FLIGHT OF BIRDS The problem of flight has been solved (I) by insects, in which the rapidly vibrating wings are flattened outgrowths of the dorso lateral body-wall of the meso- and meta-thorax, worked by strong muscles at their base ; (2) by the extinct Pterodactyls, in which a fold of skin was extended on the greatly elongated outermost or fifth finger ; (3) by birds, in which the transformed fore-limb owes its striking surface mainly to the feathers; and (4) by bats, in which a fold of skin, beginning at the neck, is continued along the anterior surface of the arm, then between the greatly elon gated fingers and palm-bones, and onwards along the sides of the body to the hind-legs, and to the tail if that is developed. The f our solutions are obviously quite different.
It is probable that birds were given to swift running before they were able to fly, and that the beginnings of flight were long flying leaps. It is likely that they practised parachuting from tree to tree, as some reptiles, like Draco volans, still do, before there was true flight. It is likely that there was a prolonged arboreal apprenticeship, which is pointed to, for instance, by the gripping arrangements of the toes of birds, and also by the active climbing of the young of the primitive hoatzin (Opistho comus) which uses its clawed hands as well as its feet in moving from branch to branch.
In ordinary flight the wings combine the functions of propellers and planes. From a raised position, sometimes vertically above the back, they are pulled f orwards, downwards and backwards by the contraction of the largest pectoral muscle, and then raised again by the pectoralis minor, whose tendon works through a pulley at the shoulder-joint. The tip of the wing describes a curve somewhat like an asymmetrical figure eight. The downward component of the wing-stroke, dis placing a mass of air, keeps the bird up or raises it ; the backward component gives it horizontal velocity; but the resistance of the air, which tends to retard the forward movement, has an impor tant lif ting function, for it works upwards against the ventral surface of the body and the slightly concave under-surface of the wings. Between successive strokes there is bound to be some loss of altitude and momentum, but this is almost inappreciable. It is lessened by economizing energy in raising the wing. This is effected by an automatic reduction in the size of the wing when it is not pressing down against the air, by a movement of the individual feathers so that air passes between them, and by the convex upper-surface which allows the air to glide off easily. For physical reasons an increase in the rapidity of flight up to a cer tain limit lessens the proportion of energy required; it is rela tively more economical to fly quickly. Marcy calculated that the energy expended by a pigeon when first taking flight is five times as great as when it has acquired its average. Among the most important adaptations to flight are—the shape of the body and the external reduction of frictional resistance ; the light build of the skeleton which affords large surface for the insertion of muscles and feathers without great increase of weight ; the insertion of the wings high up on the thorax and the ballasting of the body with the heavier organs, such as liver and gizzard, below, so that the centre of gravity is far below the centre of suspension ; the strong development of the pectoral muscles arid of the keel on which they are ventrally inserted; the possibility of increasing "sail-area," by lengthening the feathers, without in volving much corresponding increase in weight or size of skeleton.
The secondary feathers, attached to the ulna, are of major importance in the wing-stroke; the longer primary feathers, attached to the hand, are of great use in lateral steering, hence their prominence in the insect-catching swallows and swif ts. Steering is effected mainly by the differential action of the wings, but also by altering the tilt and pose of the body and by moving the tail-feathers, which likewise serve for balancing and as a brake. A quickly flying insect like a bee may have 200-300 strokes per second; in its ordinary flight a sparrow has per second 13 strokes, a wild duck nine, a carrion-crow three–four, a stork two, and a pelican one and one-sixth.
Besides ordinary flight, there is gliding, when the bird having attained to a certain velocity rests on its oars for a while ; or having attained a certain height descends to the ground without any stroke of its wings. It is probably the most primitive mode of aerial locomotion, parachuting rather than flying. A gull or a heron or any bird with a large sail area, having attained a certain velocity, planes with motionless wings, "like an aeroplane with its engines shut off," getting the necessary "lift" from the air-resistance to its forward movement ; but this cannot continue long without loss of position, unless there is a strong up-current of air, as from the face of a sea-cliff. Guidance during gliding may be effected by movements of the tail, head and neck, and by tilt ing the outstretched wings. It would be convenient to use the term "gliding" for everyday exhibitions such as gulls illustrate. Then the term "sailing" might be conveniently retained f or ex traordinary exhibitions such as the circling of the albatross around a ship. The peculiarity in this case is that the bird sails with and against the wind without visible wing-strokes for perhaps half an hour at a time. It takes advantage of currents of air of unequal velocity at different heights. It glides down the wind, with in creasing velocity but sinking a little; it wheels and rises into a less rapid current, with reduced flight velocity, changing part of its energy of motion into energy of position. The term "spiral sailing" might be restricted to cases like vultures and storks when they rise without up-strokes in slow spirals to a great height. It implies a steady, strong up-current of air, and is most frequently seen in warm countries. It seems undesirable to continue to call this soaring, a term indissolubly associated with the lark climb ing the sky. In this case there are very rapid wing-strokes, with the downward component predominating. The soaring of the lark comes close to the hovering of the kestrel or humming-bird, in which the rapid wing beat is devoted to keeping up the bird.
In thinking of the velocity of flight it is obviously necessary to distinguish between "air-speed" and "ground-speed." A bird that seems to the observer to be station ary may be flying hard against the wind. "A bird flying at 3om. per hour in a
wind will seem from the ground to be making either 5om. or lam. per hour according to whether it flies with or against the wind." The most accurate measurements are those made from aeroplanes on birds keeping level with the machine and flying in the same direction. Thus measured by Meinertz hagen, small song-birds show an average rate of 2o-3 7m. per hour, crows 31-45, ducks 44-59, plovers 4o-51; but these may be increased in exceptional circumstances, and an occasional rate of I oom. an hour is quite credible. "Swifts of our common species, feeding 6,000ft. above Mosul, were noted as easily passing and recircling about an aeroplane which was registering 68m. an hour." The value of flight is obvious, but it may be useful to recall some of the different ways in which it has justified itself in the struggle for existence. It gave its possessors a new safety and independence ; it enabled them to follow food and to seek water over long distances; it helped them to secure the well-being of their offspring by building nests in places of ten inaccessible except to enemies able to fly or climb. The power of flight in its high development also gave birds a unique power of annihi lating distance, of evading the winter, of having two summers in the year, of having two homes, of changing their season in a night. In seeking out suitable feeding-places and breeding-places neither space nor time present obstacles to the flying bird.
While there is no known connecting link between a feather (q.v.) and an epidermic scale, the development of the two struc tures is not very different. Both arise from expanded papillae of the epidermis, with the cells transformed into horn; and both are fed, as long as they grow, by pulp-like upgrowths of the under-skin or dermis, containing minute blood-vessels. There can be no hesitation in comparing the scales on a bird's toes and in step-region, as also on the bill, with t:ie typical scales of reptiles; but it should be noted that what is moulted in reptiles is the outermost dead layer of the epidermis covering the scales, whereas in birds the whole feather is thrown off. Feathers are unique struc tures whose evolution is obscure.
An ordinary feather shows the following parts : (I) the cylindrical hollow barrel or calamus, whose base is embedded in a follicle of the skin ; (2) the main shaft or rhachis, filled with white pith, and somewhat quadrangular in cross-section, but convex externally and concave internally; (3) the vane, consisting of a bilateral web, which in the case of the pinions serves to strike the air, and is built up of biserial barbs united by barbules and microscopic barbicels; and (4), it may be, an aftershaft or hyporhachis, which arises to the inner side at a little pit marking the junction of calamus and rhachis, and usually consists of a tuft of barbs, often with barbules. In a few cases, such as the cassowary, the aftershaft is as long as the shaft, and the feather looks as if it were double. The most im portant structural fact is the webbing of the vane, so that it forms in the pinions an efficient instrument for striking the air. In feathers not used in flight the coherence of the parts of the vane helps to keep air imprisoned below, to lessen the loss of heat and to protect the skin from being wetted. A bird's feathers occasionally occur all over the skin (in penguins), but they are usually disposed in regular rows (feather-tracts or pterylae), which show particular patterns in different types.
Apart from the occasional presence of glands near the ear opening, the bird's skin has only the preen-gland; and even that may be absent, as in ostrich, bustard, and some parrots. It is best developed in aquatic birds, and this is popu larly associated with the need for water-proofing the plumage. Some mammals lick and brush their skin, but in the main the fur is kept in good order automatically. Birds, on the other hand, attend to the feathers individually, and they may be sometimes seen reaching back with their bill to the preen-gland, which pro duces an oily secretion. But some birds, like the scissor-bill (Rhynchops) and the pelican could not readily compress their preen-gland, and in many cases the secretion is very small in amount compared with the extent of the plumage. Moreover, some birds without a preen-gland keep their feathers in good order. It has been suggested by W. P. Pycraf t that the preen gland may be a scent-gland, helping in the recognition of kin. But most birds have a poorly developed sense of smell. It is possible that a drop or two of the oil stimulates the secretion of the salivary juice, which helps in preening. According to Paris the removal of the preen-gland makes no difference to the bird. The structure of this organ resembles that of the odoriferous glands of some reptiles; it is a bi-lobed subcutaneous mass of glandular tubes, supplied by blood-vessels and nerves, surrounded by a capsule of connective-tissue, but without muscle-fibres except at the opening.
Af ter a feather is fully formed the component cells begin to die away, and the pulp retreats into the skin, leav ing little partitions or "caps" across the interior of the calamus. It follows that there can be no repair of worn feathers, and it is therefore advantageous that there should be "moulting." Moult ing usually occurs after the fatigue of the breeding season and before the autumnal migration, but in some birds, e.g., swallows and hawks, it occurs in mid-winter. The process of shedding the old feathers and replacing them is usually spread over a con siderable time, but it may be so rapid that the bird is left very naked. Moulting geese, ducks and rails lose all their pinions at once, and are for a time unable to fly.
All birds moult once a year ; but some moult twice. There is of ten a special spring moult, when the males put on their nuptial plumage. The ptarmigan has three moults in the year. There are many other exceptional features ; thus the penguins moult their feathers in patches, not individually. The physiology of moulting is very slightly known, but there is much interest in Beebe's experiments (1914) on the males of the scarlet tanager and the bobolink, in which a brilliant summer plumage normally alternates with a totally different winter garb. At midsummer the birds were placed in small cages in a quiet room, with grad ually diminishing light and slightly increased food. The birds ceased to sing, put on fat and increased in weight. Thus the modi fied environment brought about a bodily condition very different from the thinness and fatigue that normally follow the strain and the cares of the breeding season. The autumnal moulting time passed without a single feather being shed. Early in spring some of them were gradually brought into normal conditions, where upon the spring moult took place, the birds passing directly from one nuptial plumage to another. Beebe's conclusion was that the condition of the birds as regards fatness or thinness determines whether they shall moult or not. Side experiments showed that the seasonal "pigmental changes in the blood" go on as usual, for one of the tanagers, subjected to rapidly altered tempera ture, underwent a belated moult into the green winter plumage. Thus it is plain that the succession of plumages is not rigidly predetermined ; there is a plastic correlation of internal consti tutional rhythms and external seasonal periodicities.
Feathers often illustrate an unsurpassed combination of pig mentary and structural coloration (see COLOUR OF ANIMALS).
In connection with the variety of plumage, it is useful to bring together the chief uses of feathers, which must have been subjected to selection with reference to many different advantages : (I ) the primary use was probably to form a relatively non-conducting robe which tends to retain the animal heat. This may be aided by layers of air included between and below the feathers; (2) the plumage may also save the skin from being wetted. This prevention of wetting varies in different birds; (3) the feathers made flight possible, and here may be noted the light elastic build, the resistance to wear and tear, the almost air-tight linkage seen in the vanes of the pinions, and the possibility of attaining great length without much weight; (4) the plumage often renders the bird inconspicuous in its habitual surroundings. Thus the brooding woodcock among the fallen leaves and withered herbage has a very effective cloak of in visibility. Very common is the obliterative counter-shading seen in a bird like a curlew or a sandpiper, whose plumage is lighter below and slightly darker above, the two shades blending on the sides. Another type is seen in the irregular or dazzle pattern, where variety in different parts breaks up the shape of the body, a good example being the golden plover. This type of protective coloration has the advantage of being effective against diverse backgrounds; (5) conveniently separable, perhaps, from pro tective coloration is that which masks aggressive birds in the eyes of their victims, as may be the case with the Greenland falcon and the snowy owl. Here might be included other qualities of the plumage useful in the chase, such as the softness that makes the flight of owls so noiseless; (6) when there is a profitable re semblance to other birds, the term "mimicry" may be legitimately used. A weak species may profit by being like a strong one, a predatory bird by being like a peaceful one. A. R. Wallace pointed out that the friar-birds of the Malay Archipelago, large, strong, gregarious, assertive honey-eaters, holding their own against birds-of-prey, are "mimicked" by the weaker orioles, and that the particular species of friar-bird on each island has its corre sponding mimetic oriole ! "Of aggressive mimicry, a good example is the caracara of South America, a carrion-eating hawk which also catches small birds by stealth. It is enabled to do so by its close superficial resemblance to a species of curassow, a harmless game-bird, which is common in the same region" ; (7) feathers are often auxiliary to the appeals made in courtship. They may enhance the brilliance of the male bird; they may be exaggerated into extraordinary decorations, as in birds of paradise; they may form tufts and crests erectile in excitement; they may be used to produce arresting sounds, as in the "drumming" snipe; (8) in many cases, reaching a climax in eider-duck, feathers are used in nest-making, conserving the heat around the nestlings, and making brooding more comfortable; (9) finally, there are various minor uses, such as helping in recognition of kin by kin.
The bird's skeleton affords numerous instances of adaptations, as might be expected since there are in most cases two different kinds of locomotion, flying and running, and sometimes swimming as well. The term "adaptation" is here used to express not a process but a result, a particular adjustment of structure well suited to discharge certain functions.
The lightly built skeleton is adapted for flight. This implies that the medullary cavities of the long bones are large, that they are often without marrow in adult life, that many contain air-sacs continuous with the lungs, and that they frequently shdw spongy bony tissue below the hard and com pact cortex. Thus the skeleton is built on the hollow girder prin ciple, with a relatively large surface for the insertion of muscles, yet without the corresponding increase of weight that would be involved if the bones were as substantial as those of reptiles and mammals. Long bones with large cavities also occur in bats and in Pterodactyls. In some birds, such as the albatross, practically every bone is "pneumatic" except the scapula and the hyoid.
A second general character of the bird's skeleton is the tendency of adjacent bones to fuse to gether while still in the making. This occurs in many parts of the body. Thus, except in running birds, most of the bones of the skull coalesce early, which may be in some cases adaptive when the skull as a whole is used as an instrument; e.g., as a hammer in woodpeckers, or for tearing up a victim in birds of prey. The fusion of most of the thoracic vertebrae in ordinary flying birds affords a firm fulcrum for the down-stroke of the wings. This interpretation is confirmed by the absence of fusion in 'running birds and by the tendency to coalescence in the thoracic verte brae of bats. The fusion of several vertebrae (thoracic, lumbar and caudal) with the true sacrals, to form a syn-sacrum, and the fusion of this with the entire length of the iliac portion of the pelvic girdle, must be interpreted in adaptation to bipedal pro gression. So much of the bird's body is usually in front of a perpendicular dropped from the acetabulum, with which the head of the femur articulates, that it is necessary for balancing pur poses that the hip-girdle should take a long and strong grip of the backbone. The point is emphasized if the bird's hip-girdle be contrasted with that of the jumping and swimming frog, which cannot stand on its hind legs, and has only a single sacral vertebra with which the thigh-bone has merely a ligamentar attachment. In all modern flying birds (Carinatae), though not in running birds (Ratitae), there is a fusion of about f our terminal verte brae to form the ploughshare bone or pygostyle. This is in great contrast to the long lizard-like tail of the earliest known fossil bird (Archaeopteryx), which had 20 free vertebrae. In ordinary flying birds the ploughshare bone forms a basis for the insertion of the tail-feathers or rectrices, and may thus be interpreted as adaptive. Reference will be made later to the fusions that occur in the regions of the wrist and palm and in the region of the ankle and instep. In the running birds, where the pectoral girdle is un important, the scapula and the coracoid are fused into one bone.
Some additional adaptations to flight may be summed up. The keel or carina on the sternum serves for the insertion of the pectoral muscles, and its varying prominence in relation to the body of the sternum in different types stands in correlation with the power of flight. A keel is slightly developed in bats and also in moles, both of which have strong pectoral muscles. It is absent in Ratitae. It is degenerate in the rare burrowing parrot (Stringops) of New Zealand. In contrasting the Carinate and the Ratite breast-bone, it is inter esting to notice that the former is usually carried far back, form ing a supporting floor for the viscera, of especial value in flying and swimming, whereas the latter is a broad shield, not carried backwards, the weight of the viscera being borne by the thoracic and lumbar vertebrae, and by backward extension of the rib.
The sabre-like scapula of flying birds is bound by ligaments to the ribs and backbone ; the strong coracoid is braced movably in a deep groove on each side of the front of the breastbone ; the clavicle extends crosswise in front of the heart, and has its middle piece attached by ligament or sometimes fused to the tip of the keel; except in the South American screamers (Palamedeae) many of the ribs have a backward-projecting uncinate process, which overlaps the rib behind, to which it is lashed by ligament, thus binding the rib-system into a coherent basket; the lower or sternal parts of the ribs, which reach the breastbone, are bony, not gristly as in mammals. The total result is to form a coherent springy framework (of backbone, breastbone, ribs and pectoral girdle) against which the wings can work with maximum effi ciency. The arrangements prevent any incrushing on the heart during the down-stroke of the wings, yet the alteration in the capacity of the chest cavity, as the breastbone is raised and the backbone depressed, facilitates the outrush of air from the lungs. The glenoid cavity, formed by the junction of scapula and cora coid, is much more open than in mammals, and is thus well suited for the free play of the wings. If the clavicles, forming the merrythought, are broken, the bird cannot fly. They are rudi mentary or absent in running birds, and reduced in some birds that do not fly much; e.g., certain parrots. Hilzheimer calls atten tion to the marked mobility of the joint between corticoids and sternum in humming birds, and connects it with the great rapid ity of the wing-strokes. It allows rapid expansion and compression of the chest-cavity, thus facilitating respiration. In general, it is one of the perfections of birds that flying helps breathing.
In the wing itself the chief adaptation is that there is greatly reduced mobility in the different parts, so that the feather-bearing skeleton works as a unified whole. Thus in flight the radius does not move on the ulna; there are only two free wrist-bones; the distal half of the wrist and the whole of the palm-bones—six bones in all—are fused to form the carpo-metacarpus, carrying most of the primary pinions; the thumb has considerable mobil ity, but the two other digits are stiff. But while the fore-limb may be said to act en bloc in striking the air, the other side of the adaptation is the way in which it folds up in one plane into the resting position, forming a compressed letter Z, the elbow point ing backwards, the wrist joint forwards, the tips of the digits (probably I., II., and III.) backwards. This is a space-economiz ing arrangement, useful when the bird is swimming or diving with its feet, or threading its way afoot through thick herbage.
Among the adapta tions to bipedal progression, we have already noticed the syn sacrum, which consists in a pigeon of one thoracic vertebra, five or six lumbars, two sacrals and five caudals. This gives the hip girdle a long and strong grip of the backbone, and thus facilitates balancing the body on the rounded head of the thigh-bone. The elongation of the different regions of the leg is suited for rapid running, and the most striking feature is the long, more or less vertical, tarso-metatarsus—a unique piece of skeleton. Except in the two-toed ostrich, it consists of three coalesced metatarsals (or instep bones), and to the upper end of these the distal ankle bones or tarsals are fused. With the exception named, the lower end bears three articular knobs, f or the three toes. If there are four toes, the first is turned backwards, and has a small separate metatarsal of its own. The tarso-metatarsus adds to the bird's running power and swimming power, and in the divers and their relatives it is laterally compressed into a narrow blade, which reduces friction in preparing for a stroke and concentrates the force in the region of the foot when the backward stroke is made. At the bird's ankle-joint, two rows of small tarsal bones would naturally be looked for, and they are indicated in the embryo. But just as the distal row fuses to the fused metatarsus, forming the tarso-metatarsus—six bones in all—so the upper row fuses to the lower end of the tibia, forming the tibio-tarsus. Thus all the ankle bones have disappeared as such, and this, considered in relation to the elongated tarso-metatarsus, may be of mechanical advantage when the bird springs from the ground or rapidly alights.
The skeletal adaptations of the toes are illustrated in a special plate, and it will be enough to mention the powerful talons of birds-of-prey for gripping and carrying the booty; the elongated toes of the jacana, for walking on floating leaves; the unusual arrangement in the cuckoo, the first and fourth toes pointing backwards (with the fourth reversible), the second and third forwards, for firm perching; the same appearance in trogons, but with the first and second toes backwards, and the third and fourth forwards; the forward directed four toes of the swift, for climb ing or for clinging to the nest. In divers and grebes and the ex tinct Hesperornis, there is an interesting prolongation of bone rising from the head of the tibia above the knee-joint. As a basis for strong muscles it gives much additional power to the swim ming stroke.
The surrender of the fore-limb to wing-making implies an assumption of new functions by the skull, which becomes a manifold instrument. Perhaps this is facilitated by the early fusion of most of the cranial bones in flying birds, and it is interesting to notice that similar fusion occurred in the Pterodactyls. The exaggeration of the premaxillae to form a beak is an obvious adaptation. Detailed adaptations of the bill are shown in a special plate. The lower jaw consists of six bones on each side, a point that reveals the affiliation of birds to reptiles. More important, however, from our present point of view, is the loose articulation of the lower jaw with the movable quadrate, for this increases the gape—an important feature. Thus it facilitates the rapid swallowing of large booty, and catching small insects in mid-air. The delicate nature of the infra-temporal bar may be correlated with the usual absence of mastication; and where teeth are absent it is not surprising to find the maxillae are small.
The importance of vision in birds is correlated with the large orbits, and these with the restriction of the cranial cavity to the posterior region of the skull, which is markedly broadened.
One of the most striking advances of birds, as compared with reptiles, is in the proportion between the brain-containing region and the size of the skull as a whole. The skull has great mobility on its single occipital condyle (another reptilian feature) ; and here should be noted the mobility of the neck with its character istic heterocoelous vertebrae. The bill can reach the preen gland ; the head may be seen resting between the shoulder-blades with its point towards the tail.
In birds, as in all other animals, both structure and habits are in great part concerned with the quest for food: (I) many birds are vegetarian, eating fruits, seeds, buds, leaves. Most humming birds and honey-eaters suck up nectar; geese graze on grass; grouse devour heather tips; ptarmigan often feed on the mountain lichens; (2) many birds are carnivorous, eating small mammals, other birds, reptiles and amphibians, fishes, slugs, insects and lower animals like earthworms. Kestrels destroy voles, the golden eagle sif ts the grouse, the secretary bird kills snakes, storks swallow frogs, the flying osprey catches the swimming fish in its talons, rooks let freshwater mussels fall from a height on the river gravel, the thrush breaks the shells of snails on its anvil, the cuckoo is partial to hairy caterpillars, the swallow catches small insects in mid-air, the woodcock depends mainly on earthworms. The most important ecological fact is the indispensable check that birds keep on the multiplication of insects which would other wise ruin the world; (3) many birds enjoy both vegetarian and carnivorous diet. Thus the thrush likes fruit as well as slugs; the normally fish-eating herring gull has become a devourer of turnips, potatoes and grain. Of special interest among the mixed feeders are those which give their young ones material different from that which forms the staple food in adult life. Thus young rooks, sparrows and finches are fed for a time on insects only. In many cases the diet has necessarily to vary with the seasons; (4) there are many strange oddities of diet. Thus the kea parrot (Nestor notabilis) of New Zealand has learned to settle down on a dis abled or dead sheep and dig out the fat and flesh from near the kidneys. This is a strange idiosyncrasy on the part of a bird that belongs to a race habitually vegetarian or frugivorous; and it must have arisen rapidly, for sheep were not taken to New Zea land till about oo years ago. The red-winged starlings of South Africa include in their menu the berries of the syringa, which they eat in such quantities that they become stupefied by some in cluded narcotic. Similar intoxication has been observed in birds that devour fermenting fruit. Another quaint misadjustment is seen in some sea-birds, like the albatross, which may make such a hearty meal that they cannot rise off the water.
It is ecologically interesting to select a particular kind of food, let us say, fish, and to notice the variety of ways in which it is obtained. The hovering osprey catches the fish in its talons, the swooping herring-gull with its bill, the gannet dives from a height, the cormorant turns head over heels from the surface, the scissor-bill skims the waves, the heron usually waits for a fish to swim past, the kingfisher makes a sudden plunge, the pelicans sometimes wade shorewards in a crescent, penguins pursue their booty under water, the skuas chivy the herring gulls in the air and force them to disgorge. In the ways in which food is detected, acuteness of vision counts for most; a tactile bill is important in probing birds like snipe and woodcock; of smell there is not much evidence.
Another line of enquiry may be illustrated by the way in which the bills and feet of birds are suited to particular kinds of food and food-capture. The absence of teeth in modern birds is com pensated for by the horny bill covering the jaws. It often has a sharp edge and a hooked tip; it varies from a massive crushing instrument in the toucan to a delicate probe in the humming bird. The variety of type should be considered in the light of the fact that the fore-limb has been surrendered to making a wing, and that the bird's jaws have to discharge duties which in mammals usually fall to the hands.
The bones covered by the horny bill are especially the premaxillae above and the complex lower jaw below. It may be convenient to keep the word "bill" for the horny sheath and "beak" for bill and jaws together. The more primitive type of bill-structure is seen in birds like the albatross and the puffin, where there are several distinct horny plates, homologous with reptile's scales. In most birds these fuse into one sheath. There is an autumnal moulting of part of the bill in the puffin, and as the plates are not replaced for some time, the puffin's bill in winter differs in size and appearance from the enlarged and decorative summer form. In his Evolution Theory (1904), Weismann discusses the regeneration of both bone and horn in the bill of the stork, and correlates this with the combats of males, in which serious injury is apt to occur. Similar regenera tion is recorded for fighting cocks and for a parrot.
A generalized bill may be illustrated by the crow and its allies —strong, pointed, somewhat triangular in section. Shortening, sharpening and curving of such a bill would yield the predatory type of hawk and eagle—quick to give the death-blow by piercing the skull or cutting the jugular vein, also well suited for rapid skinning and plucking, and for drawing out strips of flesh from the body. It is interesting to see an eagle deftly run down the backbone of a fish with its bill, dislocating the vertebrae. Some what of the same type are the bills of the owls and the parrots, though the former are carnivorous and the latter vegetarian. The parrot's beak has a movable hinge between the premaxillae and the front of the skull, and there are often strong file-like rough nesses to the inside of the bill, which serve in gnawing nuts or grinding hard seeds. Not very remote are the broadly conical strong bills characteristic of the finches, suited for breaking capsules and splitting seeds.
On another line are the delicate slender types, familiar in warblers, suited for dealing with small insects, with a climax in the elongated slender bills of humming birds. Not far removed. though used in a different way, are the elongated narrow bills of curlews, snipe, avocet and woodcock, for probing in mud, sand and the like. In a very different direction ate the broad bills of ducks and geese, for sifting the mud and cropping vegetation, and on this line might be placed the broad bills of some storks and herons, and the spatulate type of the spoonbill. Also broad ened out, but kept very short, are the insect-catching bills of nightjar and swift. A slight specialization of the rook-type leads to the sharp form of bill seen in snake-birds, herons and bitterns. Lateral compression in puffin and razor-bill reaches an extreme in the long, high knife-blade of the skimmer (Rhynchops), a trop ical relative of the terns. This bird flies very close to the sur face and skims the water with the mandible which is longer than the upper jaw. When the knife edge of the mandible touches a small fish or crustacean it tilts the booty into the open mouth. The skimmer or scissor-bill is also said to skim soft mud. There are many strange types, such as that of the cross-bill, where the points of the upper and lower halves of the beak cross one another when at rest—a position that occurs as an abnormality in some birds, such as crows. The peculiarity is utilized by the crossbill in the exceedingly rapid extraction of the seeds of fir-cones, a deft manipulation in which the tongue assists. In the wry-bill plover of New Zealand the bill is turned to the right-hand side, and it is interesting to notice that a one-sided twist is occasionally seen in an individual oyster-catcher. In the flamingo the whole beak is bent almost at right angles on itself, half-way down its length, and is thrust upside down into the mud, where it searches for small molluscs and the like. The kiwi is the only bird in which the nostrils open at the tip of the bill. In the huia, another New Zealand rarity, there is sex-dimorphism in the bill. According to Alfred Newton, the male uses his short, strong, almost straight bill to chisel holes in decaying wood where grubs may lurk, while the female uses her much longer, decurved and slender bill to probe into crevices. But when he, having discovered a grub in his excavations, is unable to reach it, she utilizes the opportunity.
Before leaving bills and their adaptations we may make three general notes: (a) the adaptations to feeding habits often show great nicety, but it should always be asked whether a bird with a peculiar bill may not have sought out a peculiar kind of food, as in crossbills; (b) the most striking features of a specialized bill are not exhibited till the young bird begins to fend for itself. Thus the very young flamingo shows nothing of the strange deflection of the bill. As in many other cases, a hereditary character may have to await its appropriate liberating stimuli; (c) the same kind of bill may occur in birds which are not nearly related, as in the case of swifts and swallows, parrots and birds of prey. This illus trates "convergence" or "homoplastic resemblance"—similar structures having arisen independently in unrelated types in adaptation to similar habits or conditions of life.
The tongue is often adapted to assist in food getting. Thus that of sap-sucking wood peckers ends in a brush, while that of insect-eating woodpeckers bears spines. The tongue of the humming bird ends in two deli cate brushes, suited for nectar-sucking and for capturing small insects. In many cases there are processes on the tongue which help to guide the food backward to the gullet, or to guard the opening of the glottis, or to strain the mud, or to grip slippery booty, such as small fishes. In some fish-eating birds, like peli cans, which swallow their prey whole, the tongue is very small.
Many peculiarities in the feet of birds have to do with locomo tion. Thus the webbing of the toes is adaptive to swimming, the scaling and clinching to perching; the elongation and spreading of the toes may afford a large surface in jumping off and alighting, or may facilitate movement on floating plants. But other peculi arities of the feet have directly to do with catching and handling the food. Thus a parrot sometimes uses its foot as a hand ; the owls bend their fourth toe backwards beside the first one, form ing an effective arrangement for catching, crushing and carrying mice; fowl-like birds use their strong blunt claws for scratching the ground; the secretary bird strikes the snake with its foot, and sometimes kills with a single forward kick. These are but a few of the hundreds of instances of specially adapted feet and toes.
It will be convenient now to consider the digestion of the food.
The salivary glands, opening into the mouth, do not seem to have great importance in birds as far as their usual function of digesting carbohydrates is concerned. For most birds bolt their food. In insect-eating woodpeckers the glue like saliva on the tongue must be useful; in the sea-swifts (Col localia) the salivary secretion, mostly consisting of mucin, is used to make the "edible" nest. Through the muscular gripping pharynx the food passes into the elastic gullet, with mucus glands lubricating it internally. In many cases the gullet is enlarged into a crop which serves for storage. It is not digestive except in so far as salivary juice from the mouth may follow the food ; but there may be bacterial fermentation. In the hoatzin the crop is strongly muscular and is used to squeeze the juice out of succu lent leaves. In pigeons and a few parrots there is a fatty degen eration and desquamation of epithelial cells lining the crop. This forms the "milk," which is regurgitated by both male and female pigeons into the mouth of the young bird.
The stomach may be a simple sac, muscular and glandular, but in most cases it is divisible into an anterior glandular portion (the proventriculus) and a posterior muscular portion (the gizzard). The relative development of these two regions is correlated with the differences in diet. Thus a graminiv orous bird, like a pigeon, has a strongly developed gizzard, while many birds-of-prey have practically none. Of much interest is the modification of the character of the stomach in the individual bird, according to change of diet. This has been observed in nat ural conditions in the herring gull, which has a relatively hard stomach in summer, when it often eats much grain, and a rela tively soft stomach in winter, when it depends mainly on fish.
The gizzard which is no doubt part of the rep tilian legacy, as suggested by its presence in the crocodile, is marked by the following features : (a) there is on each side a strong development of muscle, numerous fibres radiating outwards from a central tendinous disc, and bringing the two sides towards one another when they contract; (b) the sac is twisted on itself so that the entrance into the duodenum is not far from the exit from the proventriculus; (c) there is a hard internal lining and (d) there are numerous small pebbles which help to triturate the food. As these grind-stones get their corners rubbed off, they slip down the intestine. There are many strange gizzards, such as that of some fruit-pigeons, which has numerous hard conical projections on the internal lining, or that of the snake-bird (An hinge), which shows a sieve of hair-like processes at the duodenal end. These prevent unbroken fish-bones and the like from pass ing into the intestine. When the male hornbill (Hydrocorax hydrocorax) passes food to his imprisoned mate, the packet is of ten surrounded in a tough homogeneous skin which arises as a secretion of the walls of the gizzard.
The most marked anatomical peculiarity of the rest of the alimentary canal in the bird is the shortness of the rectum, which is of ten not more than an inch long. This shows that there must be great efficiency of digestion in the much looped small intestine. The amount of faecal matter is relatively small. At the junction of the small intestine and the rectum there are usually two caeca, which occur in all sizes from mere vestiges, as in the pigeon, to long functional tubes, as in the duck. When well developed they serve to delay the food and facilitate absorption. The terminal chamber or cloaca receives the ureters and genital ducts, and bears a dorsal pouch, the bursa Fabricii, which func tions in young birds for the formation of white blood corpuscles.
Some of the important peculiarities are the f ol lowing : the bird's lungs, though small and hardly distensible, have through the complex branching of the bronchi, a very large internal surface for gaseous interchange ; the driving out of the air from the lungs, which are fixed to the ribs, is assisted by the strokes of the wings, expiration is the active part of the respir atory process, not inspiration as in mammals; the lungs open into a system of transparent, internally ciliated air-sacs which econ omize the work of breathing and increase the total content of air; this is useful in prolonged diving and in enthusiastic song ; they allow of a "double tide" in every respiratory act, thus in expira tion air passes out from the lungs to the exterior, but also into the lungs from the air-sacs; they promote internal perspiration, for water-vapour passes into them from the blood, thus helping to regulate the temperature of the body.
At the back of the tongue lies the glottis or opening of the windpipe, with never more than a hint of the epi glottis characteristic of mammals. The glottis leads into the lar ynx, supported, as in amphibians, reptiles and mammals, by trans f ormations of the representatives of the branchial arches of fishes. But the larynx of birds contains no vocal cords, these being sit uated at the lower end of the windpipe (trachea) in the enlarge ment known as the syrinx. This varies considerably in develop ment and is absent in ostriches, storks and some vultures. The voice is due to the rapid passage of out-breathed air over the vi brating vocal cords. The trachea is supported by bony rings. It is interesting to notice that the syrinx arises as a transforma tion of the base of the trachea and of the beginning of the two bronchial tubes. In contrast, again, with mammals, there is no true diaphragm, shutting off the chest from the visceral cavity.
The bird has a four-chambered heart, with a complete separation of arterial and venous blood, as in mammals; but the aortic arch, carrying pure blood to the body, turns to the right in birds, to the left in mammals. The blood of birds has more red blood corpuscles per ounce than that of any other ani mal, and this is an adaptation to intensity of metabolism and to aerial habits. The red blood corpuscles are elliptical, slightly bi convex, nucleated discs, like those of lower vertebrates, but unlike those of mammals, which are non-nucleated, slightly biconcave and generally circular in outline. On the whole the red blood cor puscles of birds are large as compared with those of mammals while the diameters in man, dog, horse and ox are 7.5
7 µ, 6.5 µ, and 5.5 µ, in the high-flying vulture 17.7 µ by 8 µ (µ is
of a millimetre, about 26
in.) . There are several kinds of white blood corpuscles or leucocytes. Of special interest are the phagocytes, mobile amoeboid cells, able to leave the blood altogether and to migrate into the tissues, where they engulf microbes, devour degenerating tissue, transport material from one area to another, and help to repair injuries.
As might be expected in animals of predominant muscularity and intense metabolism, the temperature of the bird's body is high, from 2 °-14 ° higher than that of mammals. Regulation of the temperature is automatically effected by a thermotaxic nerve centre in the corpus striatum of the cerebral hemispheres. If the temperature of the blood changes, the centre, being stimulated by the blood-stream, sends nervous impulses to the muscles, the skin and other parts, regulating the production and loss of heat. These arrangements have not been completed in nestlings, which are therefore apt to suffer from over-heating or over-cooling.
In organisms so active as birds there is neces sarily a large production of nitrogenous waste-products, due partly to the wear and tear of the protein framework of the cells, and partly to the nitrogenous residue of the amino-acids into which the food-proteins are changed by digestion. The elimina tion of the poisonous nitrogenous waste is prepared for in the liver, and completed in the tri-lobed kidneys which lie below the hip girdle. The result of the filtration is a clear fluid which passes from the kidneys by the ureters to the cloaca; but there, by losing water, it turns into a semi-solid mass of urates. This nitrogenous waste forms the guano of bird-islands, such as those of Peru.
It is impossible to understand the bird's bodily life without taking account of the regulatory system of endocrinal or ductless glands—such as the paired thyroids near the base of the neck, the yellowish supra-renals lying on the front part of the kidneys, the pituitary body on the under-surface of the brain, and the glandular tissue in the reproductive organs. These structures produce hormones which are distributed through the body by the blood, and help to secure a harmonious regulation of function. Those produced by the endocrinal tissue in the go nads are important in activating or in inhibiting such structures as decorative plumes and such activities as song.
The bird's sense of sight is highly developed, and the power of rapid accommodation surpasses that of all other verte brates. The rods of the retina, which have to do with discrimina tion of form, far outnumber the cones, which are believed to have chiefly to do with colour-sensation. Experiments show that birds cannot see blue or violet, but they distinguish colours towards the other end of the spectrum. The usual six muscles of the eye are reduced in size, and the spherical shape of the eyeball, adapted to ready mobility, is usually departed from, though approached in ostrich and vulture. It is likely that the reduced mobility of the eye is compensated for by the freedom of movement of the neck. The eyes are markedly to the sides of the head, command ing two visual fields. The vision is strictly monocular.
The sense of hearing is acute, but it has been ob served that many kinds of birds are indifferent to even loud noises. Attention is most readily given to sounds which stimulate or in terest either an inborn equipment or an acquired association. The parental danger note stimulates the crouching instinct of the young partridge ; a bird will sometimes answer back to an artificial kin-call. The semicircular canals of the ear show some correla tion with the perfection of flight ; thus they are better developed in a swallow than in a swimming bird. Birds resemble reptiles in having no ear-trumpet or pinna, such as is seen in most mam mals. Its absence in birds may be correlated with the mobility of the head and with the advantage of reducing friction ; but the close affinities of birds and reptiles must be kept in mind.
The sense of smell has been strictly demonstrated in nighthawks, owls, magpies and some other birds, but not in many. It is by sight, not by smell, that the vultures gather to the carcase. A few birds have themselves strong odours, but recognition of kin is mainly by sight. According to some natur alists, the odour of a bird—e.g., a partridge—is reduced to a minimum when brooding. As to tactility, there is not very much, for there is little exposed skin ; but there are nerve-endings at the base of some of the feathers; the tip of the tongue, as in ducks, may serve as a touch-organ ; and there may be a strong develop ment of tactile nerve-endings in the white cere near the nostrils, or at the tip of the probing bill, as in woodcock. Taste papillae have been demonstrated on the tongue of a few birds, but this sense is usually slight. Little is known of the other sensory "recep tors," such as those responsive to temperature and pressure.
The brain, compared with that of reptiles, fills a rela tively large cranial cavity, and both the cerebrum and the cerebel lum are large, the former covering the region of the optic thalami and the latter hiding the medulla oblongata. The optic lobes, which are on each side of the middle line in the young embryo, are displaced to the sides by the disproportionate growth of cerebrum and cerebellum. It may be that the large size of the cerebellum, which is transversely grooved, has to do with the bird's loco motor achievements, for this part of the brain has in part to do with the control of movements. The cerebrum, the seat of the higher mental functions, differs from a typical mammal's in being without convolutions and in having no more than a trace of the important transverse commissure, the corpus callosum. The roof of the cerebrum is relatively thin and the main mass consists of corpora striata.
When a nestling opens its mouth at the touch of food in its mother's bill, that is a reflex action. It is dependent on pre-arranged linkages between certain nerve-cells—(r) sen sory, (2) associative, and (3) motor—and (4) certain muscle cells. The linkage is part of the characteristic hereditary organi zation ; no learning is required for a reflex action ; the higher nerve-centres are not concerned. There are many of these reflex actions in birds, as in other higher animals ; and sometimes they are complex or serial, one leading on to another. Thus the nest ling's opening of its mouth is followed by other reflexes of grip ping and swallowing the food. It is probable that the movements made by a young bird just before hatching are mainly reflex. In born reflexes, dependent on pre-established neuro-muscular link ages, must be distinguished from individually acquired associa tions, as when a bird acts in a particular way in response to a par ticular sound or sight. This may become automatic.
When the activity evoked by a stim ulus is a chain of acts on the part of the creature as a whole, fol lowing a definite routine and requiring no apprenticeship, it is called instinctive behaviour. From the physiological side, instinc tive behaviour is like a chain of compound reflex actions, each pulling the trigger of its successor ; but to many naturalists it seems necessary to postulate a psychical side also. In contrasting it with intelligent behaviour, emphasis should be laid on the fact that the routine performance as in swimming or diving when tumbled into the water, does not require apprenticeship, and may be exhibited in perfection the first time. Moreover, it is limited by the apparent absence of any understanding of the situation.
Instinctive behaviour is not all at the same level, for there is a gradation from short and simple activities, as in pecking, scratch ing, swimming, diving, flying, crouching and lying low, to long and complicated chains as in nest-building. Care is necessary to dis criminate inborn capacities from the results of early education on the mother's part. This may be done by hatching the young birds in an incubator. Thus Prof. Lloyd Morgan found that his chicks, incubated in the laboratory, paid no attention to their mother's cluck when she was brought outside the door. Although thirsty, and willing to drink from a moistened finger-tip, they did not instinctively recognize water even when they walked through a saucerful. Only when they happened to peck their toes when standing in water did they appreciate water as what was needed to relieve a state of dissatisfaction. But then, the bill being moist ened, they drank eagerly, lifting the bill to the sky in the instinc tively prescribed fashion. In natural conditions the mother-bird may play an important part in supplying stimuli which liberate instinctive predispositions; thus the great crested grebe takes the young ones on her back and then sinks beneath the water, leav ing them afloat.
Instinctive behaviour is often very precise and even specific, thus the cormorant's dive involves a complex series of move ments, yet it has not to be laboriously learned by the young bird. But in the big-brained bird there is more freedom than in the small-brained bee, and this is seen (a) in the control of instinctive capacities in response to parental training, (b) in a "trial and error method" of using the instinctive reactions, in novel situa tions, and (c) in the modification of instinctive behaviour in the light of intelligence. Lloyd Morgan's chicks stuffed their crops with worms of red worsted, but they did not make this mistake more than a few times. The captive cormorants in the Amsterdam Zoological Gardens learned to associate a line of bubbles rising to the surface with the possible presence of fishes.
In training educable birds like the weaver-bird (Ploceus baya) advantage is taken of the capacity to form associations. Given a particular visual signal, the bird picks out a particular card. Given a particular auditory signal, a parrot will utter a particular collocation of sounds, with an appropriateness often misleading to those who know nothing of the training. Very remarkable is the homing of pigeons, but there is no doubt that a graduated ap prenticeship educates an inborn aptitude. Very interesting and apart from definite training is the spontaneous establishment of useful associations, which may sometimes involve some awareness of the significance of the situation. A moorhen chick for which Lloyd Morgan used to dig worms, learned to run to him from a distance when he took the spade in hand. It need not be supposed that the bird had any intelligent appreciation of the spade as a digging instrument, the spade was simply an item in the mental registration of a pleasant experience. In natural conditions this type of association-forming must be common and invaluable.
As an instance of intelligent be haviour the following may be suggested. • Herring gulls lift sea urchins and clams in their bills and let them fall on the rocks below, thus breaking the shells. It is not necessary to credit the birds with thinking out the expedient, but it is difficult to evade the conclusion that what may have been discovered by chance, as often happens in mankind, is afterwards used intelligently.
The general impression left by a survey of bird-behaviour in such activities as nest-building and f ood-capture may be stated as follows. There is an instinctive basis, varying greatly in defi niteness in different types; on this is built up a superstructure due to parental education, imitation, simple trial and error experiment ing, and the establishment of simple associations ; but above this there are instances of genuine intelligence, which cannot be de scribed without crediting the bird with some appreciation of the relations of things, some power of perceptual (not conceptual) inference, in the light of which the behaviour is adjusted.
The circle of a bird's life intersects many other circles. Thus there are nutritive interrelations, such as the check that carniv orous birds, notably hawks and owls, impose on the multiplica tion of small mammals, other birds, some reptiles and amphibians, many fishes, molluscs, insects and worms. Similarly, there is the part that plant-eating birds play in destroying buds and young shoots, in digesting some seeds and scattering others. The list of "ornithophilous" flowers pollinated by humming-birds, honey eaters, sun-birds, etc., is a long one. In estimating from the human point of view the gains and losses involved in the nutri tive interrelations of birds, it is important to envisage the com plexities of the case. A bird that does much damage in a f ruit growing country may be useful elsewhere. An estimate of the desirability of a bird from man's point of view should take account of its activities throughout the whole year, especially when it has two main kinds of food, such as seeds and insects. Taking a broad view, the great majority of birds do far more good than harm.
ORNITHOLOGY, ECONOMIC.) Food Supply and the Abundance of Birds.—The oscilla tions in the abundance of birds in connection with changes in the food-supply and in the number of enemies are of much in terest. During a protracted plague of field-voles the number of short-eared owls has been observed to increase greatly; con versely, of course, the shooting down of owls and the like is apt to be followed by increase of field-voles. Destruction of rabbits may make foxes harder on pheasants; destruction of squirrels has been followed by great increase in the ranks of wood-pigeons, whose squabs are of ten eaten by the rodents. A great "lemming year" in Greenland keeps the foxes well fed, and the ptarmigan increase in numbers. But next year, when the lemmings are scarce and the foxes many, the ptarmigan suffer in proportion. In scores of ways—often very subtle--the swaying balance in fluences the surrounding fauna and flora.
Careful watch should be kept on changes in the feeding habits of birds, f or these may rapidly give rise to serious results. Thus the depredations of the kea parrot in New Zealand have been al ready referred to, and large numbers of sheep have been destroyed in California of recent years by an exaggerated carnivorousness on the part of magpies. Since the beginning of the 2oth century the herring gull and the lesser black-backed gull have enormously increased their ravages on agricultural land in the north of Scot land. In the harvest time they settle on the sheaves and gorge themselves with grain. They work systematically along the rows of turnips, gouging out the interior, besides making wounds which open the way to fungoid attack. In many cases a change of diet follows a rapid increase of numbers.
A few instances of intri cate interrelations may be given. Liver-rot in sheep is due to the presence of the fluke-worm (Distomum hepaticum) in the liver; the larval stages are parasitic in the small water-snail (Limnaea truncatula), whose numbers are reduced by such birds as the pied wagtail. Similarly, water-birds check the increase of various water-snails (Planorbis, Melania, Isidora, etc.) which are the vehicles of the young stages of the serious human parasite known as Bilharzia. Bubonic plague in India, due to Bacillus pestis, of ten begins in mills, where the workers eat their frugal meal in the courtyard. The "crumbs" attract rats, in whose blood the bacillus is at home. A rat-flea, with fouled mouth-parts, leaves the rat and infects man with its bite. But if there was a dovecot, whose inmates would promptly look af ter the crumbs, there would be fewer rats and less plague. Darwin tells of the large clodlet he took from the leg of a red-legged partridge (Caccabis rufa): "The earth had been kept for three years, but when broken, watered and placed under a bell-glass, no less than 82 plants sprung from it; these consisted of 12 monocotyledons, including the common oat and at least one kind of grass, and of 7o dicotyledons, which consisted, judging from the young leaves, of at least three distinct species." Small animals also, such as water-fleas, water-mites, wheel-animalcules, sponge-gemmules, and even small bivalves like Sphaerium, are of ten transported on the feet of birds from one pond to another.
Along with interrelations the parasites of birds must be included. The most important ectoparasites are: (I) the biting-lice or Mallophaga which feed on delicate portions of the feathers; (2) the quite unrelated true lice, belonging to the order Hemiptera, which suck blood; and (3) the skin-mites and scale-mites belonging to the Acarine order of Arachnida. Among endoparasites, besides microscopic Protozoa, such as the Trypano some found in the blood of owls, there are frequent representa tives of the three great classes of parasitic worms. Thus the Tre matodes may be illustrated by Distomum macrostomum in var ious Passerine birds; the Cestodes by Taenia anatina, the common tapeworm of the duck; and the Nematodes by the transparent Trichostrongylus pergracilis of the grouse. The three examples mentioned live in the food-canal, but other organs of the bird's body may be affected. The abundant Nematode of the grouse may serve to illustrate the important point that numerous parasites may be present without doing any appreciable harm. Every grouse has hundreds of these minute threadworms, but a mutual modus vivendi seems to have been established between parasite and host. On the other hand, if the grouse should be enfeebled by continuous bad weather, by lack of food, or by close inbreed ing within a weak stock, then the parasites may multiply enor mously and bring about fatal results in the body.
Some parasites occur in several different kinds of birds, but it is more common to find a particular species of parasite in a par ticular species of bird. This seems to illustrate the part that isola tion may play in the establishment of a species. A bird's parasites are adapted to it and cannot readily pass to a different kind of bird. Moreover the infection of the bird is usually bound up with specific feeding habits. On the other hand, there is need of cru cial experiments to test whether the successful transference of a parasite, say a Nematode, to a new host may not be f ollowed by the modificational assumption of structural features at present regarded as inborn and specific characters of another species.
(See also ORNITHOLOGY ; MIGRATION OF BIRDS ; BIRD SANCTU ARIES.) BIBLIOGRAPHY.-L. Stejneger, Birds (1888) ; Hans Gadow, "Ayes," Bibliography.-L. Stejneger, Birds (1888) ; Hans Gadow, "Ayes," in Bronn's Tierreich (1891) ; Alfred Newton, Dictionary of Birds (1893-96) ; F. W. Headley, Structure and Life of Birds (1895) ; A. H. Evans, "Birds" (Cambridge Natural History, 1899) ; W. P. Pycraft, Story of Bird Life (19oo), and A History of Birds (i9io) ; C. W. Beebe, The Bird, its form and function (1907) ; M. Hilzheimer and O. Haempel, Vogel: in Biologie der Wirbeltiere (Stuttgart, 1913) ; J. Arthur Thomson, The Biology of Birds (1923) ; G. M. Allen, Birds and their Attributes (Boston, 1925) ; A. Landsborough Thomson, Birds: an Introduction to Ornithology (1927). (J. A. TH.)
