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Webfeet and waders.

Marsh birds tend to be a bit different. The dimensions and structure of their bills and legs set them apart from other birds. Although essentially water birds, they perhaps should be termed "wetland birds," since they are really birds of flooded dry land. As well as flying, they also have to swim or walk on submerged substrates and feed on them. This means their beaks show many different forms and adaptations, which depend on the characteristics of their main food sources and habitats. Their legs are as long as the water they feed in is deep, although this may be a response to needs for propulsion when swimming. Marsh birds display a fascinating range of morphological adaptations to the exploitation of this unique habitat.

First, let's consider the marsh birds that fish. All possess beaks well-suited to the task of hunting. Perhaps the weirdest of all belong to the pelicans (Pelecanus). They have an enormous beak, complete with a lower mandible that forms a distensible pouch. They locate their fish prey from the air and plunge down on them with unerring accuracy. Less bizarre predators include cormorants (Phalacrocorax, Nannopterum), grebes (Podiceps, Aechmophorus, Podilymbus, Tachybaptus), and sawbills (Mergus), named for the serrated bills they use to clasp slippery fish. Lastly, some birds, the ardeids, prefer to harpoon their prey with their long pointed bills; they include the herons (Ardea) and especially the anhinga (Anhinga), tropical and subtropical birds that are found in freshwater and resemble cormorants. However, the anhingas have long, thin necks and swim with their whole bodies in the water. Other marsh birds, such as spoonbills (Platalea), sweep lagoon bottoms with their spoon-shaped bills for slow-swimming fish, their main prey. The more important marsh passerines include the kingfishers (Alcedo, Halcyon, Ceryle), which observe the water from a perch on an overhanging branch until they spot a fish, and then they plunge down and catch it. Some species are capable of hovering over the water if there is no handy perch. Finally, birds such as the green heron (Butorides striatus) have learned to fly fish. It perches on a branch and throws insects onto the water until a fish rises to the bait.

Some marsh birds feed on zooplankton. The best known examples, the flamingoes (Phoenicopterus, Phoenicoparrus), capture small crustaceans through filtration by swinging their perfectly adapted bill back and forth in a zig zag; the beak is at the end of a long mobile neck and can thus reach almost anywhere. The five different species of avocet (Recurvirostra) are found everywhere except arctic areas. They filter mud and silt by moving their beaks from side to side in the water as they walk. Their bills are curved upwards so they do not get stuck in the mud, making them look very strange when they lift their heads. Their appearance is thus unusual, but not without purpose. These birds usually have long legs to keep their bodies out of the water while they filter. Some filter-feeders also dive: shovelers (Anas clypeata) are specialists adapted to shallow water rich in zooplankton.

Almost all the birds that live in open water can swim and have characteristically short legs with webbed feet. This is true for pelicans (Pelecanus), cormorants (Phalacrocorax), grebes (Podiceps), ducks, geese, swans, (Anser, Branta, Anas, Tadorna, Netta, Aythya, Bucephala, Oxyura), and coots (Fulica). Four of the pelican's and cormorant's toes are joined by a membrane, while ducks only have a membrane between the front three toes. Coots and grebes possess small lobes around each toe, but they are not fused together.

There are also a few marsh birds that are specialized walkers. One group can walk on floating leaves. The best adapted are the jacanas (Jacana) of subtropical areas in South America, Africa, and Asia, which possess exceptionally long toes to stabilize them and help spread out their weight; in temperate areas, moorhens (Gallinula chloropus), gallinules (Porphyrula), and crakes (Laterallus) are also capable of exploiting similar environments. A second group consists of the birds that normally walk on muddy or silty surfaces. They tend to have long toes to provide necessary stability. Those that frequent deeper water have longer legs, while those that remain on the muddy or silty margins often have quite short ones. Flamingoes have probably the most exaggeratedly long legs of all, followed by storks (Ciconia), herons (Ardea), and stilts (Himantopus). Some egrets and herons specialize in fishing from the branches of trees (Egretta, Nycticorax) and consequently have shorter legs. Another specialist with shorter legs, the cattle egret (Bubulcus ibis), has tended to abandon wetlands in favor of drier pastures and grasslands, where they follow herds of wild or domesticated animals.

One more group should be mentioned: the many species of passerines that breed in reedbeds and similar formations. Their feet exhibit the typical structure of non-terrestrial passerines, with three toes pointing forward and one backwards, enabling them to grasp branches and twigs, even vertical ones.
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Publication:Encyclopedia of the Biosphere
Geographic Code:1USA
Date:Oct 1, 2000
Previous Article:3 Life on tideless coastlines.
Next Article:4 Humans and the sea.

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