Читать книгу Territory in Bird Life - Henry Eliot Howard - Страница 7

In his Manual of Psychology Dr Stout reminds us that "Human language is especially constructed to describe the mental states of human beings, and this means that it is especially constructed so as to mislead us when we attempt to describe the working of minds that differ in a great degree from the human."

The use of the word "territory" in connection with the sexual life of birds is open to the danger which we are here asked to guard against, and I propose, therefore, before attempting to establish the theory on general grounds, to give some explanation of what the word is intended to represent and some account of the exact position that representation is supposed to occupy in the drama of bird life.

The word is capable of much expansion. There cannot be territories without boundaries of some description; there cannot well be boundaries without disputes arising as to those boundaries; nor, one would imagine, can there be disputes without consciousness as a factor entering into the situation; and so on, until by a simple mental process we conceive of a state in bird life analogous to that which we know to be customary amongst ourselves. Now, although the term "breeding territory," when applied to the sexual life of birds, is not altogether a happy one, it is difficult to know how otherwise to give expression to the facts observed. Let it then be clearly understood that the expression "securing a territory" is used to denote a process, or rather part of a process, which, in order to insure success to the individual in the attainment of reproduction, has been gradually evolved to meet the exigencies of diverse circumstances. Regarded thus, we avoid the risk of conceiving of the act of securing a territory as a detached event in the life of a bird, and avoid, I hope, the risk of a conception based upon the meaning of the word when used to describe human as opposed to animal procedure.

Success in the attainment of reproduction is rightly considered to be the goal towards which many processes in nature are tending. But what is meant by success? Is it determined by the actual discharge of the sexual function? So many and so wonderful are the contrivances which have slowly been evolved to insure this discharge, that it is scarcely surprising to find attention focused upon this one aspect of the problem. Yet a moment's reflection will show that so limited a definition of the term "success" can only be held to apply to certain forms of life; for where the young have to be cared for, fostered, and protected from molestation for periods of varying lengths, the actual discharge of the sexual function marks but one stage in a process which can only succeed if all the contributory factors adequately meet the essential conditions of the continuance of the species.

Securing a territory is then part of a process which has for its goal the successful rearing of offspring. In this process the functioning of the primary impulse, the acquirement of a place suitable for breeding purposes, the advent of a female, the discharge of the sexual function, the construction of the nest, and the rearing of offspring follow one another in orderly sequence. But since we know so little of the organic changes which determine sexual behaviour, and have no means of ascertaining the nature of the impulse which is first aroused, we can only deal with the situation from the point at which the internal organic changes reflect themselves in the behaviour to a degree which is visible to an external observer. That point is reached when large numbers of species, forsaking the normal routine of existence to which they have been accustomed for some months, suddenly adopt a radical change in their mode of behaviour. How is this change made known to us? By vast numbers of individuals hurrying from one part of the globe to another, from one country to another, and even from mid-ocean to the coasts; by detachments travelling from one district to another; by isolated individuals deserting this place for that; by all those movements, in fact, which the term migration, widely applied, is held to denote. Now the impulse which prompts these travelling hosts must be similar in kind whether the journey be long or short; and it were better, one would think, to regard such movements as a whole than to fix the attention on some one particular journey which fills us with amazement on account of the magnitude of the distance traversed or the nature of the difficulties overcome. For, after all, what does each individual seek? There may be some immature birds which, though they have not reached the necessary stage of development, happen to fall in with others in whom the impulse is strong and are led by them—they know not where. But the majority seek neither continent nor country, neither district nor locality is their aim, but a place wherein the rearing of offspring can be safely accomplished; and the search for this place is the earliest visible manifestation in many species of the reawakening of the sexual instinct.

The movements of each individual are then directed towards a similar goal, namely, the occupation of a definite station; and this involves for many species a distinct change in the routine of behaviour to which previously they had been accustomed. Observe, for example, one of the numerous flocks of Finches that roam about the fields throughout the winter. Though it may be composed of large numbers of individuals of different kinds, yet the various units form an amicable society actuated by one motive—the procuring of food. And since it is to the advantage of all that the individual should be subordinated to the welfare of the community as a whole there is no dissension, apart from an occasional quarrel here and there.

In response, however, to some internal organic change, which occurs early in the season, individuality emerges as a factor in the developing situation, and one by one the males betake themselves to secluded positions, where each one, occupying a limited area, isolates itself from companions. Thereafter we no longer find that certain fields are tenanted by flocks of greater or less dimensions, while acres of land are uninhabited, but we observe that the hedgerows and thickets are divided up into so many territories, each one of which contains its owner. This procedure, with of course varying detail, is typical of that of many species that breed in Western Europe. And since such a radical departure from the normal routine of behaviour could scarcely appear generation after generation in so many widely divergent forms, and still be so uniform in occurrence each returning season, if it were not founded upon some congenital basis, it is probable that the journey, whether it be the extensive one of the Warbler or the short one of the Reed-Bunting, is undertaken in response to some inherited disposition, and probable also that the disposition bears some relation to the few acres in which the bird ultimately finds a resting place. Whilst for the purpose of the theory I shall give expression to this behaviour in terms of that theory, and speak of it as a disposition to secure a territory, using the word disposition, which has been rendered current in recent discussion, for that part of the inherited nature which has been organised to subserve a specific biological purpose—strict compliance with the rules of psychological analysis requires a simpler definition; let us therefore say "disposition to remain in a particular place in a particular environment."

But even granting that this disposition forms part of the hereditary equipment of the bird, how is the process of reproduction furthered? The mere fact of remaining in or about a particular spot cannot render the attainment of reproduction any less arduous, and may indeed add to the difficulties, for any number of individuals might congregate together and mutually affect one another's interests. A second disposition comes, however, into functional activity at much the same stage of sexual development, and manifests itself in the male's intolerance of other individuals. And the two combined open up an avenue through which the individual can approach the goal of reproduction. In terms of the theory I shall refer to this second disposition as the one which is concerned with the defence of the territory.

Broadly speaking, these two dispositions may be regarded as the basis upon which the breeding territory is founded. Yet inasmuch as the survival value of the dispositions themselves must have depended upon the success of the process as a whole, it is manifest that peculiar significance must not be attached to just the area occupied, which happens to be so susceptible of observation; other contributory factors must also receive attention, for the process is but an order of relationships in which the various units have each had their share in determining the nature and course of subsequent process, so that, as Dr Stout says, when they were modified, it was modified.

Now the male inherits a disposition which leads it to remain in a restricted area, but the disposition cannot determine the extent of that area. How then are the boundaries fixed? That they are sometimes adhered to with remarkable precision, that they can only be encroached upon at the risk of a conflict—all of this can be observed with little difficulty. But if we regard them as so many lines definitely delimiting an area of which the bird is cognisant, we place the whole behaviour on a different level of mental development, and incidentally alter the complexion of the whole process. It would be a mistake, I think, to do this. Though conscious intention as a factor may enter the situation, there is no necessity for it to do so; there is no necessity, that is to say, for the bird to form a mental image of the area to be occupied and shape its course accordingly. The same result can be obtained without our having recourse to so complex a principle of explanation, and that by the law of habit formation. In common with other animals, birds are subject to this law in a marked degree. An acquired mode of activity becomes by repetition ingrained in the life of the individual, so that an action performed to-day is liable to be repeated to-morrow so long as it does not prejudice the existence or annul the fertility of the individual.

Let us see how this may have operated in determining the limits of the area acquired, and for this purpose let us suppose that we are observing a male Reed-Bunting recently established in some secluded piece of marsh land. Scattered about this particular marsh are a number of small willows and young alder trees, each one of which is capable of providing plenty of branches suitable for the bird to perch upon, and all are in a like favourable position so far as the outlook therefrom is concerned. Well, we should expect to find that each respective tree would be made use of according to the position in which the bird happened to find itself. But what actually do we find—one tree singled out and resorted to with ever-increasing certainty until it becomes an important point in relation to the occupied area, a headquarters from which the bird advertises its presence by song, keeps watch upon the movements of its neighbours, and sets out for the purpose of securing food. We then take note of its wanderings in the immediate vicinity of the headquarters, especially as regards the direction, frequency, and extent of the journeys; and we discover not only that these journeys proceed from and terminate in the special tree, but that there is a sameness about the actual path that is followed. The bird takes a short flight, searches a bush here and some rushes there, returns, and after a while repeats the performance; we on our part mark the extreme limits reached in each direction, and by continued observation discover that these limits are seldom exceeded, that definition grows more and more pronounced, and that by degrees the movements of the bird are confined within a restricted area. In outline, this is what happens in a host of cases. By repetition certain performances become stereotyped, certain paths fixed, and a routine is thus established which becomes increasingly definite as the season advances.

But while it would be quite untrue to say that this routine is never departed from, and equally profitless to attempt to find a point beyond which the bird will under no circumstances wander, yet there is enough definition and more than enough to answer the purpose for which the territory has, I believe, been evolved, that is to say the biological end of reproduction. Again, however, the process of adjustment is a complex one. Habit plays its part in determining the boundaries in a rough and ready manner, but the congenital basis, which is to be found in the behaviour adapted to a particular environment, is an important factor in the situation. For example, if instead of resting content with just a bare position sufficient for the purpose of reproduction, the Guillemot were to hustle its neighbours from adjoining ledges, the Guillemot as a species would probably disappear; or if instead of securing an area capable of supplying sufficient food both for itself and its young, the Chiffchaff were to confine itself to a single tree, and, after the manner of the Guillemot, trust to spasmodic excursions into neutral ground for the purpose of obtaining food, the Chiffchaff as a species would probably not endure. All such adjustments have, however, been brought about by relationships which have gradually become interwoven in the tissue of the race.

The intolerance that the male displays towards other individuals, usually of the same sex, leads to a vast amount of strife. Nowhere in the animal world are conflicts more frequent, more prolonged, and more determined than in the sexual life of birds; and though they are acknowledged to be an important factor in the life of the individual, yet there is much difference of opinion as to the exact position they occupy in the drama of bird life. Partly because they frequently happen to be in evidence, partly because they are numerically inferior, and partly, I suppose, because the competition thus created would be a means of maintaining efficiency, the females, by common consent, are supposed to supply the condition under which the pugnacious nature of the male is rendered susceptible to appropriate stimulation. And so long as the evidence seemed to show that battles were confined to the male sex, so long were there grounds for hoping that their origin might be traced to such competition. But female fights with female, pair with pair, and, which is still more remarkable, a pair will attack a single male or a single female; moreover, males that reach their destination in advance of their prospective mates engage in serious warfare. How then is it possible to look upon the individuals of one sex as directly responsible for the strife amongst those of the other, or how can the female supply the necessary condition? As long as an attempt is made to explain it in terms of the female, the fighting will appear to be of a confused order; regard it, however, as part of a larger process which demands, amongst other essential conditions of the breeding situation, the occupation of a definite territory, and order will reign in place of confusion.

But even supposing that the male inherits a disposition to acquire a suitable area, even supposing that it inherits a disposition which results indirectly in the defence of that area, how does it obtain a mate? If the female behaved in a like manner, if she, too, were to isolate herself and remain in one place definitely, that would only add to the difficulties of mutual discovery. We find, however, in the migrants, that the males are earlier than the females in reaching the breeding grounds, and, in resident species, that they desert the females and retire alone to their prospective territories, so that there is a difference in the behaviour of the sexes at the very commencement of the sexual process. What is the immediate consequence? Since the male isolates itself, it follows, if the union of the sexes is to be effected, that the discovery of a mate must rest largely with the female. This of course reverses the accepted course of procedure. But after all, what reason is there to suppose that, the male seeks the female, or that a mutual search takes place; what reason to think that this part of the process is subject to no control except such as may be supplied by the laws of chance?

Now, clearly, much will depend upon the rapidity with which the female can discover a male fit to breed; for if the course of reproduction is to flow smoothly, there must be neither undue delay nor waste of energy incurred in the search—some guidance is therefore necessary, some control in her external environment. Here the song, or the mechanically produced sound, comes into play, and assists in the attainment of this end. Nevertheless if every male were to make use of its powers whether it were in occupation of a territory or not, if the wandering individual had an equal chance of attracting a mate, then it would be idle to attempt to establish any relation between "song" on the one hand, and "territory" on the other, and impossible to regard the voice as the medium through which an effectual union of the sexes is procured. But there is reason to believe that the males utilise their powers of producing sound only under certain well-defined conditions. For instance, when they are on their way to the breeding grounds, or moving from locality to locality in search of isolation, or when they desert their territories temporarily, as certain of the residents often do, they are generally silent; but when they are in occupation of their territories they become vociferous—and this is notoriously the case during the early hours of the day, which is the period of maximum activity so far as sexual behaviour is concerned. So that just at the moment when the sexual impulse of the female is most susceptible to stimulation, the males are betraying their positions and are thus a guide to her movements. Nevertheless, even though she may have discovered a male ready to breed, success is not necessarily assured to her; for with multitudes of individuals striving to procreate their kind, it would be surprising if there were no clashing of interests, if no two females were ever to meet in the same occupied territory. Competition of this kind is not uncommon, and the final appeal is to the law of battle, just as an appeal to physical strength sometimes decides the question of the initial ownership of a territory.

I shall try to make clear the relations of the various parts to the whole with the assistance of whatever facts I can command. I shall do so not only for the purposes of the theory, but because one so often finds the more important features of sexual behaviour regarded as so many distinct phenomena requiring separate treatment, whereas they are mutually dependent, and follow one another in ordered sequence. I spoke of the process as a series of relationships. Some of these relationships have already been touched upon; others will become apparent if we consider for a moment the purposes for which the territory has been evolved. Indirectly its purpose is that of the whole process, the rearing of offspring. But inasmuch as a certain measure of success could be attained, and that perhaps often, without all the complications introduced by the territory, there are manifestly advantages to be gained by its inclusion in the scheme. The difficulties which beset the path of reproduction are by no means always the same—all manner of adjustments have to be made to suit the needs of different species. There are direct relationships, such as we have been speaking of, which are essential to the every-day working of the process, and others which are indirect, though none the less important for they must have exercised an influence throughout the ages. These latter are furnished by the physical—the inorganic world, by climate, by the supply of the particular kind of breeding stations, by the scarcity or abundance of the necessary food and by the relative position of the food supply to the places suitable for breeding. Why does the Reed-Bunting cling so tenaciously to an acre or more of marshy ground, while the Guillemot rests content with a few square feet on a particular ledge of rock? The answer is the same in both cases—to facilitate reproduction. But why should a small bird require so many square yards, whilst a very much larger one is satisfied with so small an area? The explanation must be sought in the conditions of existence. The Reed-Bunting has no difficulty in finding a position suitable for the construction of its nest; there are acres of waste land and reedy swamps capable of supplying food for large numbers of individuals, and the necessary situations for countless nests. But its young, like those of many another species, are born in a very helpless state. For all practical purposes they are without covering of any description and consequently require protection from the elements, warmth from the body of the brooding bird, and repeated supplies of nourishment. A threefold burden is thus imposed upon the parents: they must find food for themselves, they must afford protection to the young by brooding, and they must supply them with the necessary food at regular intervals. And their ability to do all this that is demanded of them will be severely taxed by the brooding which must perforce curtail the time available for the collection of food.

Let us then suppose that the Reed-Buntings inhabiting a certain piece of marsh are divided into two classes, those which are pugnacious and intolerant of the approach of strangers, and those which welcome their presence. The nests of the former will be built in isolation, those of the latter in close proximity. In due course eggs will be laid and incubation performed, and thus far all alike will probably be successful. Here, however, a critical point is reached. If the young are to be freed from the risk of exposure, the parents must find the necessary supply of food rapidly. But manifestly all will not be in a like satisfactory position to accomplish this, for whereas the isolated pairs will have free access to all the food in the immediate vicinity of the nest, those which have built in proximity to one another, meeting competition in every direction, will be compelled to roam farther afield and waste much valuable time by doing so; and under conditions which can well be imagined, even this slight loss of time will be sufficient to impede the growth of the delicate offspring, or to lead perhaps to still greater disaster. If any one doubts this, let him first examine one of the fragile offspring; let him then study the conditions under which it is reared, observing the proportion of time it passes in sleep and the anxiety of the parent bird to brood; and finally let him picture to himself its plight in a wet season if, in order to collect the necessary food, the parents were obliged to absent themselves for periods of long duration.

Now take the case of the Guillemot. Its young at birth are by no means helpless in the sense that the young Reed-Bunting is, and food is readily procured. But breeding stations are scarce, for although there are many miles of cliff-bound coast, yet not every type of rock formation produces the fissures and ledges upon which the bird rests. Hence vast stretches of coast-line remain uninhabited, and the birds are forced to concentrate at certain points, where year after year they assemble in countless numbers from distant parts of the ocean. If, then, different individuals were to jostle one another from adjoining positions, and each one were to attempt to occupy a ledge in solitary State, not only would the successful ones gain no advantage from the additional space over which they exercised dominion, but inasmuch as many members that were fitted to breed would be precluded from doing so, the status of the species as a whole would be seriously affected. The amount of space occupied by each individual is therefore a matter of urgent importance. A few square feet of rock sufficient for the immediate purpose of incubation is all that can be allowed if the species is to maintain its position in the struggle for existence.

Our difficulty in estimating the importance of the various factors that make for success or failure arises from our inability to see more than a small part of the scene as it slowly unfolds itself. The peculiar circumstances under which these cliff-breeding forms dwell does, however, enable us to picture, on the one hand, the precarious situation of an individual that was incapable of winning or holding a position at the accustomed breeding station, and, on the other, the plight of the species as a whole if each one exercised authority over too large an area. With the majority of species it is difficult to do this. So many square miles of suitable breeding ground are inhabited by so few Reed-Buntings that, even supposing certain members were to establish an ascendency over too wide an area, it would be impossible to discover by actual observation whether the race as a whole were being adversely affected. Competition doubtless varies at different periods and in different districts according to the numerical standing of the species in a given locality and according to the numerical standing of others that require similar conditions of existence; at times it may even be absent, just as at any moment it may become acute. These examples show how profoundly the evolution of the breeding territory may have been influenced by relationships in the inorganic world, and they give some idea of the intricate nature of the problem with which we have to deal.

I mentioned that the first visible manifestation of the revival of the sexual instinct was to be found in the movements undertaken by the males at the commencement of the breeding season. Such movements are characterised by a definiteness of purpose, whether they involve a protracted journey of some hundreds of miles or merely embrace a parish or so in extent, and that purpose is the acquirement of a territory suitable for rearing offspring. They are thus directly related to the territory, and the question arises as to whether their origin may not be traced to such relatedness. So long as we fix our attention solely upon the magnitude of the distance traversed the suggestion may seem a fanciful one. Nevertheless, if the battles between males of the same species are directly related to the occupation of a position suitable for breeding purposes, if those which occur between males of closely related forms can be traced to a similar source, if the females take their share in the defence of the ground that is occupied, if, in short, the competition is as severe as I believe it to be, and is wholly responsible for the strife which is prevalent at the commencement of the breeding season—then such competition must have introduced profound modifications in the distribution of species; it must have even influenced the question of the survival of certain forms and the elimination of others; and since the powers of locomotion of a bird are so highly developed it must have led to an extension of breeding range, limited only by unfavourable conditions of existence.