Popular Science Monthly/Volume 77/July 1910/Instinct and Intelligence in Birds II
|INSTINCT AND INTELLIGENCE IN BIRDS—II|
THE cyclical instincts of birds, present, as we have seen, a well ordered series, rising and waning in due course, until the reproductive cycle is complete. Nevertheless, the order and harmony which commonly prevail are subject to many disturbances of a transient, or of a more lasting character. When variations in the cycle, whatever their nature, become regular and permanent, any consequent loss or to the species seems to be counterbalanced by the rise of new instincts in both young and adult, which may involve marked structural changes, as shown in the parasitic cuckoos of the old world and their non-parasitic relatives of the new. If transient merely, there is more or less individual loss, according to the nature and extent of the disturbance.
We shall now consider some of these variations in the cyclical series, and we may assume, though with little exact knowledge, that when any character of the sort to be described has become general or permanent this has been effected through a gradual process of selection, with or without environmental influence and other unknown agencies. We may further assume that all modern birds originally built proper nests, and there can be little doubt that many either falter or fail in this work at present through the loss of an instinct which they once possessed; but this question aside, we can be reasonably assured that all originally concealed or guarded their eggs.
The nest, in the first instance, tends to secure a more equable distribution of warmth and moisture for eggs or young; incidentally it may conceal and therefore protect both young and adult, and add to the comfort of the whole family. There would seem to be a vast difference between digging a hole in the warm, moist sand, as we see the turtle, or the, one of the brush turkeys, doing, and weaving through the unremitted efforts of many days, a beautiful pouch like the oriole's, so admirably adapted for protection, both by its form and by its position. Yet it is by no means certain that the fundamental nest building instinct is entirely wanting in the , the peculiar habits of which will be later considered.
Nest-building of one kind or another is found in all classes of vertebrates, and the guarding and fighting instincts at nesting-time are as strong in some of the fishes as in birds, but while the practise is clearly of ancient origin, it is by no means universal; it seems in every case to be related to the needs of the animal, and to be a refinement of more simple means of securing both concealment and protection.
The causes of the disturbances, which we have to describe, are wholly obscure. We can only surmise that they may have their origin in changes in the central nervous system, which, as one result, bring about disturbances in nutrition, leading now to a premature, now to a belated development of the reproductive cells. At all events there arises what may be crudely described as an "overlap" or "blending" of instincts. Or, we may say that in the struggle of conflicting impulses victory now goes to one side, now to the other. The only facts that are really known are that the egg sometimes anticipates the nest, instead of the nest the egg, or that the migratory impulse may emerge too soon, and nip the proper parental instincts in the bud, before they have run their course. We do not doubt that the sifting process of selection would soon curb any tendency, like the last, in every species which was destined to survive.
The eccentricities of behavior, which we attribute to disturbances in the breeding cycle, will be examined under the following heads: (1) Beginning a new cycle, and scamping an old; (2) multiple and superimposed nests; (3) eccentric behavior due to conflicting instincts; and (4) premature laying of eggs, omission of nest-building and parasitism.
II. Beginning a New Cycle or Scamping the Old
When the cyclical instincts rise and wane in their proper order, they may be represented by a series of circles tangent to each other, or with but little overlap (Fig. 15). Beginning at term 3 or 4, a cycle is
completed up to term 7 for each brood successively reared. Most wild birds in this part of the world have but a single brood in the season. The success of any individual pair depends upon circumstances. Storms and predaceous animals of all kinds break down the nests or destroy the eggs, when a fresh start is usually taken.
In very timid birds like the cedar waxwing, the cycle is often abruptly ended at term 3 or 4, as a result of fear, through discovery or disturbance of the nest, and a new series is promptly begun at 3. This is the simplest type of disturbance which we can record (Fig. 16).
The old nest may be torn down by the little builders, and its materials used again, but this does not commonly happen. Since fear is rapidly depressed, with the rise of the brooding instinct, beginning at term 5, interruptions are less liable to occur after this point is reached, but wherever the thread is dropped, it is usually picked up again at stage 3.
Of far greater interest is the fact that a new cycle may be begun at the very close of the breeding season, when it seldom goes far, and is bound to fail for lack of time. Probably no stronger witness to the instinctive basis of the behavior of birds could be found than this recrudescence of the reproductive activities at a time when most must answer the fatal summons of the migratory impulse. It is typically illustrated by the great herring gulls, which toward the close of their usual cycle in mid-July begin to build new nests, and will even lay eggs in them, though all are eventually abandoned. It would not be surprising to find that many young were also left to their fate, but my observations have never extended late enough to determine this definitely. At the Great Duck Islands, Maine, where these facts were gathered, the birds arrive early in March, and depart about September 1, according to the warden and lighthouse keeper, Captain Stanley, who has found that the first eggs are laid about the middle of May, while the first young begin to appear the second week in June.
In a census of one hundred nests of this gull taken on the island July 17, 1902, at the close of the breeding season, some interesting facts were brought out, which may be summarized as follows:
|Abandoned empty nests, from which young have been reared||64|
|Abandoned nests with addled eggs||8|
|Nests with chicks outside||5|
|Nests with newly hatched chicks or pipped eggs||1|
|Nests with fresh or slightly incubated eggs||5|
|New nests, begun or completed||17|
Fig. 17. Imperfect nest of Great Herring Gull, illustrating the beginning of a new breeding cycle, when too late to be finished. Chips, grass, and roots have been brought to the new site. Great Duck Island, Maine, July 20.
Of the seventy-seven nests which had seen service in the season, eight only contained addled eggs. It was certain that none of the new nests, with or without eggs, could ever come to anything, and probably most were never finished. They are made to be abandoned, sooner or later, with the rise of stronger instincts. A new cycle is begun, but
Fig. 18. Nest of Gull, illustrating the same tendency as that shown in Fig. 17, but where the old nest was "repaired," or used as a site for the new one. The body of a dead chick was worked into this nest, and the old bird was incubating an addled egg. July 22.
Fig. 19. Gull's Nest on Rotten Log, with eggs starred and nearly ready to hatch, probably belonging to a bird which had earlier failed, but had renewed its activities in time to rear young. July 20.
stayed at terms 3 or 4. Now it might be supposed that those nests which appear in middle or late July (Figs. 17-18) were the work of young birds, or of others which for some cause had not met with earlier success, but this is certainly not always the case. For the space of several day's I watched a pair of gulls, which had large chicks to feed, and repeatedly saw them leave their young and begin the construction of a new nest about a rod from the old one. The female would split
Fig. 20. White-beaded Eagle's Nest of the First Year, built in 1900, by the owners of the eerie shown in Fig. 21, which was destroyed the previous winter. The nest is considerably broader than tall.
chips, carry them to the chosen site, and go through the instinctive moulding and turning movements in the most approved and characteristic manner. The male even mounted the female, and was borne on her back like a circus-rider, in his evident attempt to perform an act
Fig. 21. Ærie of Eagle occupied Fifteen Years, and nearly twice as tall as broad; the predecessor of the nest shown In Fig. 20 In dead sycamore, three and one half feet in diameter at base; top of nest 77 feet from ground.
which is usually necessary. In this case, however, eggs were not destined to appear, and the new nest was eventually given up. This sporadic attempt at nest-building, while there are still chicks to be nursed, illustrates what we have described as the conflict of opposing instincts.
At the beginning of the breeding season in the gull, old nests are frequently reclaimed, and possibly by the same birds, though this has not been determined, or a new site is chosen, and a new nest built. If an addled egg is left after the others have hatched, "repairs" to the nest are frequently undertaken, and the old egg is either incubated for several days longer or it is buried out of sight. A pair of gulls, which was watched from the tent, had a single chick, and this one lived only long enough to crawl out of its shell, while a second egg was bad. No sooner was the little one dead, than the work of reconstruction, that is building on the old site was begun, and the body of the chick, treated as so much nesting material, was soon buried under new layers of grass and chips (Fig. 18). This labor lasted for four days, or as long as I was able to watch it, but as in the other cases described, it was sure to be futile owing to the lateness of the season.
Fish hawks and eagles are known to return to their old nests year after year, adding fresh materials, that is, building on the old site, each season. An eagle's nest of the first year (compare Figs. 20 and 21) is broader than tall, but with the yearly increment of stubble and sticks added to its top, it gradually rises in vertical height, until becoming so much taller than broad, in certain situations it tends to topple over from sheer weight. The older of the two nests of the white-headed eagle, which are here shown (Fig. 21), was begun in the crotch of a dead sycamore, 77 feet from the ground at North Springfield, Ohio, in 1885, and occupied for fifteen years, or until January, 1900, when this ancient landmark was laid low in a storm. With the aid of photographs, taken in May, 1899, and by actual measurements which I later made on the prostrate tree, the dimensions of this great nest were exactly determined. It was nine feet high and six feet in diameter, or three feet taller than broad, and contained rather more than three cubic yards of wood, earth and stubble. The new nest (Fig. 20), which was built in the spring of 1900, was examined and photographed in June of the same year; now after the lapse of a decade, it has much the appearance of the older nest, having risen greatly in height. Such a structure might be regarded as a kind of "multiple nest," being composed of increments, corresponding in number to the years of occupation, the last "nest" being built on the site of that of the previous year.
But a more interesting fact, if true, is the statement of Audubon and others that ospreys and eagles often repair their nests in the autumn, as if in anticipation of the needs of the coming year. We can readily accept the fact, but not the interpretation, for if such a practise really occurs, it is plainly due to the rise of a new reproductive cycle, which is begun but soon checked. The sporadic return of the nestbuilding instinct at the close of the season is essentially the same in hawk or gull, and can imply no more intelligent forethought in one case than in the other.
Desertion of the Young under the Impulse of Migration
The struggle of conflicting instincts is clearly shown when the normal cycle is brought prematurely to a close by the rise of the instinct of migration, when eggs or young are left in the nest to perish.
Fig. 22. A Multiple—four-storied Nest of the Yellow Warbler (Dendroica Æstiva), with a "parasitic" cowbird's egg in each compartment, but with proper eggs in the first story only, illustrating the successive breaking of the breeding cycle through fear, the beginning of three new cycles in succession, the new nest being built in each case, and probably through association, on the site of the old, thus admirably "concealing" the successive parasitic eggs. Original in the Field Museum of Natural History, Chicago.
The migratory impulse seems to "overlap" and finally to replace the proper parental instincts. The cycle is scamped near its close.
A classical illustration of this struggle of instincts was furnished by Dr. Jenner, in his "Essay on the Migration of Birds," published in 1824, and by the more circumstantial account given by Dr. John Blackwall in 1834, who called particular attention to "the occasional desertion of their last hatched broods by the swallow and house martin." Blackwall was a keen and discriminative observer, but his work is so little known that I shall give a summary of his valuable and interesting results, under this head.
The swallow arrives at Manchester, England, about April the fifteenth, and the house martin on the twenty-fifth of the same month. They produce from two to three broods in the season, and are commonly found with nestlings in October, at a time when most of the migratory species have left the country. Many of these young which are led out of the nest, are deserted before they are able to follow their parents south, and have been found in a state of semi-or total exhaustion, late in the year. This, as Blackwall ingeniously suggests, may have given rise to the queer notion that the European swallows passed the winter season in a state of torpidity.
Blackwall's observations were begun as early as 1821, and when on November 11, 1826, twenty-two nests under the eaves of a barn in the Chapelry of Blakeley were carefully inspected, it was found that thirteen of this number contained either eggs or dead nestlings; five nests held eggs in every stage from the freshly laid to those at the hatching point, while the eight with young showed nestlings in every condition from that of hatching up to the nearly fledged state.
While the female swallow may exceptionally linger longer than the male, it should be noted that both parents commonly abandon their young at the same time. The same fatal conduct was also frequently observed in the sand martin, and Gilbert White, of Selborne, has given an interesting account of a swift, originally noted by him in 1781, which renders it practically certain that this bird may also desert its young, when the migratory impulse is strong. According to Pennant, who is quoted by Blackwall, the puffin is in like case also. The parental instincts of the puffin are strong, and the first young, which appear early in July, are guarded with the utmost care. But strong also is the instinct of migration, and when this emerges punctually at about the eleventh day of August, any young puffins which can not fly are left to the tender mercies of the peregrine falcon. This vigilant plunderer watches at the mouths of their holes, ready to seize them with mailed foot the moment hunger forces them to surrender. We may be quite sure that the young of the species enumerated above are not the only victims of the struggle of conflicting instincts. I have heard of similar behavior on the part of the domestic pigeons.
III. Multiple and Superimposed Nests
We have referred to the towering Eerie of the eagle and osprey as being, so far as instinct is concerned, a series of superimposed nests; indeed, any nest built on the site or over the ruins of a former abode, might be regarded in this light. When attachment to the site is strong, the bird, like the peasant in ancient Egypt and many of the earlier races of mankind, builds anew on the ruins of his former home, without taking care to clear the ground or raze such parts as still exist. The result is similar in either case—a series of superimposed structures of different ages, the height to which the pile may rise, depending upon the number of times the same site has been used.
The building of nest on nest, or of new nest on the site of the old, according to this interpretation, gives rise to the wonderful storied structures sometimes produced by the yellow warbler, or vireo, when plagued by the cowbird. That the intruding egg is buried out of sight is not due, however, to a feat of reason on the part of the suffering bird, but is the curious result of a nearly pure instinct, modified only by association. Fear breaks the cycle, but it is not always strong enough to break the habit of going to the old site. Instead of two or more supernumerary nests, more than one of which may contain eggs, and even stand side by side, as has been reported in the case of the phcebe, we have a series of superimposed nests, as is clearly illustrated iu the remarkable four-storied structure of the summer or yellow warbler, here shown (Fig 22). Each section of this composite, moreover, is seen to contain an egg of the parasitic cowbird, that in the first story being partially concealed by the warbler's eggs present.
According to this view, the new nest is not built to conceal the cowbird's egg, although it does so perfectly, any more than the addition of new materials to an osprey's nest in the fall is of the nature of repairs, although it may answer such a purpose admirably. The nest is built or "repaired" because the bird is at the opening of a new cycle, and is impelled to action by the rise of the building instinct. Whether the new nest is built upon the remains of the old, or close beside it, or half a mile away, must be attributed to the ordinary workings of instinct, modified by association and fear, when for some cause the normal cycle has been disturbed.
The so-called "cock nests" of the little marsh wrens may prove to be only another illustration of the supernumerary nests given above, but no opportunity has yet been offered to study these interesting structures. The fact that they may be used secondarily as sleeping apartments, if this is really the case, has no special significance. I have seen the abortive hole of a kingfisher so used, but a few rods from the bank, which was later successfully drilled and occupied by the same pair. The subject of compound nests is too long and involved for full discussion here, but from the builder's standpoint, which is that of instinct, I think there is ground for regarding such a composite structure as that reared on the cooperative plan by the ani or Savanna blackbird, as in reality a multiple nest.
IV. Eccentric Behavior; Robin offering String to Young
Under this head we shall describe a special case of what we have frequently referred to as the "overlap" or struggle of competing instincts. The incident happened in a neighbor's yard on One Hundred and Second Street, Cleveland. A pair of robins had nested in this yard, and successfully reared young, which were then hopping about in the speckled-breasted stage, and begging for food. On a certain occasion one of the parents was seen offering a long piece of twine to one of the youngsters, and trying to cram it into its throat. This robin would repeatedly gather up the string, as it would the coils of an angleworm, and offer it in the usual way, but string not being to the taste of this fledgling, it was as often rejected. After a time the old robin flew with the string into a tree.
With these facts in view, how shall we interpret such extraordinary behavior? We consider this case of the robin a most unusual and interesting exhibition of the conflict of opposing instincts, for according to this idea, the bird was at the close of an old reproductive cycle, and the beginning of a new one. She had fallen, as it were, between two stools. Impelled by the rising instinct of nidification, she gathered the string, when aroused by the calls and sight of her young she was induced to offer it; again under the sway of the building instinct, she flew with the string to a tree. We can judge of the sequel, although unfortunately observation on the robin's conduct stopped at this point. The popular interpretation that the bird was crazy gives place to something which we can measurably understand, or coordinate with other related facts. On the other hand, what a commentary such an act furnishes upon the effective intelligence of birds, when under the sway of powerful instincts. Does not the robin know a "hawk from a handsaw," or a worm from a piece of string? The behavior of the great herring gull with chicks still requiring her care, in going through all the motions of nest-building, and returning to her young again, would seem to be similar in all essential respects.
V. Premature Laying of Eggs, Omission of Nest-building and Parasitism
Lack of attunement between the appearance of the nest and eggs, or terms 3 and 4 of the cycle is very common. Too frequently the egg "anticipates" the nest. Every one who has given much attention to the activities of birds in the field must have found isolated eggs lying on the ground. Such prize packages are probably more common than we might be led to suppose, for they can not long exist wherever snakes, rodents and other prowling animals abound.
With most birds the act of prematurely dropping an egg can be only a sporadic or casual variation. Without doubt, in the course of time a proper nest is built; eggs are laid, and the normal cycle is rounded out to completion. It is quite possible, on the other hand, that all such eggs are not immediately neglected, but that they are sometimes carried away, and "concealed" by dropping them in another bird's nest, although we have no observation to support such a view directly. It is known, however, that certain birds, such as the blackbilled cuckoo, will upon disturbance remove its eggs from the old nest to a new one or to a place of safety. It is also certain that the premature egg is at times laid direct in another bird's nest, which the intruder will often strive to possess by force, and may even succeed. Thus, Davidson, who is quoted by Bendire, found a black-billed cuckoo and a mourning dove sitting on a robin's nest together. This nest was in reality double, and contained two eggs each, of the cuckoo and dove, and one of the robin. The cuckoo managed to get possession of the nest before the robin had finished her work, and filled it with rootlets, but the robin held its ground long enough to deposit an egg. The fact that the cuckoo had "filled it nearly full of rootlets" is a very interesting circumstance, for it shows how completely instinct held the reins of action. This robin's nest seems to have served as a site on which the cuckoo strove to erect one of its own. The dove, noted for its strong parental instincts, had evidently come last, and her eggs were the only ones in which incubation had not begun.
Such a case seems to present us, as in a picture, with one of the steps in the process through which the most remarkable of all the known instincts of birds, that of parasitism, has been brought about.
Certain cowbirds of the new world and cuckoos of the old steal the nests of other birds, but usually only long enough to deposit an egg of their own, which is left to its fate. If tolerated, as is apt to be the case, the stranger is hatched with the other eggs, and the owner of the nest assumes the role of nurse or foster-parent. If a cowbird, the foundling soon smothers the proper young, and if a cuckoo, it evicts them. The cuckoo seems to react to a contact stimulus of a disagreeable kind, and when from one to three days old, while still blind, it strives to get egg or nestling on its broad, depressed back, and hitching its way, thus laden, up the wall of the nest, throws them overboard. If such a bird is replaced after a time, the same movements are repeated. With the coast thus clear, the little "parasite" can monopolize the attention of its nurse, and grows apace, being attended with all the care which is bestowed on a legitimate child. As Philemon Holland has quaintly rendered the account of the elder Pliny:
It is evident that this practise of nest-stealing, somewhat ambiguously called "parasitism," could never become very popular or widespread, for it would soon break down of its own weight.
For over two thousand years, or since the time of Aristotle, who was the first to leave a permanent record of this propensity in the European cuckoo, the question has been asked, How could such a habit arise? and the answers have been various, and far from satisfactory. The key to the matter lies, as we believe, in the cyclical instincts, and in the disturbances to which they are prone. When the normal rhythm is generally disturbed or permanently changed, new instincts and even new structures may arise, which serve as a counterbalance to the changes wrought.
We believe that the instinct of parasitism got its start through lack of attunement in terms 3 and 4, of the reproductive cycle, and that it has passed through essentially the following stages: (1) The egg forthcoming before there is a nest ready to receive it, a condition sporadic in very many, if not in most modern birds, due to unknown causes, such as lead to a premature growth of the ovary, or to a disturbance of certain instincts. There is a loss of eggs, although a nest may be eventually built, and young reared in the season. (2) The eggs are ready before the nest, and many are lost by dropping them on the ground, while others are laid in stolen nests. A proper nest is sometimes built, but whether young are ever reared, will depend upon circumstances. This stage is exemplified by the Argentine cowbird (Molothus badius), described by Hudson, which commonly wastes its eggs, scattering them in all directions, yet it will steal a nest upon occasion, or build one of its own. It even laid eggs in artificial nests, which Hudson placed in trees to test its propensities in this direction. (3) The common practise of stealing nests of other birds, but of holding them, as a rule, only for laying its own eggs, as illustrated to-day by the North American cowbird (Molothrus pecoris). The instincts of the intruder seem to be satisfied by "concealing" its eggs, or simply laying them against the wall of another bird's nest, and leaving them. At this stage the European cuckoo, we may suppose, not only frequently dropped its eggs on the ground, but occasionally tried to incubate them, and may have even attempted a rough nest. At this stage also the normal tendency to lay eggs at daily intervals was possibly disturbed, and the interval became irregular, with the gradual establishment of a longer rhythm.
At this point several roads would seem to be open, for the resources of nature are not limited to one course. Parasitic or non-brooding cuckoos have "chosen" one, so to speak, the brooding American species another, and if we are to accept the accounts, certain owls, which breed in the far north, successfully rear young in the short Arctic summer, with an interval of a week or more between each egg. Yet there can be little doubt that an undue lengthening of this interval would seriously interfere with nest-life in many species, and break the tendency to guard the egg. All would seem to depend upon the correlated instincts of parent and child. With an interval of from five to seven days, which has been credited to Cuculus canorus, self-brooding would be impracticable without a change in its instincts, for it migrates in July. While it is certain that the egg-laying interval was gradually extended in this bird, it is not known at what corresponding point the parasitic practise was finally established. Certain it is, however, that then as now, the egg, whether laid direct in a nest or dropped on the ground and subsequently conveyed to one, was abandoned. The American brooding cuckoos (Coccygus erythropthalmus and C. americanus), although suffering a similar disturbance in the brooding interval (of one to three clays), have adjusted these differences by another course. The young which are hatched in succession, also leave the nest in succession, when one week old, and enter upon a climbing stage which lasts a fortnight. In this way the brood is divided into two groups, and any untoward effects which might result from a marked difference in age of the nestlings, is avoided. The greatest disadvantage of such a mixture, in the nest of this species, would seem to lie in the fact that the oldest and strongest usually succeed in holding up most of the food. We may add that the American cuckoos have never advanced far beyond the first stage, as designated above, although they have suffered a disturbance in the normal rhythm of egg-production, and that the parental instincts are as strong with them as in passerine birds. The study of their habits gives no support to the idea advanced by Darwin, in his "Origin of Species," that they are passing along the same road to parasitism already traversed by their European relative. I do not know whether the American cuckoos ever built a better nest or not, but it is certain that the present structure is adequate to their needs, and affords no evidence of a waning instinct of nidification. (4) The final stage of the parasitic instinct among the Cuculidæ is presented by their famous European representative, Cuculus canorus, in which the instincts of both young and adult have become so specialized that to describe them at all adequately would require many pages. One hundred and nineteen different species of birds have been the prey of this parasite, the eggs of which have become reduced in size and highly variable in form and color. The commonest dupes are birds of small size, like the hedge sparrow and titlark; but one egg is laid in the same nest by the same bird, and this is often similar in size and coloring to those of the prospective nurse. The egg is deposited stealthily in the stolen nest, and in the absence of the owner, either just before or just after the proper eggs have appeared, or it is first dropped on the ground and conveyed to the nest in bill or gullet, by which the range of accessible nests is greatly increased. These and other remarkable practises of this bird have been fully described in a paper on the "Life and Instincts of the Cuckoo," shortly to appear.
All travelers who have studied the ostriches of South America and Africa in the field speak of the great numbers of their eggs which are annually wasted both in and out of season by dropping them over the plains or around their nests. If this is a secondary character, it must have come from a disturbance of the normal cycle, quite similar to what we have found in cuckoos and starlings. In this case adjustment seems to have been effected in quite a different manner, for we find the male taking upon himself almost the whole duty of incubation and care of the young. Even the wasted eggs, at least in the neighborhood of the nest, serve a secondary use as food, for the young soon break them open and devour them.
We can not discuss with much profit the remarkable breeding habits of the megapodes of Australia and the East Indies, referred to earlier in this paper, until naturalists have made more detailed studies upon the various species. The notes which follow are purely tentative, and are offered by way of suggestion. The true megapodes build huge mounds of earth and leaves, which serve as incubators for their eggs, and the young, which may or may not be subsequently tended by their parents, are in most cases able to run or fly from birth, or when they emerge from their mound. The moleos or "maleos" deposit theirs in black volcanic sand which is both clamp and warm, either by the seashore or in the vicinity of warm springs in the interior. In any case both birds instinctively secure the two requisites for successful incubation—even warmth and moisture—though in different ways.
That the brush turkeys are descended from stock which possessed the instinct of incubation is rendered probable from the fact that they are gallinaceous birds, allied to the curassows, wild turkeys and grouse, all of which build some kind of a nest and brood their young at the present time. Further, the fact that the same mound is used continuously by the same birds, whether by more than one pair or not, and is added to year after year, like the aerie of an eagle, and that in the ocellated megapode, at least, the adults remain in the vicinity of their mound and tend their young after leaving it, all suggest that this mound must be regarded in the light of a nest, however modified from the typical structure. From the stage seen in the ocellated magapode, it is only a step or two to that found in others, where the parents never see their young, for which they make ample provision, any more than does a turtle or a mud-dauber wasp.
More aberrant still, but in the same direction, is the behavior of the moleo, in which as in the parasitic cuckoos, other changes have arisen, which would render self-brooding difficult if not impracticable. Their large eggs, six to eight in number, are said to be deposited at the extraordinary interval of ten to twelve days, so that a period of three months would elapse, between the laying of the first and last. Again, unlike the fowls and birds generally, no turning of the eggs during incubation is necessary.
While nothing is certainly known concerning the history of these peculiar instincts of the megapodcs, it is not unlikely that, as in cuckoos and cowbirds, they have arisen through the modification of earlier and more uniform instincts which the ancestors of all modern birds seem to have possessed in common.
- That is, when it is limited to the direct reproductive activities, the relation of which to migration is not important in this discussion.
- By Mr. H. E. Denio, of Milesgrove, Pa., to whose kindness I am indebted for their present as well as a former use.
- Audubon speaks only of the fish hawks, which he says but seldom alight on the ground, as "when they collect materials' for the purpose of repairing their nest at the approach of autumn." "Ornithological Biography," Vol. I., p. 419, Edinburgh, 1831.
- "Researches in Zoology, Illustrative of the Manners and Economy of Animals," London, 1834; 2d ed., London, 1873.
- For the use of this photograph I am indebted to the courtesy of Mr. F. J. V. Skiff, director of the Field Museum of Natural History, Chicago.
- That other species of birds occasionally remove their eggs when disturbed can not be doubted, and they probably do it with their bills. The king penguins of the Antarctic are said to guard their single egg by carrying it in a pouch or fold of the skin, developed in either sex, between the legs.