World-wide webs
BIRDS which live mainly in an aquatic environment have developed many adaptations to enable them to survive. Few are more striking than those aids to locomotion when in the water, namely webs and lobes. Looking at the structure of a duck’s foot, the appearance of webbing seems to be a logical way to make swimming more effective. Interestingly, the evolution of webbed feet happened independently in several unrelated families of birds.
During early embryological development a bird’s toes are joined by webs of skin, but in most species this web dissolves during its development in the egg. Mutations may have resulted in webs being retained; if this happened in an aquatic bird it would help its survival and so lead to webs evolving as a dominant feature over time. This process occurred in many groups of birds, resulting in the development of various different webs and lobes.
Recent research suggests webs or lobes evolved independently on at least 14 different occasions. The ancestor of ducks, geese and swans had no webs, and that of Magpie Goose split at an early stage with semipalmations, leaving the rest of the group to become fully palmate (between three toes) – apart from Nene, which is semipalmate. Sungrebes and coots developed their lobed feet separately from grebes and phalaropes. Flamingos and grebes probably started with palmate feet.
The ancestor of the Charadriiformes (waders, gulls and so on) had unwebbed feet, but gulls, terns and skuas branched off and all developed palmate feet. A separate branch of waders developed semipalmated feet in many species, plus the Plains Wanderer and some coursers.
The phalaropes branched off to develop lobate feet.
The ancestor of most oceanic seabirds had palmate feet and these were retained in divers, penguins and tubenoses. The pelicans, cormorants, gannets and boobies, together with herons and ibises, all lost their webbing; the latter group stayed unwebbed, while the others developed webbing between three toes, known as totipalmate feet.
Webbed and lobed feet have other uses than for simply swimming. Dabbling ducks use their webbed feet to ‘tread water’ while they upend and reach down into the water to find food. Diving ducks propel themselves down beneath the water in search of food by closing their wings and using their webbed feet. Even the hind toe of diving ducks is lobed to assist with this. Many waterbirds will use their webbed feet as ‘flaps’ to help steer and slow down when landing by providing resistance to the air and also like water skis when landing on water.
Great Crested Grebes have a courtship display which involves paddling furiously until their bodies are almost right out of the water, vertically, while holding a beakful of weed. Courting Western and Clark’s Grebes of North America indulge in similar high-speed chases across the surface of a lake. Their lobed feet provide most of the force to enable them to do this.
Blue-footed Boobies are renowned for their delightful displays which involve waving their brightly coloured totipalmate feet. But gannets, boobies and pelicans also use their feet for incubation, rather than having brood patches, as their large webs are highly vascularised. The feet are placed on top of the eggs and the circulating blood keeps them warm. When hatching begins they put their feet under the eggs. Tropicbirds, which also have totipalmate feet and no brood patch, place their feet underneath the egg and their black colour radiates maximum heat. Emperor and King Penguins use their warm feet to keep their eggs from touching the ground or ice and so getting cold.
There is one group of non-waterbirds which have semipalmate webbing: grouse. Many grouse species grow a comb-like fringe along the edges of their toes, and webbing at the base of them produces a ‘snowshoe’ effect when walking on soft snow. These fringes are shed each summer and then regrown before winter. Chris Harbard