Best foot forward
HOW often do you look at birds’ feet? The answer is probably infrequently, unless looking for a special feature, such as colour or palmations, which might help with identification.
Birds’ feet are adapted to the habitats in which they live and feed. Wildfowl, seabirds and some other waterbirds have webs or something similar to help them swim; raptors have talons for grasping prey; and woodpeckers have widely spread toes to help them grip and climb a trunk.
Almost all passerines have four toes, three of which point forward, with the hind toe (hallux) pointing backwards, and all toes join onto the foot level with the ground.
The only exception is Three-toed
Parrotbill, a Chinese endemic, whose almost vestigial outer toe is fused to the middle one.
This arrangement of toes is called anisodactyl and of the four toes, the middle one is usually the longest, although hind claws can be very long on some ground-dwelling birds, such as larks and pipits. Aerial birds, like swallows and martins, don’t have very strong feet and their toes are relatively smaller than other passerines.
A bird’s feet are more than just the toes, though, as they include everything below the ankle, which is the joint we see part way up the leg of the bird, meaning that what we would call the sole includes the back of the leg. The flexor tendons attach to the leg muscles and run down the back of the tarsus, extending along the underside of each toe.
When a bird lands on a perch, the tendons automatically tighten, curling the toes tightly around the perch. The bird’s weight keeps its toes locked in position so it can sleep while perched. This fixed position only changes when the bird straightens its leg and opens the toes when flying or hopping down from its perch.
The skin covering a passerine’s foot is usually bare, although some martins have feathered feet. The surface of the skin is made up of thickened epidermis, in the form of flat, rounded scales. On the front and top of the foot these are regularly arranged (scutellate), while on the back and underside (the sole) they are less regular and thicker (reticulate), providing support.
The surface of the toes which come into contact with the ground or other surface are covered with pads and folds of skin. Each toe has three pads corresponding to the three parts of bones in each digit, making 12 in all, with a 13th pad in the centre of the foot. The pads are separated by folds or furrows and the organisation of these may differ depending on the lifestyle of the species, so ground-dwelling larks, trunk-climbing treecreepers and branch-gripping warblers will all be different.
Some birds use their feet for more than just perching. Woodcreepers have long, strong front claws and a long hind toe to help with climbing up vertical trunks and are different from other passerines in that their leg tendons are extremely ossified and inflexible, which helps them maintain their vertical position. Nuthatches and treecreepers also have long toes with strong claws. Many passerines preen their head and neck feathers using their feet, some will hold food, such as seeds and nuts, with their feet and some build nests with the help of their feet to hold twigs and branches.
When you start to look closely at birds’ feet there are some interesting differences even between closely related species. The feet of Goldcrests and Firecrests differ because of the way they feed. Goldcrests cling onto small twigs and even pine needles when feeding and so the soles of their feet have deep furrows to help them to hold on. Firecrests, on the other hand, spend more time feeding on the tops of branches and so have much smoother soles. Chris Harbard