Popular Science Monthly/Volume 62/December 1902/The Significance of the Condition of Young Birds at Birth
|THE SIGNIFICANCE OF THE CONDITION OF YOUNG BIRDS AT BIRTH.|
IT is a matter of common knowledge that the young of birds are ushered into the world in very different degrees of development, according to the species to which they belong. The helplessness of the callow young of the crow-tribe, for example, stands in strong contrast to the activity displayed by the young of the game-birds. Again the young of birds are remarkable for the very wide degrees of variation which obtain in the matter of clothing on their escape from the shell, variations which range from absolute nakedness to abundant feathering, albeit feathering of a peculiar type.
Out of these commonplace facts the systematic ornithologists, on the one hand, and the philosophical zoologists, on the other, have woven theories which have undergone many changes; but, so far, we venture to think, all have missed the point. A survey of the work of the systematic ornithologist will show that on more than one occasion the condition of the young at birth has been made either the corner-stone of a classification, or one of the main supports thereof.
For the one purpose or the other these young have been duly labeled and classified. In consequence, they may be contemplated from two different points of view: (1) According to their helplessness or otherwise, and (2) according as they are clothed or otherwise. When the young emerge from the shell in a fully active state, they are known as nidifugous or præcocial; those, on the contrary, which are quite blind and helpless on leaving the shell are known as nidicolous or altricial young. The nidicolous young may be, as we have already remarked, absolutely naked, in which case they are said to be ptilopædic. If, on the other hand, they are clothed, they are said to be ptilopædic. All nidifugous young are ptilopædic. The nidicolous young, being helpless and often blind, are assiduously fed by the parents, whilst among the nidifugous types, the young either feed themselves under the guidance of their parents, or accompany them in the search for food, and are fed by the way as the food is procured.
Two very different standards of thought have inspired those who selected the condition of the young at birth as a basis of avian classification. The older naturalists, adopting what we now regard as the purely artificial standards of their time, grouped birds according as the young were 'altricial' or 'præcocial,' nidicolous or nidifugous, thus creating an entirely arbitrary system of the same value as those other systems based upon the form of the bill, shape of the wing and so on. The post-Darwinians, working on the lines of evolution, see in these very different conditions, a phylogenetic significance, and regard the nidifugous as more reptilian than the nidicolous young. Consequently, those groups which have nidifugous young are to be regarded as standing comparatively low in the scale, whilst those with nidicolous young must be considered to have risen to a higher plane. This newer view is undoubtedly an improvement on the older, but il must, we think, give place to a yet wider interpretation.
If we turn from the purely systematic point of view to the more philosophical side of the question, as it at present stands, we shall, I think, find an equally unsatisfactory state of affairs. The two most recent text-books of zoology may be cited as authorities.
According to Jordan and Kellogg ('Animal Life,' 1901), 'those animals are highest in development, with best means of holding their own in the struggle for life, that take best care of their young,' and 'among the lower or more coarsely organized birds, such as the chicken, the duck, and the auk, as with reptiles, the young animal is hatched with well-developed muscular system and sense organs, and is capable of feeding itself,' but the offspring of the 'more highly organized forms, such as the thrushes, doves, and song-birds generally' are hatched in a wholly helpless condition, with ineffective muscles, deficient senses, and dependent wholly upon the parent. Similarly Shipley and MacBride write: "The manner the young are cared for is a most important feature. . . . The just-hatched young of the Pheasant and Game-birds are able to run about and look after themselves, whereas those of the Passeres or Songsters, require constant care and attention for a long time. These last are considered. . . to be the most highly developed of all birds, both as to their intellects and their flying powers, so that it is hardly too much to say that the increasing sacrifice of the parents on behalf of the young has had its reward, in the improvement it has brought about in all the faculties of the race."
Those who are responsible for the views just enunciated appear to have forgotten that the cormorants, for example, also bring their young into the world blind, naked and helpless, and not infrequently rear them in a nest of sticks on tree tops; yet the warmest admirer of these birds can not claim for them either a high degree of organization or great intelligence among birds. Their near allies, the darters, gannets, tropic and frigate birds also have helpless young, which in the case of the frigate birds and darters are also reared in nests in trees. We might multiply instances, but these are sufficient for our present purpose. They show at least that 'sacrifice of the parents on behalf of the young' has not been uniformly rewarded 'in the improvement ... in all the faculties of the race..' More than this, they seem to me to show that this factor has had little or nothing to do with either inclination or structural development.
The real explanation of the matter seems rather to turn upon a question of expediency, designed, so to speak, to reduce infant mortality.
We shall show presently, on evidence well nigh incontrovertible, that the nidifugous condition is indeed a primitive one, but associated with a strictly arboreal habitat. This is an important point, as the nidifugous condition is commonly regarded as peculiar to, and possible only in, a terrestrial habitat. Let us assume for the moment that the former is an established fact.
One great disadvantage attendant on precocious development of the young whose nursery is the tree top is obvious the nestlings would be constantly in danger of falling to the ground, and a large number would indeed meet this fate. Some would fall through weakness, the habit of dispersing themselves among the branches of trees in which the nest was placed resulting in a loss of regular food supply, owing to the difficulty of being on the spot when the parents returned with food. Thus the more sedentary members of the family would stand the best chance of being regularly fed, but among these the danger of falling by accident would be an ever present one. Once on the ground it is probable they would perish speedily, for it is almost certain that the earliest birds were entirely arboreal, and either would not or could not seek for lost offspring amid the thick undergrowth.
Now two courses were open whereby this infant mortality could be reduced. Either the eggs could be deposited on the ground, or the activity of the young curtailed. The game-birds, ducks and geese, rails, cranes and plovers may serve for examples of those species which have descended from the trees to the ground for nesting purposes, and although, as a consequence, the young have undergone considerable modifications in adaptation to the new environment, these changes are not so striking as those which have taken place among the young of the tree-dwelling species to be described presently.
The modifications which we should expect to find in the offspring of those species which, instead of curtailing the activity of the young, descended to the ground to breed, would be (1) peculiar habits of concealment aided by protective coloration; and (2) a reduction in the size of the wings and feet, now no longer required solely as grasping organs.
Protective coloration and peculiar habits of concealment are obviously direct responses to the increased need of escaping enemies, and hence we find these devices have been universally adopted. It is possible, however, that protective coloration was preceded by the development of precocious powers of flight, which have since been discarded by all save the game-birds. Probably this rejection was brought about because excessive activity on the ground was found to be as fatal as in the trees; since the young, in escaping from one danger, would be liable to run into another, or to stray too far away to render return possible.
At the present day, though the young of all the game-birds are protectively colored they have yet preserved more or less perfectly their earlier precocial powers of flight, the birds, when escaping danger, using their wings either like the ostrich, as an aid in running, or in actual flight, and there is evidence to show that the broods in consequence suffer. As an example, the observations of Mr. Ogilvie Grant on this subject may be cited. In writing of the common pheasant, he tells us that the mother, on alarm, with a warning note to the young, at once flies off and leaves them to take care of themselves. This they do by scattering in all directions, and then squatting down and trusting to their protective coloration for safety. Quiet restored, the parent returns, often only to recover but three or four of her chicks, the rest having strayed to such a distance that they are left to perish.
Thus, then, the hypothesis of precocious flight seems by no means an improbable one. Its development will be easy to understand when it is remembered that the raw material therefor is furnished by that aberrant member of the game-birds—the hoatzin. The life-history of this bird will be discussed later.
We may pass now to a consideration of those species which, retaining their arboreal nesting habits, have adopted the method of curtailing the activity of the young. This process was accomplished by reducing the food-yolk within the egg, and thus inducing an earlier hatching period. We may approximately measure the extent to which this reduction has been carried by the degree of helplessness displayed by the newly hatched bird, and by the nature and extent of its clothing.
The number of species which have adopted this expedient outnumber those which have not, and this speaks volumes for its success. As examples, we may instance the passerine or song-birds, parrots, cuckoos, birds of prey, cormorants and their allies, and the storktribe. The young of these are all born extremely helpless, many perfectly naked, others enveloped in a thick coat of down, whilst in some down is developed soon after hatching, and, in a few, not at all.
Having once however reduced the amount of food-yolk, return to the older fashion of nidifugous young became impossible, and this explains why nidicolous young are still born to those parents which have adopted the practice of depositing their eggs upon the ground. It proves that the arboreal habit has been forsaken since the specialization. Some, like the cormorants, herons and certain of the gull-tribe, for example, build as occasion demands, either on the ground or in trees. Now it is interesting to note that among these birds the young cormorants and herons are completely nidicolous, the young gulls only partially so, whilst the near and less specialized allies of the gulls, the plovers, have nidifugous young. This indicates that in the gulls the food-yolk is in process of reduction. To species breeding in large colonies, or on ledges of precipitous cliffs, the reduction of the food-yolk and helplessness of the young are obviously advantageous.
That this is so may be seen in the case of the colony-breeding species, since it would be impossible for the parents to recognize their own offspring, if nidifugous, when running about amid those of their neighbors. In consequence a large number would almost certainly go unfed and soon starve, whilst great activity among the young of the cliff-breeding species would be accompanied by an enormous mortality, owing to falls from the cliff.
It is contended that the facts so far submitted amply justify the interpretation put upon them, but the following instances should carry conviction. In the aberrant South American game-bird, the hoatzin, we have probably a direct survival of the protoavian type of nestlings. They are, of course, nidifugous, but they differ from all other nidifugous young in the prehensile character of their wings, which are armed with large claws borne upon the thumb and index digit. Claws on the wing are common among birds, and hitherto they have been regarded merely as vestiges—indices of a reptilian ancestry. The part which they play, however, in the life-history of the hoatzin, coupled with certain correlated modifications to be discussed presently, shows that they have a wider significance than this.
The adult hoatzin is an absolutely arboreal bird, inhabiting the dense scrub and trees bordering the lagoons and river banks of British Guiana and the Amazon Valley. Its powers of flight are extremely limited, and it has never been observed to alight upon the ground.
The young, like those of other nidifugous birds, are clothed in down, conspicuous, in the present instance, for its hair-like appearance and sparse distribution. But whilst the locomotion of the nidifugous young of other birds is bipedal that of the hoatzin must be described as quadrupedal, the wings as well as the legs being brought into requisition as the birds make their way along the branches. Even the beak is sometimes used, as in the parrots.
In a wing used as a prehensile organ we should expect to find certain peculiarities which would not be observable in the normal wing. These are not wanting. One of the first points which attract attention in the examination of such a wing is the great length of the hand, which is considerably longer than the forearm. The thumb is found to be unusually long, and to extend beyond the level of the tip of the third digit. Both thumb and finger are armed with large claws. The index finger is furthermore remarkable in that it is produced beyond the fold of skin which runs along the hinder or post-axial border of the wing for the support of the quill-feathers. Examined further, the palmar surface of the thumb and second finger are found to be swollen into little cushions resembling the cushion-like undersurface of the finger-tips of the human hand. Next the budding quill-feathers attract attention, and if a series of young is being examined, probably the first point to be noted is the fact that the development of the quill feathers of the hand were peculiar inasmuch as in the older forms whilst the inner quills are found to have pushed their way some considerable distance beyond the post-axial border of the wing, the outer quills are only represented by simple down-feathers. Thus, a long, free finger-tip is left beyond the quills. The thumb also, as yet, bears only down-feathers, the future quills being conspicuous by their absence. On a little reflection the meaning of this becomes clear.
The arrested development of the quills of the thumb and the tip of the finger is an adaptation to the bird's peculiar needs, albeit a deepseated character, dating from the dawn of avian development. If all the quills were to grow at an equal rate, a stage would soon arrive when the wing would be useless as a climbing organ, by reason of the developing feathers and so expose the bird to constant danger of falling before the quills had sufficiently developed to break the force of such a fall. Thus then the arrested development of the quills begins to look as if it might have a definite meaning, and this becomes a certainty when still older specimens are examined. In them we find that as soon as the inner quills of the second digit have grown sufficiently long to enable the bird to recover itself in falling the hand begins to shorten, and the claw to diminish, till at the time of puberty the hand has become shorter than the forearm, the claws both of the thumb and finger have disappeared, the thumb no longer extends to the level of the third digit, and the second finger no longer projects beyond the hinder wing fold (post patagium).
That the structural peculiarities observable in the wing of the hoatzin are not recently acquired characters can not be doubted. The presence of the claws is almost sufficient to prove this, for having once become vestigial it is unlikely they would reacquire their primitive size.
But we have other evidence affording the strongest confirmation of the contention that the wing of the hoatzin represents an ancient order of things once common to all birds. This evidence is 'writ large' upon the wing of those allies of the hoatzin, the common fowl, the turkey or the pheasants, for example. Herein we find the same developmental stages, but with certain modifications easy to interpret.
In the limb itself—as considered apart from the appendages, the incipient quills or flight-feathers—one of the first things to attract attention is the hand, which although relatively shorter than that of the hoatzin, is still longer than the forearm; next the cushion-like pads of the thumb and second finger are missed, as also are the claws. That of the thumb, however, is generally present though in an extremely reduced condition, and in the index finger it appears only during embryonic life.
The flight feathers again reveal some very interesting features, inasmuch as the inner quills develop at a rate relatively much greater than in the hoatzin, so that they become functional earlier in the lifehistory, whilst the outer quills, three in number, are still only represented by delicate down feathers, thus, be it noted, leaving a free fingertip as in the hoatzin. The abrupt changes from quill feathers to nestling down observable in the wing of the chick and turkey seem to show that the quills have undergone a process of forcing or accelerated development, in which the inner quills have developed at an excessively rapid rate, so as to out-distance their fellows at the distal or outer extremity of the wing, which as yet are only represented by nestling down. The rapid development of the inner quills is probably due to the fact that the terrestrial mode of life demanded the aid of the wings for the purposes of flight at an earlier period than would be the case if they dwelt, like the hoatzin, in comparative security among the trees.
The developmental history of the wings of the fowl and its allies seems to leave but little room for doubt that the ancestors of these birds, like the hoatzin, were hatched in trees and crawled about among the branches. Moreover, the change from an arboreal to a terrestrial nursery seems to have taken place comparatively recently. On no other assumption can we explain the free finger-tip and the arrested development of the outer quill feathers. Nevertheless, a sufficient time has elapsed wherein to bring about the suppression of the claws. That of the index digit, being no longer useful, appearing later and later in development, has now entirely ceased to put in an appearance save only during embryonic life; in other forms, separated by a still greater lapse of time from their tree-crawling ancestors, even the embryonic claw has ceased to be.
Of considerable importance is the fact that whilst in the hoatzin and the fowl and its allies the quill feathers appear long before the contour feathers of the body—that is to say, whilst the body is still clothed in down—in the nidifugous young of ground-breeding forms such as the plovers, for example, which seek protection by concealment alone, unaided by flight, the quill feathers appear together with the contour-feathers of the body and at a much later date than in the above. So also with the nidifugous young of aquatic forms.
The accelerated development of the quills is probably a remnant of an earlier phase in the life-history before protective coloration was adopted. As we have already shown, precocious flight is attended by too many perils to prove an effective means of escape from enemies.
In conclusion we may say a word about the young of the megapodes. The eggs of the megapode are, as is well known, hatched in decaying vegetables heaps, or in hot sand, instead of being incubated by the parents. To this end the amount of food-yolk within the egg has been enormously increased, enabling the normal nestling period to be passed within the egg, the young passing through the downy stage during embryonic life, and emerging from the shell, fully fledged.
That the ancestral megapode was originally hatched in trees like the young hoatzin, there can be no doubt, since like the latter the wing of the young shows a free finger-tip and an arrested development of the outer quill feathers, characters which, as we have already seen, are direct adaptations to the peculiar locomotion of tree-climbing nestlings. We may be almost certain that the increase in the food-yolk, just referred to, did not take place until some time after the descent to the ground for breeding purposes, since the wing of the young megapode forms an exact counterpart of that of the young fowl and turkey, and their allies, whilst, had the increase taken place earlier, the wing would have resembled that of the hoatzin in the possession of large claws. The latter are present now only during embryonic life.
The increase in the food-yolk, allowing the earlier nestling stages to be passed within the shell, must be accounted for by supposing the adult megapode to have been obliged to adopt this expedient to avoid perils attendant on normal incubation, perils which may since have passed away leaving no record of their nature. A return to the normal method of incubation is now impossible, the instinct therefor having been replaced by that which induces the birds to bury their eggs and leave them to be hatched by heat other than that of their bodies.
Finally, we may compare the hoatzin with the ancient archæopteryx, and the result of such a comparison will go far to prove that the former represents the most primitive of living birds.
That archæopteryx was strictly arboreal there can be no doubt—the structure of the feet indicates this much; and its long tail is a scarcely less certain index, for such an appendage is undoubtedly but ill-adapted for ground-dwelling habits. The long hand, and the large claws thereon are, so to speak, primitive characters. The period of their greatest functional activity was during the nestling stage when the young clambered about among the branches of the trees like the young hoatzin of to-day. The species was perhaps not phylogenetically old enough to eliminate the traces of nestling-life on reaching puberty, when the quills rendered claws, and climbing, not only needless, but impossible; hence then the retention by the adult of both elongated manus, free finger-tips and claws. It has been suggested, however, that the claws and elongated manus of the adult archæopteryx were periodically functional, the periods being the moulting seasons. It is well known that many existing birds, the anseres to wit, when moulting, shed all the quills at once, and in consequence are for a season flightless. It may well be that this system of losing the quill is a primitive one and obtained in archgeopteryx, in which case it is obvious that in a bird so strictly arboreal, the climbing hand of infancy would be of some service. The more usual method of moulting the quills in couples prevailing amongst modern birds so as not to impair the power of flight has probably come about by selection, and hence the reduced and clawless hand of the adult hoatzin, fowl and turkey for example; birds in which, for reasons already explained, the primitive form of manus is still temporarily exhibited by the nestling.
Thus then, through the wing of the hoatzin we have a revelation of a phase of bird-life hitherto unsuspected; inasmuch as its peculiar developmental stages, each with its period of functional activity, enable us to interpret the hitherto meaningless and puzzling characters seen in the wing of the fowl and turkey, and their allies. These constitute wellnigh invincible proofs of an earlier and universal arboreal existence, extending back to the time of the earliest known bird archæopteryx. Certainly the skeleton, especially the wing, lends the strongest support to this view. This carries us further back still, and suggests the conclusion that the reptile stock from which the aves are descended was probably also arboreal.
That too much stress has been laid by systematists on the condition of the young birds at birth is admitted. It is further maintained here that its significance has been misunderstood, and that the facts now brought forward are strong enough, on the one hand, to refute the older views, and on the other, to justify the theory, firstly, that birds were originally arboreal and their young nidifugous; secondly, that nidicolous habits and helplessness of young birds are specialized adaptations to an arboreal or gregarious mode of life; and, thirdly, that the young of gallinaceous birds form a link in the chain of the evolution of nidifugous habits. The free finger-tip and arrested development of the outer quill-feathers point to a prior arboreal habit; whilst the accelerated development of the inner quill-feathers indicates an adaptation to enable the young to escape from the enemies surrounding a terrestrial nursery. The third and last stage is represented by the protective coloration, a device which has been almost universally adopted by nidifugous birds, owing to its greater effectiveness.