Territory in Bird Life Part 5

If there were always sufficient breeding ground to support the offspring of all the individuals of each species, if the individuals were always so distributed that there was no possibility of overcrowding in any particular area, and if the conditions of existence of different species were so widely divergent that the presence of this one in no way affected the interests of that, no opportunity would be afforded for the development of so complex a system as is involved in the "territory" and all that appertains to it. But the available breeding ground is by no means unlimited. The supply of food, which is a determining factor in the environment, is always fluctuating according to the climate and according to the changes in the earth's surface; and so the distribution of the bird population in any given area, though it may be suitably adjusted for one year or even for a period of years, is bound in the course of time to require readjustment. Now there cannot be readjustment without compet.i.tion, nor compet.i.tion without combat. But the appeal to physical force is only a means to an end, and, since no male can endure incessant warfare and the perpetual strain of always being on the alert, without experiencing such physical exhaustion as might affect his power of reproduction, its direct effect upon the combatants cannot be otherwise than harmful--in fact it is a necessary evil which for the good of the species must be kept strictly within bounds. Bearing in mind, then, these two facts, namely that the distribution of the males is never stable and that overmuch fighting may defeat the end in view, we can appreciate the importance of any factor which will lead to a more uniform distribution and at the same time insure security by peaceable means.

The proximate end of the male's behaviour is isolation--how is it to be obtained? If, after having occupied a territory, the bird were to remain silent, it would run the risk of being approached by rivals; if, on the other hand, it were merely to utter the recognition call of the species, it would but attract them. In neither case would the end in view be furthered, and isolation would solely depend upon alertness and the capacity to eject intruders. Supposing, however, that the song, just as it serves to attract the females, serves to repel other males, a new element is introduced deserving of recognition; for those males that had established themselves would not only be spared the necessity of many a conflict, but they would be spared also the necessity of constant watchfulness, and so, being free to pursue their normal routine--to seek food, to rest, and, if migrants, to recover from the fatigue of the journey, they would be better fitted to withstand the strain of reproduction; and those that were still seeking isolation in an appropriate environment, instead of settling first here and then there only to find themselves forestalled, would avoid and pa.s.s by positions that were occupied, establis.h.i.+ng themselves without loss of time in those that were vacant. Without the aid of something beyond mere physical encounter to regulate dispersal, it is difficult to imagine how in the short time at disposal anything approaching uniformity of distribution could be obtained. Hence, both in the direction of limiting combat, of insuring accommodation for the maximum number of pairs in the minimum area, and of conserving energy, the song, by conveying a warning, plays an important part in the whole scheme.

And if this be so, if the song repels instead of attracting, it follows that the more distinct the sounds, the less likelihood will there be of confusion; for supposing that different species were to develop similar songs, whole areas might be left without their complement of pairs just because this male mistook the voice of that, and avoided it when there was no necessity for doing so. So that just as from the point of view of "recognition" each female must be able to distinguish the voice of its own kind, so likewise the warning can only be adequate providing that the sounds are specifically distinct. A point, however, arises here in regard to closely related forms. Some species require similar food and live under similar conditions of existence; they meet in compet.i.tion and fight with one another; and, if they did not do so, the food-supply of a given area would be inadequate to support the offspring of all the pairs inhabiting that area. Generally speaking, the more closely related the forms happen to be, the more severe the compet.i.tion tends to become; and it may be argued that in such cases a similar song would contribute to more effective distribution and in some measure provide against the necessity of physical encounter; that, in fact, it would stand in like relation to the success of all the individuals concerned, as does the song to the individuals of the same species. But we must bear in mind that the primary purpose of song is to direct the females to those males that are in a position to breed; and to risk the possibility of prompt recognition in order that the males of closely related species should fight the less, would be to sacrifice that which is indispensable for a more remote and less important advantage.

What meaning does the song convey to a male that is unestablished? Does the bird recognise that it is forestalled; does it foresee and fear the possibility of a conflict, and conclude that the attempt to settle is not worth while? I do not imagine that it thinks about it at all. How then does the warning warn? We will endeavour to answer this question, but, in order to do so, we must review the stages by which a territory is secured.

We take as our starting point the internal organic changes which are known to occur. These changes are correlated with other changes, manifested by a conspicuous alteration in behaviour--to wit, the disappearance of sociability and its replacement by isolation. Having found a station which meets the requirements of its racial characteristics, the male establishes itself for a season, becomes vociferous, displays hostility towards others of its kind, and in due course is discovered by a female. The whole is thus an inter-related whole, a chain of activities which follow one another in ordered sequence. Now we have seen that it is neither pugnacious nor vociferous until the territory is actually occupied; we have seen that the fact of occupation is the condition under which the instincts of pugnacity and of song are rendered susceptible to appropriate stimulation; we have discussed the nature of the stimulus in each case, and we wish to know the sort of meaning that the song conveys to an individual which is still in the preliminary stage of seeking a station. In sequential order we have the following: (1) internal organic changes which lead to isolation, (2) the appropriate environment which gives rise to an impulse to remain in it, (3) the occupation of a territory which is the condition under which the instincts are rendered susceptible to stimulation, (4) the various stimuli. Each is dependent upon that which precedes it, and no part can be subtracted without failure of the biological end in view, neither can the different stages be combined in different order. So that, in considering the significance of song to an unestablished male, we are dealing with the situation at a point at which all the latent activities have not been fully felt, for all that so far has occurred is the change from sociability to isolation determined by internal organic changes. The bird has not established a territory because it has not come into contact with the appropriate environment, and it is not pugnacious because the condition which renders its instinct susceptible is absent; and so, as it wanders from place to place and hears the voices of males here or males there, it merely behaves in accordance with that part of its nature which predominates just at that particular moment--the impulse to avoid them.

But given the appropriate environment, given, that is to say, just that combination of circ.u.mstances which might bring into functional activity all the latent instincts of the intruder, and no matter how vociferous the occupant of a territory might be, it would not be preserved from molestation. The advantage of the song, biologically considered, is then this, that it will often prove just sufficient to preclude males in search of isolation from coming into contact with the environmental conditions adequate to supply the stimulus to their latent activities and to convert them into rivals.

If this interpretation be correct, if we are right in attributing the withdrawal solely to the fact that the first stage only in the relational series has been reached, it follows that the effect of song upon males that have reached subsequent stages in that series must be of a very different kind. We have dealt with the male when in the preliminary stage of seeking isolation, we must deal with it now when eventually it occupies a territory. How does it behave when it hears, as it is bound to do, the voices of rivals in its neighbourhood? You may remember that some allusion was made to the fact that an outburst of song from one individual was followed, not unfrequently, by a similar outburst on the part of other individuals in the immediate locality. For example, silence may reign in the reed-bed except for an occasional note of the Reed-Warbler or Sedge-Warbler. Suddenly, however, a dispute arises between two individuals, accompanied by a violent outburst of song, and forthwith other males in the vicinity begin to sing excitedly and continue doing so for some minutes in a strangely vigorous manner, the tumult of voices affording a striking contrast to the previous silence. Spasmodic outbursts of this kind, stimulated by an isolated utterance, are by no means uncommon. But not only does song stimulate song; under certain conditions it has the still more remarkable effect of arousing hostility. The boundary that separates two adjoining territories is by no means a definite line, but rather a fluid area wandered over by this owner at one moment, by that at another. Now so long as the bird is silent while in this area, the probability is that it will escape detection and remain unmolested; let it however sing--it often does so--and it will not merely be approached but attacked, and consequently this area is the scene of much strife. The point to be noticed here is that the song brings about no withdrawal; it elicits a response, attracts instead of repelling, and, in short, arouses the impulse that is always predominant in the nature of the male when eventually it occupies a territory--the impulse of self-a.s.sertiveness.

Therefore it seems clear that the different stages in the process of reproduction mark the appearance of different conditions, each of which renders some new impulse susceptible to stimulation, and that the significance of song depends upon the stage which happens to have been reached. Hence when we speak of song acting as a "warning," we do not mean that it arouses any sensation of fear; it is but a stimulus to that part of the inherited nature of the hearer which predominates at the moment.

Are we then justified in the use of such terms as "warning,"

"significance," or even "meaning," when it is but a matter of stimulus and response? In what does the impulse to avoid other males consist?

There is no reason to suppose that there is any sensation of fear in the first stage, and the course of behaviour demonstrates that there is none in the later stages. But it is difficult to conceive of an impulse which has, as its end, the isolation of the individual from members of its own s.e.x and kind, without some feeling-tone, the reverse of pleasurable, entering into the situation; just as it is difficult to believe that the female experiences no pleasurable sensation when she hears the voice of the male that directs her search. So that the song may be actually repellent in the one case and attractive in the other; and it is none the less repellent when, as in the later stages, it attracts a neighbouring male, for the attraction is then of a different order, determined by the presence of the condition which renders the pugnacious nature susceptible and leads to attack. In a sense, therefore, we can speak of "meaning"--though not perhaps of "significance"--and of "warning," when we refer to the prospective value of the behaviour.

So much for the purpose of "song"; there still remains the more difficult question--the question of origin. Let me make clear what I mean by origin. As we have already seen, there is infinite diversity in the s.e.xual voice of different species; some are harsh and others monotonous, and some strike the imagination by their novelty whilst others are melodious; and to the naturalist each, in its particular way and in a particular degree, probably makes some appeal according to the a.s.sociations that it arouses. But just why a Marsh-Warbler is gifted with a voice that is so beautiful and varied, whilst the Gra.s.shopper-Warbler must perforce remain content with a monotonous trill; just why the tail feathers of the Snipe have developed into an instrument, whilst the Pied Woodp.e.c.k.e.r has developed muscles which enable it to make use of a decayed branch as an instrument--we know no more than we do of the nature of the forces which lead the Reed-Warbler to weave its nest to reeds, or the caterpillar of the Elephant Hawk Moth to a.s.sume so peculiar an att.i.tude when disturbed. When therefore I speak of the origin, I do not refer to the mode of origin of variation; I take for granted that variations somehow arise, and I seek to ascertain whether there is anything in the phenomena which we have explored which might reasonably be held to determine the survival of this one in preference to that.

When we reflect upon the problem of song and consider the numerous and diverse forms in which it is manifested, we are apt to draw a comparison between the sounds we hear and those produced by musical instruments, and hence to conclude that each bird is gifted with a special instrument in virtue of which it produces its characteristic melody. But there is a very remarkable phenomenon connected with the singing of birds which shows that this is really not the case--I mean the phenomenon of imitation. There are plenty of good imitators amongst our native species, and the power of imitation is not the exclusive property of those which have reached a high degree of vocal development, nor, for the matter of that, of song-birds at all. Even the Jay, than which few birds have a more raucous voice, that "hoots" like the Wood-Owl, or copies the sounds produced by the tail feathers of the Snipe, will occasionally imitate the most melodious strains of some other species; and the Red-backed Shrike, whose s.e.xual call is princ.i.p.ally a few harsh notes rapidly repeated, bursts at times into perfect imitations of the song of the Swallow, Linnet, or Chaffinch.

Nevertheless it is amongst such typical songsters as the Warblers that we find the greatest volume of imitation, and no limit seems to be placed upon their capacity. The Marsh-Warbler can utter the call of the Green Woodp.e.c.k.e.r, or sing as the Nightingale does, with as much facility as it sings its own song; and the Blackcap is well-nigh as proficient in copying the cries and melodies of surrounding species--and so, if it were necessary, we might proceed to add to the list.

These examples demonstrate that different songs are not represented by a corresponding number of different physiological contrivances; for if the difference were really attributable to some structural peculiarity, then the range of sounds embraced in the call-notes and the s.e.xual call of any given species, must be the measure of the capacity of its instrument; and no matter how great its power of imitation may be, it follows that it will only be capable of copying those sounds which fall within that range. There is plenty of evidence to show that the power of imitation is almost unlimited, at all events that it is not confined within such narrow limits as are here demanded. Hence it seems clear that the diversity of song is not to be sought in structure, but in some innate capacity to play one tune in preference to another; and if this be so, and if out of the same instrument, which has been primarily evolved to further the biological end of intercommunication, all manner of diverse sounds can be made to proceed, the problem of the origin of song is to that extent simplified.

We must next inquire into the nature of song, and endeavour to ascertain whether all the individuals of a species are alike proficient, or, failing this, whether there is any quality which can be observed to be constant under all conditions. I watch the Reed-Buntings in a marsh and find that there are three males occupying adjoining territories. Two of them are fully mature and their plumage is bright: that is to say the crown is black, the collar and breast are white, the flanks are dull white spotted with black, and the mantle is reddish-brown. The third is immature: the crown, instead of being black, is suffused with brown; the collar, instead of being white, is mottled with brown; and the flanks are more heavily streaked with brown. These three birds take up their positions in February, and, as is their wont, sing incessantly each day at daybreak. The song of the first two is normal, including the usual number of phrases which flow in no definite sequence, but are combined and recombined in different order, and the tone is pure; that of the third, the immature bird, is, however, very different; for just as in comparison its plumage is dull, so the phrases of its song are limited and reiterated with great monotony, the tone is impure, and the whole performance is dull and to our ears unmusical. I watch them from February to June, and observe the order in which they are mated--first a mature male; next, after a short interval, the immature male; and finally, after a still longer interval, the remaining bird gets a mate.

As the season advances, still keeping watch on the development of the plumage and of the voice of the immature male, I observe that no very definite change takes place--that the colours remain dull, that there is a conspicuous absence in the song of certain phrases, and that the notes lack purity of tone.

If now, instead of Reed-Buntings in a marsh, I watch Yellow Buntings on a furze-covered common, I find that, establis.h.i.+ng themselves early in February, they sing persistently, and in a few weeks are paired. But what arrests my attention more particularly is the quality of the song; for although the voice is unmistakably the voice of the Yellow Bunting, yet it is incomplete and lacks the variety of phrases and musical notation which we customarily a.s.sociate with the bird. Nevertheless, as the season advances, there is a progressive development in both these directions, and by the end of March or the beginning of April the song possesses all those qualities which appeal to us so forcibly.

There is one other fact to which attention must be drawn--the variation in the song of the same species in different districts. As an ill.u.s.tration let us take the case of the Chaffinch. In Worcesters.h.i.+re the bird sings what I imagine to be a normal song--the notes are clear and the phrases are distinct and combined in numerous ways. With the notes fresh in mind I leave them and go to the west of Donegal, where I am at once conscious of a difference; not a subtle difference that perplexes the mind and is difficult to trace, but a change so remarkable that one is conscious of a pa.s.sing doubt as to whether after all the voice is the voice of the Chaffinch; the song is pitched in a lower key, certain phrases are absent, the notes lack tone and are sometimes even harsh, and the bird seems wholly incapable of reaching the higher notes to which I am accustomed.

Now the immature Reed-Bunting, though to our ears its song is but a poor representation of that of the adult, gains a mate; the Yellow Bunting pairs, and the discharge of the s.e.xual function may even have taken place before its voice attains what we judge to be its full development; and there are no grounds for supposing that the Donegal Chaffinch, with its less musical notes, has on that account any the less chance of procreating its kind--facts which demonstrate that the biological value of song is neither to be sought in the purity of tone, nor in the variety and combination of phrases, nor, indeed, in any of those qualities by which the human voice gains or loses merit, and which leave us with no alternative but to dismiss from our minds all aesthetic considerations in the attempt to estimate its true significance.

What, then, determines its value? Are there any qualities which, whether the bird is mature or immature, whether it is untrained or has acquired fuller expression by practice, whether it inhabits this district or that, are alike constant? Well, no matter how great the variation, no matter how much this voice falls below or exceeds the standard, judged from the human standpoint, attained by that, even we, with our duller perception, have no difficulty in recognising the species to which the owner of the voice belongs; in other words, the song is always specific, and this is the most noticeable, as it is the most remarkable, characteristic.

There is still, however, another quality to which I would draw attention--that of loudness. The sounds produced are on the whole alike penetrative, and the individuals of any given district, even though the climate by affecting their vocal muscles may have modified the character of the song, are at no disadvantage in this respect; neither are the females on the same account the less likely to hear the undeveloped voice of the immature male.

We have then the following considerations: firstly, there is the widespread and remarkable phenomenon of imitation, from which we can infer that the diversity of song is not due to structural differences but must be sought in some innate capacity to play one tune in preference to another; secondly, not all the individuals of the same species play a similar tune--we find that there is in certain directions a noticeable variation which nevertheless does not seem to affect the question of success or failure in the attainment of reproduction; in the third place, in contrast with this variation, we can observe a striking uniformity in two important particulars, namely in the specific character and penetrative power of the song--qualities which we know are essential for the purposes of "recognition" and "warning"; and finally, from the general course of our investigation, we can infer that if a male had no certain means of advertising its position, the territory would not be brought into useful relation in its life. Have we here sufficient ground on which to construct a theory of origin; in other words, has the evolution of song been incidental to, and contributory to, the evolution of the territory?

We have all along spoken of the song and of the call-notes as if they were manifestations of separate emotional states having their respective and well-defined spheres of usefulness; and while, speaking generally, this is a true statement of the case, there is much evidence to show that the relations.h.i.+p between them is nevertheless very close. There are, for example, quite a number of cases in which a particular call-note is uttered with unusual energy during s.e.xual emotion, and is attached to the song, of which it may be said to form a part; but a still closer connection can be traced in many simple melodies which are merely compositions of social and family calls repeated many times in succession, and even in some of the more complex productions there will be found indications of a similar construction. And since this is so, since moreover, in the seasonal vocal development of such a bird as the Yellow Bunting, we can observe the gradual elaboration from simple to complex--from the repet.i.tion of single notes to phrases and from phrases to the complete melody--we have every reason to suppose that it is along these lines that the evolution of the voice has proceeded.

In all probability there was a time when vocal expression was limited to primitive social and family cries which would be called into play with special force during times of excitement, more particularly during the s.e.xual season which is the period of maximum emotional excitement.

But the excitement would express itself in all the congenital modes of behaviour peculiar to the season, and thus the repet.i.tion of these cries would become a.s.sociated with combat, with extravagant feats of flight, and with other forms of motor response. Now the more emotional individuals would be the more pugnacious, and all the more likely therefore to secure territory and so to procreate their kind; and, being of an excitable disposition, they would at the same time be the more vociferous. Hence variations of the hereditary tendency to vocal expression, even though in themselves they were not of survival value, would be fostered and preserved, so long as they were not harmful, in virtue of their a.s.sociation with pugnacity. But if, instead of being neutral, they helped to further the biological end of combat, the relations.h.i.+p between the voice and pugnacity would be of a mutually beneficial kind; and those individuals in which variation in both directions happened to coincide, would have a better chance of success in the attainment of reproduction.

A territorial system, closely corresponding to that which we have discussed, forms part of the life behaviour of certain mammals, and of its existence much lower in the scale of life evidence is not wanting; from which we can infer that it is not of recent origin, but that the conditions in the external environment demanded such a system at a remote period of avian development. Now even in its incipient stages the system must have involved a separation of the s.e.xes, and howsoever slight the degree of separation may have been in comparison with that which can be observed to-day, inasmuch as the power of locomotion was then less highly developed, mating could only have proceeded satisfactorily providing that males fit to breed had some adequate means of disclosing their positions. Thus there is reason to think that from the very commencement of the process variations of emotional disposition expressed through the voice would have been of survival value.

But expressed in what direction, in loudness and persistency of utterance, these are the qualities which, I imagine, would have been more likely to have facilitated the search of the female? Yet if she were uncertain as to the owner of the voice, neither loudness nor persistent repet.i.tion would avail much; and as species multiplied and the compet.i.tion for the means of living became increasingly severe, so the necessity of a territory would have become intensified, and so, too, with the extension of range, would the separation of the s.e.xes have been an ever-widening one; and as with their multiplication, irregularities and delays in mating, arising from the similarity of the calls, would have increased in frequency, so a distinctive call, which would have tended to minimise these risks, would have come to possess biological value.

Here we have a theory of origin, but origin of what? Of certain characteristics of song--nothing more; and therefore to suppose that it furnishes a complete explanation, which satisfies all the requirements of scientific logic, of so wonderful an intonation as that, for example, of the Marsh-Warbler, or that no other relations.h.i.+ps, except that of the territory, enter into the total emotional complex, simplifying here or elaborating there to meet the exigencies of diverse circ.u.mstances--to suppose this would be foolish. That there are many relations.h.i.+ps which even to-day are leading to modifications in important particulars, but which at the present time are beyond our cognisance, of this there can be no doubt.

There is one process by which song may have attained a fuller development, and which would account in some measure for the elaboration, inexplicable merely in terms of "recognition." It is this: the effect of the s.e.xual call upon the female cannot well be neutral, it must be either pleasurable or the reverse--it must, that is to say, be accompanied by some suggestiveness, and by suggestion I mean the arousing of some emotion akin to that of the male; and if there are degrees of suggestiveness, which well there may be, some males will mate sooner than others and some will remain mateless--this is the theory of s.e.xual selection. The question to be decided here is whether the biological emphasis is on loudness, or specific distinctness, or pitch, or modulation, or the manner in which the phrases are combined--that is, on some qualities in preference to others--or whether the emphasis is on the whole. We have already seen, and it is well known, that there is much variation in the voices of different individuals of the same species, and thus the first condition of the theory is fulfilled. Now the conditions which lead to variation are threefold--immaturity, seasonal s.e.xual development, and isolation. Of the three, the variation in the case of the immature bird is the most instructive; the tone is not so pure, the combination of phrases is incomplete, and elaboration is imperfect, and yet, notwithstanding all these imperfections, we can observe that the bird pairs as readily as does the adult. But even if we lacked this demonstrative evidence, we should still be justified in a.s.suming that such must be the case, for we know from experience in the preservation of game, where there is no surer way of reducing the stock than by leaving too high a percentage of old c.o.c.ks, that for the young bird to be at any disadvantage in compet.i.tion with the adult is detrimental, if not disastrous, to the species. So that while there is plenty of evidence of variation in those particular qualities which appeal to our aesthetic faculties, there is at the same time evidence which demonstrates that such variations exercise no influence on the course of mating; and inasmuch as it is difficult to conceive of any voice departing more from the normal type in these particular qualities than the immature does from the adult, if there be degrees of suggestive influence, we must seek it in some other direction. There remain the two other characteristics which we found to be constant under all circ.u.mstances, namely, loudness and specific distinctness; and if, in addition to serving the purpose of disclosing the positions of the males, they serve to evoke some emotion in the female, which helps to further the biological end of mating, so much the more reason is there for their survival.

There can be no question that this ingenious and attractive theory, if it were true in its special application to song, would immensely simplify interpretation, and moreover that preferential mating would contribute not a little to the success of the whole territorial system.

No one can deny the strength of the argument: that the s.e.xual instinct, like all other instincts, must require a stimulus of an appropriate kind; that the effect of the s.e.xual call upon the female cannot be neutral; and hence the probability that stimulation varies too; no one, I say, can question the strength of this evidence, and, one might add, of the evidence derived from the a.n.a.logy of the human voice. But when we have said this, we have said all; and our acceptance of the hypothesis, so far as song is concerned, must remain provisional so long as the evidence remains but secondary evidence.

CHAPTER V

THE RELATION OF THE TERRITORY TO THE SYSTEM OF REPRODUCTION

In the first two chapters I tried to show that the inherited nature of the male leads it to remain in a definite place at a definite season and to become intolerant of the approach of members of its own s.e.x, and that a result is thus attained which the word "territory" in some measure describes. But the use of this word is nevertheless open to criticism, for it denotes a human end upon which the highest faculties have been brought to bear, and consequently we have to be on our guard lest our conception of the "territory" should tend to soar upwards into regions which require a level of mental development not attained by the bird. It is necessary to bear this in mind now we have come to consider the meaning of the territory, or rather the position that it occupies in the whole scheme of reproduction.

Relations.h.i.+p to a territory within the interrelated whole of a bird's life serves more than one purpose, and not always the same purpose in the case of every species. We have only to glance at the life-histories of divergent forms to see that the territory has been gradually adjusted to suit their respective needs--limited in size here, expanded there, to meet new conditions as they arose. Now some may think that the theory would be more likely to be true if the territory had but one purpose to fulfil, and that one the same for every species; and they may see nothing but weakness in the multiplication of ways in which I shall suggest it may be serviceable. But such an objection, if it were raised, would arise from a mistaken conception, a conception which, instead of starting with a relations.h.i.+p and working up to the "territory," sees in the "territory" something of the bird's own selection and thence works back to its origin. Holding the view that it is nothing but a term in a complex relations.h.i.+p which has gradually become interwoven in the history of the individual, I see no reason why the fact of its serving a double or a treble purpose should not be a stronger argument for its survival. I now propose to examine the various ways in which the territory may have been of use in furthering the life of the individual, and the circ.u.mstances in the inorganic world which have helped to determine its survival.

The purpose that it serves depends largely upon the conditions in the external environment--the climate, the supply of food, the supply of breeding-stations, and the presence of enemies. Hence its purpose varies with varying conditions of existence. But before we proceed to examine the particular ways in which it has been modified to suit the needs of particular cla.s.ses of species, and the reason for such modifications, we must inquire whether there is not some way in which it has been serviceable alike to every species, or at least to a large majority of them.

Success in the attainment of reproduction depends upon the successful discharge of the s.e.xual function; and the discharge of the s.e.xual function depends primarily upon an individual of one s.e.x coming into contact with one of the opposite s.e.x at the appropriate season and when its appropriate organic condition arises. Now the power of locomotion is so highly developed in birds that it may seem unreasonable to suppose that males and females would have any difficulty in meeting when their inherited nature required that they should do so, still less reasonable to suggest that this power might even act as a hindrance to successful mating. Nevertheless, if we try to picture to ourselves the conditions which would obtain if the movements of both s.e.xes were in no wise controlled, and mating were solely dependent upon fortuitous gatherings, we shall come, I fancy, to no other conclusion than that much loss of valuable time and needless waste of energy would often be incurred in the search, and that many an individual would fail to breed just because its wanderings took it into districts in which, at the time, there happened to be too many of this s.e.x or too few of that. And as the power of locomotion increased and the distribution of the s.e.xes became more and more irregular, so the opportunity would be afforded for the development of any variation which would have tended to facilitate the process of pairing, and by so doing have conferred upon the individuals possessing it, some slight advantage over their fellows.

What would have been the most likely direction for variation to have taken? Any restriction upon the freedom of movement of both s.e.xes would only have added to the difficulties of mating; but if restriction had been imposed upon one s.e.x, whilst the other had been left free to wander, some order would have been introduced into the process. That the territory serves to restrict the movements of the males and to distribute them uniformly throughout all suitable localities, there can be no question; and since the instinctive behaviour in relation to it is timed to appear at a very early stage in the seasonal s.e.xual process, the males are in a position to receive mates before the impulse to mate begins to a.s.sert itself in the female.

We will take the Ruff as an example. According to Mr. Edmund Selous, pairing, in this species, is promiscuous--the Ruffs are polygamous, the Reeves polyandrous. Suppose, then, that upon this island of some few miles in circ.u.mference, whereon his investigations were made, the movements of neither Ruff nor Reeve were subject to control, that the birds wandered in all directions, and that the union of the s.e.xes were fortuitous, would the result have been satisfactory? We must remember that the Reeve requires more than one Ruff to satisfy her s.e.xual instinct; we must also bear in mind the possibility that the functioning of her instinct may be subject to some periodicity, and we ask whether, under these circ.u.mstances, accidental gatherings would meet all the requirements of the situation. Now, manifestly, she must be in a position to find males when her appropriate organic condition arises.

But in the absence of any system in the distribution of the s.e.xes, how could delay be avoided, or how could a uniform discharge of the s.e.xual function be a.s.sured? There is, however, a system. In the first place, there are the a.s.sembly grounds to which the birds repair season after season; and then, on the a.s.sembly grounds, there are the territories, represented, as Mr. Selous tells us, by depressions where the gra.s.s by long use has been worn away, and each depression is owned by one particular Ruff. The a.s.sembly grounds have the effect of splitting up and scattering the birds, and the number of Ruffs at any one particular meeting place is limited by the territories; with the result that Ruffs fit to breed are evenly distributed and always to be found in certain definite places, and the Reeves know by experience where to find them.

The advantage of this territorial system is therefore apparent. Instead of this district being overcrowded and that one deserted; instead of there being too many of one s.e.x here and too few of the other s.e.x there; instead of a high percentage of individuals failing to procreate their kind, just because circ.u.mstances over which they have no control prevent their discovering one another at the appropriate time--each s.e.x has its allotted part to play, each district has its allotted number of inhabitants, and the waste of energy and the loss of time incurred in the process of mating is reduced to a minimum.

Let us return again to the question of fortuitous mating, and consider the position of a male and female that have discovered one another by accident and have paired; what will be the subsequent course of their behaviour? We are a.s.suming, of course, that a territory forms no part of their life-history. If the discharge of the s.e.xual function takes place immediately and the ovaries of the female are in an advanced state of seasonal development, the construction of the nest will proceed without delay--and the nest will answer the same purpose as the territory in so far as it serves to restrict the movements of the birds and tends to make them remain in, or return to, its vicinity; but if not, there will be an interval during which both s.e.xes will continue to wander as before, guided only by the scarcity or abundance of food. In the first case, there will be the attraction of the nest to prevent any untimely separation; in the second, there will be nothing in the external environment to induce them to remain in any particular spot. Now if we turn to any common species and observe the sequence of events in the life of different pairs, we shall find that pairing is seldom followed by an immediate attempt to build; that an interval of inactivity is the rule rather than the exception, and that this interval varies in different species, in different individuals, and in different seasons.

Our imaginary male and female will therefore be faced with considerable difficulty; for with nothing in the external environment to attract them and with no restriction imposed upon the direction or extent of their flight, their union will continue to be, as it began by being, fortuitous. Next, let us consider their position were a disposition to establish a territory to form part of the inherited nature of the male.

Each one will then be free to seek food when and where it wills and to a.s.sociate with other individuals without the risk of permanent separation from its mate; and, no matter how long an interval may elapse between mating and nest-building, each one will be in a position to find the other when the appropriate moment for doing so arrives. Hence, while preserving freedom of movement for each individual, the territory will render their future, as a pair, secure.

No doubt the course of behaviour, as we observe it to-day in the lives of many species, is the outcome of, rather than the condition which has led to, the evolution of the territory. Thus, in many cases, we find that early mating is the rule rather than the exception; we find that the s.e.xes frequently separate to seek their food, and fly away temporarily in different directions; and, under exceptional climatic conditions, we find that they even revert to their winter routine and form flocks; only, however, to return to their territories, as pairs, under more congenial conditions. Yellow Buntings, for example, pair comparatively early in the season--some in the latter part of February, others in March, and others again in April; and some build their nests in April, others in May. There is a gorse-covered common which I have in mind, a favourite breeding resort of this species. Between this common and the surrounding country, the birds constantly pa.s.s to and fro. If you watch a particular male you will observe that it sings for a while in its territory, that it then rises in the air and disappears from view, and finally that it returns to the tree, bush, or mound which const.i.tutes its headquarters, where it again sings. Meanwhile the female, with which there is every reason to believe that this male has paired, behaves similarly; she, too, flies to the surrounding country and in time returns with equal certainty. Sometimes male and female accompany one another--that is, they leave simultaneously and likewise return; at other times, though they depart together, the male returns alone; or the male may disappear in one direction whilst the female does so in another--and, on the whole, there is a sameness in the direction of flight taken by the same pairs on different occasions. An interval of nearly two months may thus elapse between mating and nest-building, during which the s.e.xes are not only often apart but often separated by a considerable distance.

What does this species gain by the individuals belonging to it mating so early in the season? If the appropriate condition which leads the females to seek males were to arise in each individual at a late date, the first stage in the process--mating--would not be completed before the second--the discharge of the s.e.xual function--were due to begin.

Thus, instead of having ample time, the females would have but a short period in which to discover males; and this in some cases might lead to delay, in others to failure, and in others again to needlessly severe compet.i.tion, entailing physical exhaustion at a critical moment in their lives. Hence those females in which the appropriate organic condition developed early in the season would not only be more likely to find males, but would be in a position to rear more broods than those in which it developed late; and they would have a better chance of leaving offspring, which, in their turn, would reproduce the peculiarities of their parents. Moreover, within certain limitations, the more these successful females varied in the date of their development, the less severe would be the compet.i.tion, and the more uniformly successful would the mating of all the individuals in a given district tend to become.

But all of this renders an interval of s.e.xual inactivity unavoidable; an interval which must const.i.tute a danger unless there were something in the external environment to prevent the male and female from drifting apart. Inasmuch, then, as the occupation of a territory serves to remove all possibility of permanent separation, I suggest that its evolution has afforded the condition under which this beneficial procedure has developed--free to mate when they will, free to seek food where they will, free to pursue their normal routine of existence, and to meet all exigencies as they arise in their ordinary daily life--whilst free to do this, their future, as a pair, is nevertheless secure.

Thus far we have considered the territory in its relation to the discharge of the s.e.xual function. In many of the lower forms of life, the success or the failure of reproduction, so far as the individual is concerned, may be said to end with the completion of the s.e.xual act--the female has but to deposit her eggs in a suitable environment and then her work is done, because in due course and under normal conditions of temperature the young hatch out, and from the first are able to fend for themselves. And so, when we come to consider the question of reproduction in the higher forms of life, we are apt to focus attention too much upon the s.e.xual function and too little upon the contributory factors, the failure of any one of which would mean failure of the whole. For a bird, success in the attainment of reproduction does not merely imply the successful discharge of the s.e.xual function; much more is demanded; it must find somewhere to build its nest and to lay its eggs, it must s.h.i.+eld its young from extremes of temperature and protect them from enemies, and it must be in a position to supply them with food at regular intervals. And, consequently, every situation is not equally favourable for rearing young; there must be a plentiful supply of food of the right kind in the immediate vicinity of the nest, and it must be in greatest abundance just at the moment when it is most urgently needed--that is to say, during the first few weeks after the birth of the young. Success, therefore, depends upon manifold relations.h.i.+ps which centre in the station, and these relations.h.i.+ps vary in intensity with the conditions of existence.

First, then, let us examine the problem from the point of view of the food-supply. There are many species whose success in rearing offspring is largely dependent upon the rapidity with which they can obtain food; and it makes but little difference which species we choose out of many--Finch, Bunting, Warbler, or Chat. I shall choose the Buntings, as their life-history in broad outline conforms to the general type, and, moreover, their behaviour is fresh in my mind. The young are born in a very helpless state; they are without covering--fragile organisms, ill-fitted, one would think, to withstand extremes of temperature, and wholly incapable of protecting themselves from enemies of any description. For the first three days after they are hatched the female spends much of her time in brooding them, and, when she is thus occupied, the male sometimes brings food to her, which she proceeds to distribute or swallows. But all the young cannot be fed, neither are they ready to be fed, at the same moment; and the parents have besides to find food for themselves, and the nest has to be cleaned--all of which necessitates the young being exposed to the elements at frequent intervals. Now it is impossible to observe the instinctive routine of the parents, when the young need attention, without being impressed with the conative aspect of their behaviour. Why, we ask, are the movements of the female so brisk; why does she seek food and clean the nest so hurriedly; why, if her instinctive routine is interrupted, do her actions and her att.i.tude betray such bewilderment? I take it that the only answer we can give to these questions is that the part of her inherited nature which predominates just at this particular time is to brood. But why is brooding of such importance? Partly to maintain the young at the proper temperature, and thereby to induce sleep--and sleep for offspring newly hatched is as important as food--and partly to protect them from the risk of exposure to extremes of temperature. This latter danger is no imaginary one. Examine a young bird that has recently left the egg; observe its nakedness; and consider what it has to withstand--a temperature that may rise to 70 F. or may fall to 40 F., the tropical rain of a thunderstorm or the persistent drizzle of many hours' duration, the scorching effect of a summer sun or the chilling effect of a cold north-easterly wind, and, constantly, the sudden change of temperature each time that the parent leaves the nest.

One marvels that it ever does survive; one marvels at the evolution of a const.i.tution sufficiently elastic to withstand such changes. But, however much the const.i.tution may give us cause to wonder, it is clear that much depends upon the parents. A slight inefficiency of the instinctive response which the presence of the young evokes, a little slowness in searching for food or sluggishness in returning to the nest, might lead to exposure and prove fatal. And, however much is demanded of the parents, it is clear that much also depends upon the relations.h.i.+ps in the external environment; for no matter how sensitive or how well attuned the instinctive response of the parent may be, it will avail but little in the presence of unfavourable conditions in the environment.

Everything turns upon the question of the effect of exposure. And in order to ascertain how far extremes of temperature are injurious, I removed the nests of various species containing newly hatched young, and, placing them in surroundings that afforded the customary amount of protection from the elements, I made a note of the temperature and the atmospheric conditions and then observed the condition of the young at frequent intervals. Details of these experiments will be found at the end of the chapter.

The experiments with the Blackbirds and the Whitethroats gave the most interesting results. Both broods of each species were respectively of much the same age, yet one brood of Blackbirds survived for five, and the other only for two and a half hours, and one brood of Whitethroats lived for twelve hours whilst the other succ.u.mbed in a little over an hour. This difference is rather remarkable; and it seems clear that the power of resistance of the young diminishes rapidly when the temperature falls below 52 F. It must be borne in mind, however, that the conditions under which the experiments were made were, on the whole, favourable--the weather was dry, the temperature was not unusually low, nor was the wind exceptionally strong or cold; and even in those cases in which the young succ.u.mbed so rapidly, the atmospheric conditions could by no means be regarded as abnormal.