The hindlimbs of modern birds are more than just the sum of their parts. They have evolved for effective locomotion and many other vital behaviors like foraging, feeding, and even nesting. Birds are diverse creatures, and so are their hindlimbs.
While each is made up of the same basic building blocks, the final product varies hugely, shaped by time and the Earth's diverse habitats.
In this guide, we’ll unpack the anatomy of the avian hindlimb. Join us as we uncover the inner workings of birds’ legs and feet.
Birds have evolved unique, fused leg bones with superior strength to endure the rigors of take-off, landing, and many different modes of locomotion.
Like dogs, horses, and many other mammals, birds are digitigrade, which means only their toes make contact with the ground while walking.
Whether swimming, walking, hopping, climbing, or sprinting, birds require powerful muscles to move their legs. The major avian leg muscles are located high up on the leg, along the femur and tibiotarsus, with long tendons controlling the movements of the foot.
These muscles are particularly well-developed in birds that favor swimming and terrestrial locomotion and make up nearly a third of the Ostrich’s body mass!
In contrast, the leg muscles of Hummingbirds make up less than 2% of their body mass. However, muscles aren’t only for locomotion. Birds of prey rely on powerful muscles in their lower legs for catching, killing, and carrying their prey.
A Bald Eagle. Whether swimming, walking, hopping, climbing, or sprinting, birds require powerful muscles to move their legs
Bird claws and talons are keratin-based structures that cover the outer phalanx (toe bone) of the foot. They are composed of specialized skin that grows constantly throughout the bird’s life. Claws are important for grip in many birds, but they perform a more specific function in Owls and raptors.
Birds of prey have exceptionally large, curved talons that are used to grasp their prey. These needle-sharp weapons puncture the skin, muscles, and organs of the prey animal, immobilizing it and causing massive injury.
All birds of prey have relatively large talons, although their shape and size do vary depending on the species’ hunting strategy and prey size.
Vultures, for example, have relatively blunt, small, and weakly curved talons, while raptors like the Harpy Eagle that hunt large prey have much larger, sharper, and strongly hooked talons.
There are variations even among species that target large prey. Accipiters (e.g., Coopers Hawk) and Falcons have longer toes to increase the spread of their talons, which is helpful for catching fast-moving birds. Meanwhile, large Owls have straighter talons since they have more maneuverable toes and greater grip strength than Hawks and Eagles.
Raptors like the Harpy Eagle that hunt large prey have much larger, sharper, and strongly hooked talons
Birds rely on their lower leg bones for terrestrial and aquatic locomotion, foraging, preening, and many other vital functions. You may be surprised to learn that their lower legs are also essential for flight in many species.
While getting airborne is much easier when starting from a perch or other elevated position, most species can take off from the ground, and this relies heavily on leg strength.
Avians have evolved strengthened and fused shin bones called tibiotarsi and fused ankle bones called tarsometatarsi. Birds generate tremendous power and speed when extending the joints of these long lower-limb bones to leap into the air before flapping their wings.
They use their wings and tails to slow their trajectory before landing, but they also extend their legs before impact and flex their joints to absorb the shock.
The length and strength of the tarsometatarsus and the tibiotarsus vary greatly between bird species and their habitats. Stilts, for example, have incredibly long and delicate lower legs for wading through shallow wetlands. In contrast, Ruddy Ducks have very short, robust legs for improved swimming performance in open water.
Long, delicate legs would be really cumbersome for perching forest birds, which tend to have short to medium-length lower limbs for hopping through the canopy. Then there are birds that barely use their legs at all. Hummingbirds, Swifts, and Swallows have very short legs because they rarely walk.
Ruddy Ducks have very short, robust legs for improved swimming performance in open water
The size, shape, and arrangement of birds’ toes vary depending on their habitat and foraging strategies. Most birds (nearly 90%) have anisodactyl feet complete with four toes—just one digit, the hallux, projects backward.
Anisodactyl feet are well adapted for hopping, gripping, and walking. Syndactyl feet (e.g., Hornbills) are similar, but the inner two front toes are mostly fused. Birds with zygodactyl feet (e.g., Woodpeckers) have two forward-facing toes and two backward-facing toes for superior grip strength.
In some birds, the hind toe is greatly reduced, and others have lost this digit completely. These birds have tridactyl feet. This arrangement is seen on ground birds that do not require a backward-facing digit for grasping a perch or catching prey. Ostriches are unusual in that they have just two toes (didactyl).
Toes aren’t just for locomotion–avians also use their digits for gripping perches and prey. Birds use strong digital flexor muscles and long tendons to grip, but they also have some neat tricks up their sleeves to improve their grasp.
Although birds can use just their balance to perch, they generally rely on their long toes and powerful leg muscles to grasp their perch. Perching birds and raptors also have a ratchet-like locking mechanism of tubercles and plicae that holds their grip firmly when carrying prey or perching.
Expert climbers like Treecreepers and Woodpeckers have several adaptations to suit their arboreal lifestyle, many of which are seen in their strong legs and feet. These birds hop up and down on the tree bark using relatively short legs to keep their body close to the trunk and long toes and claws for a wide grip.
Birds are incredibly diverse animals, and many species show unique adaptations for specialized lifestyles and feeding behaviors. Let’s take a look at a few interesting examples from around the world:
Jacanas have incredibly long and slender toes. These birds walk on water lilies and other floating vegetation, relying on their elongated digits to spread their weight and keep them high and dry.
Parrots and other Psittacines are among the most dextrous birds. These intelligent avians use reversible toes to hold their food in their feet. They can stand on one leg while lifting their meal up to their bill.
Snowy and Little Egrets have very brightly colored toes, which are thought to disturb small fish and other prey into striking range as the birds wade through shallow water.
An Eurasian Treecreeper. Expert climbers like Treecreepers and Woodpeckers have several adaptations to suit their arboreal lifestyle, many of which are seen in their strong legs and feet
An African Jacana. These birds walk on water lilies and other floating vegetation, relying on their elongated digits to spread their weight and keep them high and dry
When it comes to birds, a once-size-fits-all approach is not the way. Instead, avians have evolved hindlimbs that serve specific purposes such as grasping, running, and swimming or selected for feet that combine various properties beneficial to their lifestyle.
For example, aerial feeders like Alpine Swifts have evolved small, lightweight legs and feet that can be tucked into their plumage for streamlined flight. On the opposite end of the spectrum, flightless land birds like Emus have forfeited their wings in favor of massive, robust legs for running at high speeds.
The same pressures act on birds with similar foraging strategies and habitat preferences, so birds’ feet and legs may evolve convergently. For example, many completely unrelated groups of water birds share similar webbed feet.
The fossil record provides fascinating clues into the dawn of modern bird anatomy. One of the major changes seen between modern birds and theropod dinosaurs is the angle of the femur.
Theropod dinosaurs, like Tyrannosaurus, had comparable leg structure to modern birds, although their thighs were more vertically orientated, sort of like our own. In contrast, the femur of modern birds is held at a horizontal angle and contributes relatively little to the motion of the hind limbs.
Fossil evidence from an early bird called Confuciusornis provides interesting evidence of the gradual change in femur angle relative to the spine. Their leg posture fell somewhere in between that of modern birds and their non-avian dinosaur ancestors.
An Australian Emu. Flightless land birds like Emus have forfeited their wings in favor of massive, robust legs for running at high speeds
Despite the great diversity in the size and shape of bird legs, each species has evolved its own unique hindlimbs based on the same general blueprint of relatively immobile thighs, fused shin and ankle bones, and a digitigrade gait.
Differences in toe arrangements, leg lengths, and musculature improve important functions like traction, swimming ability, wading height, stride length, and grip strength.
Birds worldwide are under increasing pressure from habitat loss, pollution, climate change, and other forms of environmental degradation. By raising awareness around their plight and supporting science-based conservation, we can protect the Earth’s birdlife now and into the future.
Brighten up your inbox with our exclusive newsletter, enjoyed by thousands of people from around the world.