Page images
PDF
EPUB

1175 ; and states that the Cirriped (Lepas anatifera), also so-called, took its name from the Bird, a kind of Goose, and not the Bird from the Cirriped.

BILCOCK, said to be a local name for the Water-RAIL.

BILL or BEAK, in Latin Rostrum. This consists of an upper, chiefly premaxillary and maxillary, and of a lower, or mandibular, half. The horny covering is to a certain extent moulded after the shape of the supporting bones. The soft cutaneous portion of the skin is frequently restricted to a thin layer between the periosteum and the Malpighian layer of the epiderm; in it run numerous bloodvessels and nerves, the latter occasionally penetrating the horny layer, and ending in tactile or sensory corpuscles.

On the other hand, in very stout beaks, the cutaneous layer forms conical elongations which project into the thick horny parts, especially into the ends of the upper and lower bill. In the broad edge of the mandible of Parrots such projections are particularly numerous and long; when they calcify, as cutaneous structures are liable to do, they bear in horizontal sections a superficial resemblance to the germs of teeth, and have been mistaken as such by various anatomists (see TEETH).

The horny sheath, or rhamphotheca, is produced by the outer layers of the Malpighian cells, and resembles in structure other horny parts, as Claws, nails, and spurs. Sometimes, as in the Anseres, the greater portion of the outer sheath of the bill is soft, and only the tip of the bill is transformed into a thick horny “neb," which contains numerous tactile organs. In some birds, especially in the diurnal Birds-of-Prey and in the Parrots, the greater portion of the distal end of the upper beak is hard, while the basal portion is thick and soft—the so-called cere. It is generally very sensitive, and encloses the nostrils. Though mostly bare, it is in some Parrots thickly covered with feathers, and then approaches in structure the ordinary skin. The neighbourhood of the nostrils is often soft, and produces an operculum by which, in some cases, the external nares can apparently be closed, although no muscles seem to exist there. Such a soft and swollen operculum is a prominent feature in Pigeons, and is very large and curled in Rhinochetus (KAGU). In the Petrels each operculum forms a more or less complete tube, which may or may not fuse with its counterpart in the middle line, and thus produce an apparently single tube with a longitudinal vertical septum, whence the name “ Tubinares.”

A leathery operculum or valve also occurs in Plovers, in Podargus, many Passeres (especially shewn in Meliphagida), and in the Humming-birds, in the last being covered with feathers. In Caprimulgus each nostril is produced into a short, narrow, and quite soft tube.

Another differentiating feature in connexion with the nostrils

BILL

and the rostrum is the presence or absence of a complete vertical internasal septum. If the septum is complete, which seems to be the primary condition, the right and left nasal cavities are completely separated from each other, and birds having this structure are said to possess nares imperviæ. The septum either remains cartilaginous, or it ossifies to a variable extent. Consequently in macerated skeletons, where only the bony parts remain, this character cannot be determined. In comparatively few birds is the ossification complete, but this occurs in the Owls, in Podargus, in some Accipitres, Parrots, and others. When the septum is incomplete, the right and left nostrils communicate with each other, forming nares perviæ, as in Phaethon, among the Steganopodes, in the Herons, Grebes, Divers, Gralla (except Rhinochetus), Gaviæ, Limicolæ, Storks, Flamingos, Anseres, Cathartida (but not in the Vulturidæ and Falconidae), and in many Passeres, especially in the Meliphagidæ. In some Steganopodes, for instance in the Cormorants, the nostrils are reduced to narrow slits, and this condition is carried to an extreme in the Gannets, the external nostrils being absolutely closed, and the greater portion of the nasal cavity obliterated or filled with cancellated bony tissue; however, the olfactory apparatus is well developed, the inner nostrils or CHOANÆ being very wide, and in open communication with the mouth, enabling the Gannet to smell its food when in the mouth.

Various parts of the rostrum have received special names : culmen, the dorsal ridge of the upper bill; apex or tip; dertrum, in which it often terminates; gonys, or more correctly genys, the prominent ridge formed by the united halves of the under jaw, e.g. in Gulls ; tomia, the cutting edges of the bill.

The form of the bill exhibits almost infinite variations in size, shape, and structure, of which only the most striking modifications can here be dealt with. Generally shape and size stand in obvious correlation with the mode of feeding, but sexual selection seems also to play a great part, and leads to formations which it is often impossible to understand.

The horny sheath of the bill sometimes consists of a number of pieces more or less separate. In the Ostriches and Tinamous there is a lateral pair and an unpaired piece for each jaw; in the Tubinares on the upper jaw at least one pair of lateral or maxillary pieces, an unpaired piece which covers the culmen and is continued into the prolonged nasal tubes, and an apical hook, strongly curved and pointed : each half of the under jaw is covered by one ventral, one dorsal, and one terminal piece, the latter partly fusing with that of the other side into a strong scoop. Indications of such a compound rhamphotheca are, however, found in other birds, especially in the Steganopodes, in some Herons, like Nycticorax and Scopus, and in Penguins; the culminar or dorsal BILL

unpaired piece being more or less separated from the lateral pieces. In the majority of birds the horny covering forms one coherent sheath.

Frequently the edges of the mandibles and of the maxillæ are serrated to secure a firmer hold upon the food, for instance in Toucans. In the Anseres these tooth-like serrations are arranged in the shape of numerous transverse lamellæ, and hence the name “ Lamellirostres,” which, especially in the Shoveler, form an elaborate sifting apparatus.

The bill of the Flamingos is likewise furnished with such sifting lamellæ ; the two halves of the under jaw are considerably enlarged, so that the comparatively narrow upper jaw closes upon a wide cavity. In addition to this the whole bill is bent downwards, in some species rather abruptly; these long-necked birds being thus enabled to sift the soft mud of lagoons with their bill in an inverted position, the dorsal surface of the bill being turned towards the bottom. Undoubtedly this most peculiar bill is a secondarily acquired character, referable to the mode of feeding, which again is connected with the long neck and legs. This view is strengthened by the fact that very young Flamingos still have straight and short bills, which very gradually and only comparatively late assume the final shape.

Fine sifting lamellæ occur also in Prion (WHALEBIRD), and as a dense brushlike mass on the inside of the premaxillary region in Anastomus. The jaws of this genus have the further peculiarity that they do not shut completely, being slightly curved in opposite directions.

In the Spoonbilled Sandpiper, Eurinorhynchus pygmæus, the end of the upper and lower bill is of a peculiar spatulate and heartshaped form.

The broad and flattened spatulate bill of the SPOONBILLS, the boat or shoe-shaped bill of the Whale-headed STORK, Balæniceps, and of the Cancroma (BOATBILL), the long bills of the Ibis and the Whimbrel, curved downwards, and upwards in the Avoset, need no further comment but that they all are illustrations of the adaptation to a special mode of life, and therefore not necessarily indicative of relationship, as rather analogous than homologous structures.

The beak of Parrots is extremely strong, and well adapted to the breaking open of nuts by sheer force. The mandible ends in a transverse blunt edge, which presses against a corresponding horny prominence of the upper beak. In the large Microglossa (COCKATOO), which lives on the stone-hard fruit of the kanari-tree (Canarium commune), the beak bears a striking resemblance to a sledge-hammer. Transverse ridges, like those of a file, are common in front of the prominence of the upper jaw, the bird using them as a rasp— no Parrot swallowing anything but absolutely comminuted particles of hard substance, or pulpy and soft food—and also for filing or sharpening its mandible.

In the SKIMMER, Rhynchops, the bill forms two sharp vertical blades, which somewhat gape asunder, with the further peculiarity that the mandibular sheath and the supporting bone itself is considerably larger than the upper portion. A vertically compressed bill is also common in the Alcidæ, and is often vividly coloured during the summer. In the PUFFINS the outermost bright layers of the horny sheaths, and the horny excrescences at the gape of the mouth and above the eyes are cast off periodically, these parts being developed for the breeding season (Bureau, Bull. Soc. Zool. France, 1877, p. 377 ff.)

In many birds the covering of the bill, especially near the base of the culmen and the forehead, is swollen, and forms various protuberances, horns, knobs, and other apparently ornamental excrescences. In the Coors and in Musophaga (PLANTAIN-EATER) the coating of the culmen is produced backwards over the forehead, overlapping the latter as a conspicuous white or yellow soft plate. Often the underlying bones, especially the nasals and the adjoining premaxillary parts, are also swollen, and form a light and extremely spongy meshwork of cancellated bony tissue, a peculiarity which attains its highest development in the HORNBILLS and in the TOUCANS. Similar swellings are the knobs on the bill or on the forehead of the SCOTER and Mute SWAN, of Globicera among Pigeons, of certain Cracidæ, and of Macrocephalon (MEGAPODE). In most of these cases the swellings are very light; rarely, as in the Helmet-HORNBILL, the bones of the forehead are greatly enlarged, and, although much cancellated, of great weight and strength; moreover, the horny epidermal covering of the forehead is three quarters of an inch thick, and of the hardness and weight of ivory.

Another deviation is constantly found in the CROSSBILL'S beak, the sharply-pointed and hooked ends of the upper and lower jaws crossing each other in an individually varying way, there being an equal number of right and left-billed specimens. This crossing begins to shew itself before the young birds are fledged, increases with age, and ultimately leads to an asymmetrical development of the masticatory muscles and of the bones of the occipito-quadrate region.

In Anarhynchus frontalis (WRYBILL) the terminal half of the bill is turned towards the right side, an abnormality which exists in a marked degree even in the very young birds. The right edges of the premaxilla and of the mandible are thin and strongly turned inwards, so that the right and left sides are asymmetrical in section. The left nostril and the groove which is continued towards the terminal third of the bill remain in their original position, but the right nostril, and still more the groove, are perceptibly slanting towards the right, as can be ascertained by viewing the bill from the dorsal side.

Sexual DIMORPHISM is mostly restricted to peculiarly shaped bills; for instance, the horn of the male Hornbills is often larger, and differs in shape from that of the female. In the males of Pelicans several unpaired excrescences are formed entirely by the horny coating of the premaxilla ; they sometimes reach a height of three to four inches, and are again cast off after the breeding season, resembling in the latter feature the Auks, as described above.

The most striking example of dimorphic bills is that of the NewZealand HUIA, Heterolocha, the bill of the female being slender, about four inches long, and much curved, while that of the male is nearly straight, stout, and scarcely half that length. The knobs or swellings in the Gallinæ are mostly restricted to the males; the same applies to Edemia (SCOTER). Sexual differences in colour are common. For instance, in the male Scoter the bill is black and orange, in the young and in the female it is simply grey, and without the knob. The bill of the adult male BLACKBIRD is orangeyellow; that of the young of both sexes and of the adult males of Buceros malayanus (HORNBILL) is white, but becomes black in the adult female, forming thus an interesting exception to the general rule that the young agree with the females, and that aberrant coloration is confined to the males. The colour of the bill is deposited as a diffused pigment in the horny cells of the epidermal coat, but is occasionally restricted to the deeper layers, or even to the Malpighian layer itself, then shining through the outer transparent layers.

In connexion with the bill is to be mentioned the “egg-tooth," which is developed in the embryos of all birds as a small whitish protuberance or conglomeration of salts of calcareous matter, deposited in the middle layers of the epidermis of the tip of the upper bill, without being connected with the premaxilla itself. The sharp point of this “tooth” soon perforates the upper layers of the horny sheath, and then files through the eggshell, a slight crack in the latter being sufficient to enable the young bird to free itself. A similar egg-tooth exists in Reptiles, and is, as in Birds, cast off after hatching. The wearing away of the growing and constantly renewed horny layers of the bill can be easily observed in the pealing beak of a Parrot.

BIRD (etymology unknown; but in Old English Brid), originally the general name for the young of animals; 1 then, as the

1 As in Wyclif's translation of Matth. xxiii. 33, "eddris, and eddris briddis” (A.V. “serpents” and “generation of vipers "); Trevisa, Barth de P. R. xii. v.

« EelmineJätka »