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Birds pay a price for the advantages of flight. They must commit their forelimbs almost entirely to that enterprise. As a result the bill often must assume responsibility for diverse functions for which many mammals use their forelimbs -- grasping, carrying, scratching, fighting, and digging. The bill (or "beak") consists of the upper and lower jaws (mandibles), ensheathed in a layer of toughened skin. The horny outer layer tends to be especially thick near the tip, where the most wear occurs. The edges of the bill may be sharpened for cutting, or serrated for grasping, but the edges of some bills, including those of ducks, are blunt and relatively soft except at the tip, which is hardened. Ducks often must sort insects and seeds from murky water, and the edges of their bills are richly supplied with touch receptors that help them to detect their food. In most birds the upper mandible is perforated by nostrils, although in some high-diving birds like gannets the external nostrils are missing; gannets avoid flooding by being "mouth breathers" and keeping their mouth shut when they hit the ocean. Similarly the nostrils of woodpeckers are protected from being flooded with "sawdust" by feathers or by being reduced to narrow slits. In the marine Procellariiformes (albatrosses and their relatives) the nostrils are a tube (storm-petrels) or pair of tubes (albatrosses, shearwaters, and fulmars) on top of the bill. In most birds the horny sheath exfoliates (peels) and is continuously replenished from underneath. Sometimes the sheath develops special protuberances that are used in courtship and subsequently shed. The large, eye-catching grooved bill of the breeding Atlantic Puffin returns to its smaller and duller appearance after the fancy scales peel away at the end of the reproductive season. As tools, bills are not used just for eating food, but also for catching it, prying up bark that conceals it, filtering it from water, killing it, carrying it, cutting it up, and so on. Bills also serve for preening, nest building, excavating, egg turning, defending, attacking, displaying, scratching, hatching, climbing, and so on. Small wonder that bill size and shape are characteristics that vary enormously from species to species and among major groups. And small wonder that the adaptations of bills to these various functions have long fascinated ornithologists. The most obvious adaptations of bills are those related to feeding. Birds that catch fishes with their bills must maintain a tenacious grip on slippery prey. Thus albatrosses and pelicans have hooked upper bill tips, and mergansers have serrated margins. Most waders hunt by probing in mud and sand, and have long, slender, forceps-like bills for finding and grasping their prey. Avocets, however, tend to feed more at the water's surface and swing their upward-curved bills from side to side. Oystercatchers have especially stout bills designed for hammering and prying open recalcitrant mollusks. Hummingbirds also probe, and their fine bills are well designed for finding the nectar in deep tubes formed by the fusion of flower petals (corolla tubes). In tropical species the bills may have closely coevolved with specific flowers. The straight 4-inch bill of the Swordbill -- the length of the bird's body and twice as long as the bill of any other hummer -- permits it to drink nectar from (and pollinate) a passion flower with a corolla tube 4.5 inches deep. The half-arc bill of the Sicklebill hummers fits exactly in the sharply curved corollas of Heliconia flowers (South American relatives of Strelitzia, the "bird-of-paradise" flower). Whip-poor-wills and their relatives have a wide-gaping bristle-fringed bill that acts as an aerial vacuum cleaner, sweeping in insects during flight. And tyrant flycatchers, such as kingbirds, pewees, phoebes, Myiarchus and Empidonax flycatchers, have ligaments connecting the upper and lower jaws that act as springs to snap the gaped jaw shut when an insect is snared. Used for hunting and excavating nest cavities in wood, the powerful bill of a woodpecker is shaped like a pickaxes and has an end like a chisel. The apparatus that supports the use of the bill is impressive: strong, grasping feet that work in concert with stiff tail feathers to form a triangular brace allowing the bird to position itself for its strenuous pecking against trunks or branches. Its very long, sensitive "tongue" (actually a complex extensible bone-muscle apparatus with a short tongue on the end of it) may wrap all the way around the bird's skull under the skin when it is retracted and is used to extract insects from holes and recesses. Birds such as warblers and creepers that glean foliage or bark for insects tend to have slender bills that may or may not be down-curved. Those subsisting on seeds, such as sparrows, buntings, and other finches, have short, stout bills adapted for cracking and husking seeds. The stout, crossed mandibles of crossbills have evolved for the job of extracting seeds from conifer cones. The bills of omnivores like crows have an intermediate shape between those of insectivores and those of seed-eaters.
Interestingly, the bills of passerines that move about actively searching for bugs on leaves in our deciduous forests are shorter and wider than those of tropical forest species that feed in the same way. It has been proposed that the difference is related to differences in the insects that make up the major food sources in the two habitats. Highly mobile Orthoptera (katydids, crickets) and Blattodea (roaches) are abundant in the tropics. They are thought to be best grabbed with the long, slender, fast-closing bills of the tropical birds -- bills that are also handy for deftly removing the spiny legs of such prey. In the temperate forest sluggish caterpillars abound. They require no dexterity to catch, but a stout forceps to hold them while they are beaten into immobility. Thus a great deal can be surmised about birds' feeding habits simply from examination of their bills. One should always keep in mind, however, that bills do serve other functions. Skimmers have one of the most interesting bills of all. Since, when foraging, they fly with their lower mandible slicing through the water, the mandible would be quickly eroded away by friction if it did not grow at roughly twice the rate of the upper mandible. Skimmers in zoos, deprived of the opportunity to skim, soon have lower mandibles much, much longer than the upper. SEE: Flamingo Feeding; Feet; Bird Communities and Competition; Coevolution. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye. |
