temperature
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Bell C (1847) Die Hand und ihre Eigenschaften (translated from the English by F Kottenkamp; original: The hand, its mechanism and vital endowment as evincing design). Expedition der Wochenbände, Stuttgart
Berndt R, Meise W (1959) Naturgeschichte der Vögel. Band 1. Allgemeine Vogelkunde. Kosmos, Stuttgart.
Bouchard LC, Anderson MJ (2011) Caribbean Flamingo resting behavior and the influence
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Chang Y-
Clark GA 1973 Unipedal postures in birds. Bird Banding 44: 22-
Dawson WR, Whittow GC (2000) Regulation of body temperature. In: Whittow GC (Hrsg)
Sturkies´s Avian Physiology: 343-
Flamingo file (1991): New Scientist Nr. 1782 vom 17. August 1991, Letters: Flamingo file. URL: http://www.newscientist.com/search?doSearch=true&query=Flamingo+file.
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Hertel F, Campbell KE Jr (2007) The antitrochanter of birds: form and function in
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Kahl MP Jr (1963) Thermoregulation of the wood stork, with special reference to the
role of the legs. Physiol Zool 36: 141-
Herzog K (1968) Anatomie und Flugbiologie der Vögel. Fischer, Stuttgart.
Langer K (1859) Über die Fussgelenke der Vögel. Zweiter Bericht. Zur vergleichenden
Anatomie und Mechanik der Gelenke. Denkschriften der kaiserlichen Akademie der Wissenschaften,
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McFarland JC, Meyers RA (2008) Anatomy and histochemistry of hindlimb flight posture
in birds. I. The extended hindlimb posture of shorebirds. J Morphol 269:967-
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of birds: evidence of a function as a sense organ which is involved in the control
of walking. J Comp Physiol A: 439-
Necker R (2010) Stehen der Vögel auf einem Bein: Mechanismen und mögliche Funktionen
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Randler C (2007) Foot preferences during resting in wildfowl and waders. Laterality
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Schaller NU, Herkner B, Villa R, Aerts P (2009) The intertarsal joint of the ostrich
(Struthio camalus): Anatomical examination and function of passive structures in
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Stanley E (1835) A familiar history of birds. Their nature, habits, and instincts. Parker, London.
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Stiefel A (1979) Ruhe und Schlaf bei Vögeln. Neue Brehm-
Stolpe M (1932) Physiologisch-
Stresemann E (1934) Sauropsida: Aves. W Kükenthal, Th Krumbach (eds) Handbuch der Zoologie. Band 7, 2. Hälfte. Gruyter, Berlin.
Fig. 4: Gray goose standing on one leg
Bell (1847) described in detail a snapping motion in the ostrich intertarsal joint (for explanation see Fig. 6). Bell (1847) claims that the mechanism described by him explains the peculiar "springy" gait of ostriches. However, the ostrich does not rest on one leg and Bell (1847) does not mention that the snapping mechanism might be useful when standing on one leg.
Fig. 6: Snapping mechanism in the intertarsal joint of the ostrich. On the left: extended leg; on the right: flexed leg. In the extended state the lateral ligament B rests relaxed in a groove caudal to an elevation A of the condylus of the tibiotarsus. During flexion the ligament is stretched when gliding over the elevation and relaxes again in the rostral position. An additional effect is a stretching of muscle C which improves the force exerted on the leg when pushing the body forward during walking. After Bell (1847)
Langer (1859) cites Bell (1847) but describes a somewhat different snapping mechanism in the ostrich. Due to the geometry of the tibiotarsal condylus, coming from an extended state, flexion has to overcome a point of resistance, which involves a stretching of the lateral ligament (Fig. 7). The point of resistance is at an angle of about 125° and amounts to about 3°. Langer (1859) did observe the elevation in the ostrich described by Bell (1847) but argues that it cannot be the only source of a snapping mechanism since he observed such a mechanism and hence a springy gait in bustards, storks and flamingos all of which do not have such an elevation. Cutting the lateral ligaments eliminated the snapping mechanism.
Fig. 7: Scheme to show the elongation of the lateral ligament during flexion as described by Langer (1859). Filled circles represent points of fixation of the ligament. These points are fixed in the scheme while the length of the ligament is adapted to the state of flexion (constructed by copying the tarsus to different positions). Inset on the lower right compares the lengths of the ligament according to the three states of flexion (copied from the corresponding schemes). Scheme of the intertarsal joint redrawn from Fig. 6 (Corel Draw 11®).
Stresemann (1934) refers in detail to Langer (1859) and compares the snapping mechanism
to a pocket knife. Furthermore, Stresemann (1934) mentions that the snapping mechanism
helps long-
In a recent study of the intertarsal joint of the ostrich (Schaller et al. 2009)
a snapping mechanism which involves ligaments and bony structures of the tibiotarsus
has been described with fresh material. Coming from a fully extended state of the
joint (168°) there is an increase in resistance (measured as joint moment in Nm)
up to 140° and a transition point to relaxation at 115°. It seems that this mechanism
(named “engage-
The most detailed study on the functional organization of hindlimbs of birds, which also deals with the problem of standing on one leg is by Stolpe (1932). Fig. 8 shows a parasagittal section through the lateral condylus of the intertarsal joint of the flamingo (Phoenicopterus ruber). There is a process in the tarsometatarsus which fits into a groove of the condylus of the tibiotarsus in the fully extended joint (see arrows in Fig. 8). Although this looks like a locking mechanism, Stolpe (1932) claims that it only prevents the joint from overstretching. Similar specializations are found in cranes and storks but not in herons (Stolpe 1932).
Stolpe (1932) mentions that Langer (1859) described a snapping mechanism in long-
The possible function of standing on one leg
There are many speculations but few experimental studies or quantitative behavioral observations why birds stand on one leg. A couple of possible functions are published in Flamingo file (1991). Two possible functions which have been tested by quantitative observations in recent publications (Anderson & Williams 2009; Bouchard & Anderson 2011; Anderson & Laughlin 2014) will be dealt with in detail: a thermoregulatory function and relaxation of muscle fatigue in the retracted leg.
Thermoregulatory function. Sleeping positions and the occurrence of sleeping on one
leg among many orders of birds are described in detail by Stiefel (1979). When sleeping
while standing on one leg the head is often hidden in the plumage on the back (Fig.
11). The head is usually positioned on the side of the ground foot or above the center
of gravity. This supports a stable balance. The retracted leg is hidden in the breast
feathers or under the wing. Hiding non-
Reduction of muscle fatigue of the retracted leg. Humans tend to put the body weight
on one leg when standing for longer times. This serves to reduce muscle fatigue in
one leg. Accordingly, Clark (1973) suggested that standing on one leg in birds may
serve a similar function. However, retraction of one leg needs muscle activity. This
is confirmed by an own observation of a resting nile goose (Alopochen aegyptiacus)
whose retracted leg dropped again and again. However, when sleeping with its head
on the back and with closed eyes, the leg was continuously hidden in the plumage.
That leg retraction needs energy is supported by the casual own observation of a
woolly-
Humans of some tribes which use to rest on one leg (e.g. Australian Aborigines or African Bushmen) the retracted leg rests on the supporting leg and equilibrium may be stabilized by leaning on a stick. A casual own obervation shows that storks may use the same technique (Fig. 14).
The question of reduction of muscle fatigue was addressed by Anderson & Williams (2009) in flamingos by measuring the latency of initiating a forward movement. This latency was longer following resting on one leg as compared to the latency when resting on two legs. The authors conclude that this result discounts the possible function of reducing muscle fatigue or enhancing predatory escape.
In most bird species there is no preference of the side (left/right) they use for standing on one leg (Randler 2007; Anderson & Williams 2009; Anderson & Laughlin 2014) and there is an about equal use of left and right leg in individual flamingos (Anderson & Williams 2009). However, it would be interesting to know whether there is a regular shift from one leg to the other leg in order to reduce muscle fatigue in both legs. However, it is not clear whether a reduction of muscle fatigue of the supporting leg or the retracted leg is intended. Given that retracting a leg is energy consuming (see above) one might assume that changing the retracted leg may serve reduction of muscle fatigue of the retracted leg.
Fig 12B: Mute swan on the water with left leg stretched backwards
Fig. 14: Woolly-
The legs of birds are an important site of heat exchange (Steen & Steen 1965; Dawson
& Whittow 2000). In a warm environment heat of the body is dissipated via the legs.
In cold ambient temperature blood supply to the legs is reduced and there is a counter-
In recent investigations it was shown that flamingos spend more time standing on one leg in the water (facilitates heat loss) as compared to standing on one leg on the ground. Furthermore, flamingos stand more often on one leg at cool temperatures.(Anderson & Williams 2009; Bouchard & Anderson 2011; Anderson & Laughlin 2014). These quantitative behavioral observations support a thermoregulatory function of standing on one leg. There are, however, observations which show that in the range of 8 to 19 °C, i.e. at low temperatures, the incidence of standing on one leg is reduced (Harker and Harker 2010). This is supported by a casual own observation of flamingos at 2 °C. The authors argue, that the birds tend to rest/sleep only at higher temperatures. For a detailed discussion of the temperature/rest/sleep topic on standing on one leg see Anderson 2016.
Standing or resting on one leg does not necessarily mean an inactive state. Birds may practice intense preening while standing on one leg (Fig. 12A). Even aggressive behavior to neighbors was seen in flamingos while standing on one leg (own observations). Often resting birds just lift one leg partially without hiding it in the plumage (see Fotos). In preliminary own observations at the zoo of Dortmund it is shown that lifting one leg partially occurs more often at higher temperatures (27 C) than at lower temperatures (20 C). On the other hand complete retraction of one leg was observed more often at the lower temperatures (Fig. 13). This supports the assumption of a thermoregulatory function of standing on one leg: an only partially retracted leg means less heat conservation.
Fig. 13A: Sleeping flamingos: Occurence of standing on both legs, with one leg retracted partially and one leg retracted completely. Blue column: 20 °C (observed in 2008); red column. 27 °C (observed in 2013).
Fig. 13B: Percentage of standing on one leg with the leg retracted only partiallly. Left column: two observations (20 and 23 °C); right column: two observations (27 and 31 °C)