Bill

Bill, in Natural History, the horny, toothless, and lipless jaws of birds. (a) Structure.—The foremost bones of the skull (mainly, the premaxillæ, but also the maxillæ and nasals) are elongated in variable degree and form, and are covered with a horny sheath. The same is true of the lower jaw or mandible, and the result is a bill. The upper and lower portions are generally equal, but the upper is longer in birds of prey. The edges, usually more or less sharp, sometimes (e.g. in goosander) exhibit saw-like notches not unlike teeth. In the majority of cases the bill is not sensitive, the distribution of nerves being restricted to a wax-like patch (the cere) at the base of the upper part of the beak. In aquatic birds, such as ducks and snipes, which feed for their food in the mud, this sensitive skin is carried forward almost to the point; and the bill of the woodpecker is also very richly supplied with nerves. The nostrils are usually far back near the cere, but often lie farther forward, most markedly perhaps in puffins. (b) History.—No living birds have teeth. With the evolution of the class these structures have been lost. The fossil birds of America (Odontornithes) exhibit teeth, within sockets in Ichthyornis, simply in grooves in Hesperornis. But even in these forms the characteristic bones of the upper part of the bill—the premaxillæ—were toothless, and probably insheathed in a horny beak. The primitive bird Archæopteryx was also provided with numerous uniform teeth. From the beginning of the Tertiary Period onwards no toothed birds are found. Traces of teeth, however, still persist. Geoffroy St Hilaire (died 1844) observed small tooth-papillæ in the embryos of certain parrots, and this observation has been repeatedly confirmed. The presence of the characteristic tooth-substance dentine has even been asserted. The tortoise tribe have also lost their teeth, and here again the presence of rudimentary teeth in embryos (Trionyx) proves that the toothless state is secondary. That birds get on well without teeth is evident enough, but it is difficult to give a physiological explanation of their absence. The very spongy character of bird bone, obviously inconsistent with teeth, the sharp antithesis between the skin structures of birds and those of reptiles, the disuse resulting from the developed parental care which supplies the young with food, the general nature of the nutrition, and the freedom from fighting which birds must at first have enjoyed, ought probably to be taken into account, but some deeper physiological reason is still required. This is peculiarly difficult when we recall the very varied nature and habit of toothless vertebrate animals—e.g. pipe-fish, toads, turtles and tortoises, birds, duckmole, echidna, ant-eaters, and some whales. (c) Relation to Function.—The bill is chiefly used in seizing and dividing the food, but also for fighting, preening, nest-building, &c. It varies most with the nature of the food. It is hardest in birds which live on fruits, seeds, and flesh; sharpest in the birds of prey; more delicate in insectivorous forms; and softest in those which pick their food out of the mud. It is shortest in the graminivorous, and longest in the marsh-birds. In birds of prey the beak is hooked, and often notched; in fishing-birds it is usually very large; in those which catch flying insects the gape is particularly wide. Water birds often exhibit cross plates at the margins, between which the water taken in with the food is allowed to escape. Wallace has noted an interesting correspondence between the length of a humming-bird's bill and the depth of the flower-tube which it visits. Finally, it is worth noting that beaks sometimes exhibit (e.g. in fowls and sparrows) pathological variations, which closely resemble what in other birds are the normal forms. See BIRDS.