CHAPTER VI
THE WARFARE BETWEEN DIFFERENT SPECIES AND ITS RELATION TO THE TERRITORY
We have now considered the various ways in which the territory is useful in furthering the life of the individual. We have seen that, in some cases, there is competition for stations where the egg or eggs can be deposited and incubated in safety; that, in others, there is competition for stations capable of furnishing an adequate supply of food for the young; and that the establishment of "territories" not only renders the attainment of reproduction for the individual secure, but serves so to regulate the distribution of pairs that the maximum number can be accommodated in the minimum area. This being so, the question arises as to whether competition for territory is strictly limited to individuals of the same species, or whether it may not occur also between different kinds of birds, providing always that similar conditions of existence are required. First of all I shall relate a number of facts which will serve to show the nature and extent of the warfare, and I shall then give the reasons which lead me to believe that the fighting not only bears some relation to the "territory," but that it is an important factor in contributing to the attainment of that which for biological interpretation is the end for which the whole territorial system has been evolved.
Those who have studied wild life on one of the rocky headlands, which are so numerous round our coasts, will probably be familiar with the rivalry that exists between the Raven and certain birds of prey. Where the Raven finds shelter for its nest, there, too, the Peregrine has its eyrie—and so it happens that these two species are continually at war. Now the warfare occurs not only during the season of reproduction but continues throughout the greater part of the year, and can even be observed in the late summer or early autumn—the period when we should expect to find the instinct least susceptible to appropriate stimulation. But it is of a more determined kind early in the spring, and it is then that we often witness those remarkable exhibitions of flight, the skill of which excites our admiration. The Falcon rises above the Raven, stoops at it, and when it seems no longer possible for a collision to be avoided, or, one would imagine, for the Raven to escape destruction, the Raven skilfully turns upon its back and momentarily faces its opponent, and the Falcon with equal skill changes its course, passing upwards and away. The attack, however, is soon repeated, and 
Peregrine Falcon attacking a Raven
Emery Walker ph.sc.
though no collision may actually take place, yet the fact that the Raven, when it turns to face its adversary, is obliged to drop the stick which it carries, is not only an indication of the character of the struggle, but it shows that a definite end is gained—that the efforts of the Raven to build in that particular locality are hampered. But the Falcon is not the only enemy that the Raven has to face; Buzzards are just as intolerant of the presence of Ravens in their^neighbourhood as the Ravens are of them, and consequently there is incessant quarrelling wherever the same locality is inhabited. As a rule, the fighting occurs whilst the birds are on the wing; the Buzzard rises to a considerable height, and, closing its wings, stoops at the Raven below, and when within a short distance of its adversary, swerves upwards and gains a position from which it can again attack. The Buzzard, however, is by no means always the aggressor; I have watched one so persistently harassed by a Raven that at length it left the rock upon which it was resting and disappeared from view, still followed by its rival. Thus it seems as if they were evenly matched, and, when they occupy the same locality, it is interesting to notice how the initiative passes from the one to the other according to the position occupied by the birds in their respective territories.
That there is constant warfare between the Green Woodpecker and the Starling is well known, the purpose of the Starling being to gain possession of the hole which the Woodpecker with much skill has drilled for itself. As far as my experience goes, the Starling is always the aggressor, and there is only too good reason to fear that, in the course of time, the Green Woodpecker will disappear as a result of the greater fertility and tenacity of its enemy. The Martin suffers a similar kind of persecution from the House-Sparrow, and here again there is reason to believe that the greater virility of the Sparrow will hasten the extinction of its rival. In cases of this description the purpose of the fighting is clear, and one can understand why such divergent species should be hostile to one another; yet others, equally remote in the scale of nature, are hostile when no such ostensible reason can be assigned for their hostility. Few birds are more pugnacious than the Moor-Hen, and the determined manner in which different individuals fight with one another is notorious. But the intolerance it displays towards other species is no less remarkable, and its pugnacious instinct seems to be peculiarly susceptible to stimulation by different individuals belonging to widely divergent forms. At one moment a Lapwing may be attacked, at another a Thrush or a Starling, harmless strangers that have approached the pool to drink; even a Water-Rail, as it threads its way through the rushes, may fail to escape detection; and, which is still more curious, a covey of Partridges will evoke response if they approach the pool too closely. Here is a curious instance of apparent waste of energy. A pair of Magpies built their nest in an ilex tree. Early one morning there was a commotion in the tree, much flapping of wings and a medley of sounds which told of large birds engaged in a struggle—the Magpies were attacking a pair of Wood-Pigeons. There was no question as to the genuineness of the struggle, nor any doubt as to the proximate end for which the Magpies were striving, for their efforts continued so long as the Wood-Pigeons remained in the tree, and only ceased when they had succeeded in driving them away.
Turning next to species which are less distantly related, we find that instances of intolerance are more numerous and that a wider range of species is involved. The hostility that the Lapwing displays towards the Snipe calls for special remark. It often happens that the marshes or water meadows, that are such favourite haunts of the Lapwing, are also resorted to by Snipe for the purpose of securing food, or it may be even for the purpose of reproduction. In such places both species are often abundant; the meadow is divided up into Lapwings' territories, and early in the season the Snipe wander over it in small parties, singly, or in pairs. Now, if it were only on isolated occasions that the Lapwing paid heed to the Snipe, one would not perhaps attach any peculiar significance to the fact; but the pugnacious instinct of the bird responds to the presence of this intruder almost as freely as it does to that of another Lapwing. Again and again, day after day, the Snipe are attacked and driven off in a manner which would be fittingly described as persistent persecution, for the Snipe has neither the physical capacity nor apparently any instinctive tendency to retaliate. Thus a Lapwing may come suddenly upon a small party of Snipe hidden from view in a dyke where they are probing the ground for food; the Snipe immediately rise and fly away and there is momentary confusion as the Lapwing darts first at this one, then at that; or, espying a Snipe at rest at the opposite end of its territory, it will first of all run rapidly towards it, and then fly after it, as, with twisting flight, it darts hither and thither a few feet above the ground; or again, it will attack and rapidly pursue solitary individuals as they skim across its territory and attempt to settle. Is this intolerance merely an exuberant expression of an instinct which is serviceable in another direction? The behaviour of the Lapwing scarcely justifies such a conclusion, for all its actions denote a striving towards some end which we can describe, and it seems to gain satisfaction only when the ejection of the intruder has been accomplished.
Many of the Warblers display irritation when approached by other birds which we should scarcely expect would arouse their hostility. The Hedge-Sparrow, for example, is frequently regarded with suspicion, and it is by no means unusual to see it attacked by so small a bird as the Chiffchaff. The Wood-Warbler is also pugnacious, and will even attack a pair of Chaffinches. Between the Tit family and some of the smaller Warblers there are constant exhibitions of hostility; even the Great Tit is liable to be driven away, but the Blue Tit is especially marked out for persecution, though doubtless it is well able to hold its own.
The following incident will show how real is the antagonism between these two families. A Chiffchaff occupied the corner of a small osier bed, and was particularly aggressive towards other closely-related forms in its immediate neighbourhood. On two mornings in succession ten Blue Tits invaded its ground, passing from end to end of it as they wended their way from tree to tree in search of food. Their presence evoked the usual hostile response, yet, withal, aroused the fear of the Chiffchaff, which, at times, appeared to be swayed by conflicting impulses. Now, in attempting to interpret the nature of the instinct which was evoked, one has to be guided, in a case of this description, by the similarity of the response to that which can be observed on other occasions and in other situations when the intention of the bird is clear. And on this occasion the Chiffchaff betrayed all the symptoms which normally precede an attack; it spread its tail, quivered its wings, uttered its high-pitched note rapidly, hopped from twig to twig, or flew restlessly from tree to tree, and seemed to be prevented from attacking only by the number of its opponents. This, indeed, was shown byits subsequent behaviour, for whenever a Tit became temporarily detached from its companions it hesitated no longer but forthwith attacked.
There are other species which are no less aggressive than the Warblers—the Chats for example. The Stonechat regards with suspicion almost any bird of its own size, and will even pursue a Tree-Pipit if it approaches too closely. The same is true of the Whinchat, and one would scarcely expect to find this bird attacking Buntings as it sometimes does. A Whinchat that occupied some marshy ground was constantly at war with a pair of Reed-Buntings; their territories were adjacent and in some measure overlapped, and the Whinchat drove away either sex indiscriminately, and was not only always the aggressor but seemed to be master of the situation.
Coming now to kindred forms, those, that is to say, which belong to the same family, we find that, both in intensity and extent, the warfare far exceeds anything that we have thus far considered. So frequent, indeed, are acts of intolerance, and so readily awakened into activity is the pugnacious nature of the bird, that the fighting will almost bear comparison in volume with that which occurs between individuals of the same species. Between the Thrush and the Blackbird there are incessant quarrels early in the year, and the initiative seems to pass from one to the other according to the circumstances in which they are placed. If the territory of a Thrush is invaded the Thrush is the aggressor, and, conversely, if that of the Blackbird is threatened, the Blackbird becomes the aggressor; and so, when the territories of the two birds are adjacent or overlap, as frequently they do, there is constant friction, resulting in quarrels which attract attention on account of the noisiness of the birds.
All the Warblers are exceedingly pugnacious, the fighting being especially severe between those that are very closely related. The Blackcap and the Garden-Warbler are constant rivals, and the scenes which can be witnessed when the two meet in competition are interesting from many points of view. The birds not only pursue and fight with one another, but their emotional behaviour reaches a high level of intensity—excitable outbursts of song are indulged in, tails are outspread, wings are slowly flapped, and feathers raised—in fact the attitudes assumed are similar in all respects to those which occur during the contests which are so frequent between the respective individuals of each species; and it would be difficult to point to any one item of behaviour which is not also manifest at one time or another during the battles between these rivals, and still more difficult to trace any difference iii the intensity of the excitement. And if we are satisfied that the fighting in the one case is purposive, so, too, must we regard it as having some biological purpose to serve in the other. But the Garden-Warber is not the only bird that acts as a stimulus to the instinct of the Blackcap; Whitethroats are often attacked, and the Chiffchaff is a source of irritation. Even when a male Blackcap is engaged in incubation, it will leave its nest on the approach of a Chiffchaff, and, having driven away the intruder, proceed to sing excitedly. At other times both male and female will combine to attack this small intruder.
But this does not mean that the Chiffchaff suffers persecution; it is itself most aggressive, as is shown by the fact that it will join in the Blackcap quarrels and attack the combatants indiscriminately. Its behaviour, however, requires further consideration, especially as regards its relations with its nearest of kin—the Willow-Warbler; for here we have a mutual intolerance which is somewhat remarkable, and evidence of it can be found wherever the birds occupy the same ground. Now it can be observed that the hostility is not limited merely to occasional acts of intolerance, but that there is organised warfare lasting, it may be, for many days in succession, and that the actions of the birds bear the stamp of a persistent striving towards some end. On one occasion the Willow-Warbler may be the aggressor, on another the Chiffchaff, and at times it is difficult to say which of the two is responsible for the quarrel. In size and in strength they are equal, and the "will to fight" is as strong in the one as in the other, so that it is seldom, if ever, possible to point to this one as the victor and that one as the vanquished. Success or failure probably depends more upon the cumulative effect of many combats entailing physical exhaustion, than upon the issue of any one particular battle; and whilst observation might quite well fail to distinguish any resultant change in the relative positions of the birds, or any harmful effect upon their constitutions, yet the area occupied by this one might be sufficiently curtailed to prejudice the welfare of the young, or the vitality of that one might be seriously impaired—and we should be none the wiser.
Neither the Marsh-Warbler nor the Reed-Warbler will tolerate strangers within the small space of ground over which they exercise dominion. Of the two, the Marsh-Warbler is perhaps the more pugnacious, and will attack any other Warbler that approaches too closely; Whitethroats are often pursued and driven away, and less frequently. Garden-Warblers. In one case, a male occupied the same ground as a Sedge-Warbler, and there was a constant feud between them; a willow-tree formed its headquarters, and this same tree seemed to be the headquarters of the Sedge-Warbler, so that they often met and whenever they did so they quarrelled. As a rule the Marsh-Warbler was the aggressor and had the mastery over its opponent, and when it attacked, it uttered a peculiar harsh scolding note, raised the feathers on its back, spread out its wings, and betrayed the usual symptoms of emotional excitement.
On the other hand, the Sedge-Warbler is most aggressive towards other kindred species, and when a male happens to occupy the same ground as a Reed-Warbler, there are frequent battles between them and incessant commotion; they fly at one another and meet in the air with an audible clicking of bills, or pursue one another amongst the reeds, each one uttering its characteristic scolding note.
The Tits, as a family, are notoriously pugnacious. I have seen a pair of Blue Tits attack a single Long-tailed Tit with great determination, and not only did they pursue it, but, flying at it, struck it with considerable force.
In giving an account of the domestic economy of the Carrion-Crow. Mr Edmund Selous refers to the hostility between this bird and the Magpie. "About a week ago," he says,[1] "I saw a Crow busily engaged in chasing away several Magpies, not only from three or four tall slender trees close together, in one of which it had its nest, but also from various other trees, not far off, round about. In this the Crow had a good deal of trouble, as the Magpies were always returning. After a time it was joined by another crow, which however did not take so active a part in the drama, nor did I see either of the two actually go to the nest, though I could only explain their action by supposing it was their own. This morning I saw the same thing reversed, for a pair of Magpies, with an undoubted nest, kept attacking a Crow that insisted on settling in one of a row of trees—also tall and slender—in which it was placed. Both were equally persevering—the Crow, though often chased away, always returning, and settling generally in the last tree of the row, where he would be left alone sometimes for a minute or two, but before long one of the Magpies flew at him, and put him to flight. The Crow defended itself, but not, it would seem, very successfully, and in the last attack upon him, made, with great spirit, in the air, a large black feather floated to the ground, which I made no doubt was his. Yet this did not drive him from the trees, and it was only on my approaching nearer that he finally left them. Thus we see that both species look upon the approach of the other to within a moderate distance of their nest as an intrusion."
That the Rook suffers persecution from the Carrion-Crow is a well-established fact, and there is reason to believe that it has another dangerous enemy in the Hooded Crow. According to the late Mr Ussher. Choughs will attack both Hooded Crows and Ravens. "I once saw," he says, "two Choughs energetically attacking a pair of Ravens; they shot up into the air and darted down on the latter, whose heavy flight made them helpless against their agile tormentors."
Birds of prey are often hostile to one another. The Merlin is exceptionally pugnacious, and its boldness in attacking intruders is well known. When, for example, a Kestrel approaches its territory, it leaves the tree, bush, or rock upon which it was resting, utters its characteristic cry, and soars rapidly upwards; then, rising to a considerable height, it swoops down upon the Kestrel, and by alternately stooping at and chasing its opponent, drives it away from the immediate neighbourhood.
What we have, then, to consider is, Do these battles between different species contribute towards the attainment of the end for which the whole territorial system has been evolved?
Let us take the individual and see whether we can establish any relation between the hostility it displays towards members of other species and its general disposition to secure a territory. We must remember that a male can have no knowledge of the prospective value of its behaviour, nor is it likely that it has any ulterior purpose in ejecting other males, beyond the pleasure it derives from satisfying its impulse to do so. The proximate end of its behaviour is to attack, nothing more, and this, of course, it can only do just in so far as the intruder evokes the appropriate instinct.
Now the arguments we shall employ will, on the whole, be similar to those which we made use of in the second chapter, wherein we attempted to ascertain the conditions under which a male becomes intolerant of other males of its own species, and examined more especially the claims of the "territory" as opposed to those of the "female." But here we start on firmer ground, because the one factor which introduced an element of uncertainty—the female—can be definitely excluded; at least it seems so to me, for granting even that her presence is the condition under which the pugnacious nature of the male is rendered susceptible to stimulation, it is difficult to see why a male of a different species should supply that stimulus, or what biological purpose could be served by its doing so.
When dealing with the attitude of a male towards others of its kind, we attached considerable significance to the fact that its pugnacious nature gained or lost susceptibility according to the position which it happened to occupy. We found, it will be remembered, that the same bird that was pugnacious in its own territory took no further interest in its opponent when the boundary was passed; and, moreover, that if it happened to wander into an adjoining one, it made no, real effort to defend itself when attacked, but returned forthwith to its own headquarters. It remains to be shown whether the rivalry between different kinds of birds is similarly related to the position which the opponents happen to occupy at the time.
First, then, there is the general consideration, namely, that the enmity occurs for the most part just at the time when the territories are in process of being established. During autumn and winter, many birds of more or less close affinity assemble together in flocks, wherever the supply of food is abundant, and are then not only sociable, but, so there is reason to believe, are mutually helpful both in discovering the necessary means of subsistence which are often none too plentiful, and in affording protection from enemies, which, on the contrary, are often numerous. That the different units of which these flocks are composed should live on amicable terms is therefore as necessary for the welfare of the whole community at this particular season as that the different individuals of the same species should do so. But just as the sociable relations, which obtain between these individuals throughout the winter, undergo a marked change at the commencement of the breeding season, so, too, do different species, which habitually associate together, suddenly become hostile to one another. This change is coincident in time with the rise of the organic condition which leads to the establishment of territories; and the hostility continues, though in diminishing degree, throughout the breeding season, and dies away the following autumn.
For example, different Warblers resort to the elders {Sambucus nigra) in September, and there pass much time feeding on the fruit which is then ripe and often abundant. In the same bush there may be Blackcaps, Garden-Warblers, Whitethroats, and Lesser Whitethroats, some preening their feathers, others searching for the berries, others again, with feathers relaxed, making feeble attempts to sing. Occasionally there may be a scuffle, perhaps between a Blackcap and a Lesser Whitethroat, or between a Garden-Warbler and a Blackcap, but it is of short duration and lacks vigour. Apart, however, from such temporary disturbances, there is no real rupture in their relations, and certainly nothing to lead one to suppose that the bickerings are determined by the functioning of any~ specific instinct. Yet only a few months previously some of them were constantly at war, and their quarrels betrayed symptoms of great persistence; and if we remember how the observed behaviour of the birds suggests the fact that they were striving to attain something definite, we shall understand the nature and extent of the change, and shall, I fancy, be in a better position to estimate its biological worth at its true value.
We can find many similar examples—flocks are to be found on arable ground, on the water meadows, and on the mud-flats; here different kinds of Thrushes feed on the berries of the yew, there different kinds of Tits travel together in parties; hosts of Finches collect in the hollies to pass the night and Buntings roost together in the gorse; and, in fact, in whatever direction we choose to look in the autumn and winter, we find various birds assembled together and living on amicable terms. All of this changes in the spring, and the relationship undergoes a gradual but noticeable alteration; so much so that whereas the outstanding feature of bird life in the winter is sociability, that of the spring is hostility.
So much, then, for the seasonal change of relationship; let us now turn to particular cases and attempt to trace the condition which accompanies such change.
Many migrants in the spring seem to follow the course of the Severn during their journey northwards through Worcestershire; and where the river bends to the north-west at Lincombe Lock[2], there they leave it, or, rather, continue in a north-easterly direction which takes them across the southern end of Hartlebury Common. As I have already mentioned, this Common is overgrown with gorse, heather, and ling, and scattered here and there are a number of dwarf oak-trees and small elder-bushes. The situation is therefore an ideal one for the smaller migrants to rest for a brief time, and, from the point of view of the observer, very suitable because it is open and the movements of the birds can be traced for some distance. Turtle Doves pass over at a great height, or skim across a few feet above the gorse; Redstarts settle for a few minutes and then disappear; Tree-Pipits, Whinchats, and Willow-Warblers pass from tree to tree or flit from bush to bush—and all in a north-easterly direction. They do not sing, they are restless, and, judging by their behaviour, they are anxious to conceal their presence, not to make it known. Yet we know that when they reach their destination, as presently they will, all this will change; that each of them will employ every means at its disposal to make itself conspicuous; and that each, as far as it is able, will resist intrusion on the part of other species.
Now the southern end of the Common is always inhabited by individuals belonging to one of these species, or to others of close affinity; so that wherever these travellers settle whilst passing across it, the chances are that they will find the ground occupied—and their behaviour under such circumstances is no less interesting than the behaviour of the bird upon whose ground they are trespassing. We will take the case of the Whinchat. It arrives from the south-west, and, flying from bush to bush, works its way in a north-easterly direction. In doing so it intrudes upon the territory of a Stonechat; and the Stonechat, becoming excited, flies towards it, and it retires for a short distance in the direction from whence it came. Here again it is followed and attacked and again moves on, and then, flying in a circle as if to avoid the territory which blocked the path, resumes its former line of flight, though still followed by the Stonechat, which after continuing the pursuit for perhaps a quarter of a mile, suddenly turns in the air and returns to its headquarters.
It is difficult to put oneself in the place of the Stonechat or of the Whinchat. But even after making due allowance for the danger inseparable from any attempt to do so, there remains the unquestionable fact that whereas the impulse to attack was strong in the one, the impulse to defend itself was wholly lacking in the other. Yet a Whinchat, when it has established itself, is most pugnacious; it not only attacks every bird of a similar size that approaches its position, but its behaviour under such circumstances bears the impress of unusual determination; and if we were to take a male and place it in the position of the Stonechat, we should find that its nature would change, that the presence of the Stonechat would evoke a hostile response, and, conversely, that the instinct of the Stonechat would not be susceptible to stimulation. Hence it is clear that the nature of a bird when on migration is not quite the same as it is when its destination is reached; that the positions occupied from time to time during the journey carry no meaning, or, rather, are not brought into relation with its life in quite the same way as is the position which it finally occupies; and further, it is clear that the interest it displays in other species undergoes a somewhat remarkable transformation when at length its destination is reached.
This altered nature of the migrant is a fact of some importance in relation to our present subject, but it does not stand alone—the same characteristic is observable in other phases of bird life. Some of the residents, the Buntings and the Finches for example, occupy their breeding ground very early in the year, and it often happens that the situations which they select are not capable of supplying them with food so early in the season, though at a later date food will be there in abundance; so that they are compelled to resort to the surrounding neighbourhood, and since, even there, the available supply is sometimes scarce or, if plentiful, limited to certain areas, they are constrained from time to time to join together again in flocks. Thus, for part of the year, they may be said to lead a double existence; for just as the Whinchat, that is sociable on migration, betrays a changed nature when it reaches its destination, so too does the nature of these residents change from hour to hour according to whether they are seeking food or occupying the breeding ground.
In the newly-sown fields of grain the birds frequently find a supply of food. Here Yellow Buntings, Greenfinches, and Chaffinches collect from the surrounding neighbourhood. The majority are somewhere in possession of territories, and not a few are paired. Between the territories and the feeding ground a highway is formed by individuals passing to and fro. Sometimes both members of the pair leave together in order to seek food, at other times they separate and the male may be in his territory whilst the female is with the flock. Apart from occasional manifestations of sexual emotion on the part of a male, there is nothing to disturb the harmony of the flock nor anything in the behaviour of the birds which would lead one to suspect that, when they return, their nature will change and that they will be no longer sociable; and, which is still more remarkable, no matter how great the provocation which an individual, when in company with the flock, may be called upon to endure, its customary hostile response will fail to be elicited. An incident which happened in the spring of 1917 will serve to make this clear. A flock of some thirty Yellow Buntings. Greenfinches, and Chaffinches were feeding in one corner of a field which had recently been sown with barley. As they sought their food they wandered outwards into the middle of the field, and in so doing, passed across the territory of a Skylark. Whereupon the Skylark became excited, uttered its call-note rapidly, and rising a few feet from the ground, attacked those members of the flock that were nearest, which happened to be the Yellow Buntings; and so determined were its onslaughts that the Yellow Buntings were forced to retire. The Skylark showed no discrimination as to sex, but attacked both males and females, and within a few minutes succeeded in driving away at least two pairs. One would have expected that the Yellow Buntings would have made some show of resistance; one would have thought that the fact of being violently attacked would have supplied a stimulus sufficiently strong to evoke a corresponding hostile response: yet there was no mistaking the lack of interest that they displayed in the contest—they made no effort to retaliate but seemed to accept the situation as unalterable and left.
So far we have examined only those cases in which the pugnacious instinct was stimulated in one of the adversaries, and in which consequently the fighting seldom reached any high degree of severity. We must now consider some others in which each of the opponents acts as a stimulus to the pugnacious instinct of the other. It is here, of course, that we find the most violently contested battles, and it is here, too, that the purpose of the fighting seems clear. The persecution which the Green Woodpecker suffers from the Starling is well known. The purpose of the Starling's behaviour is clear, namely the possession of the hole occupied by the Woodpecker. Bird for bird, the Woodpecker is more than the equal of the Starling, but persistent endeavour ultimately wins the day. The Starlings perch close beside the hole, and, whenever the Woodpecker shows itself, attack with determination; and not only do they do so but they are assisted, so there is reason to believe, by other individuals or pairs in the attainment of their end, so that no matter how stoutly the Woodpecker defends itself, in time it is almost certain to be deprived of its ownership.
In like manner different kinds of Woodpeckers contend with one another for the possession of a hole, and here the opponents are more equally matched. I have seen a pair of Lesser Spotted Woodpeckers endeavouring to drive away a Great Spotted Woodpecker. The excitement of all three birds was exceptional. Each of the Lesser Spotted Woodpeckers kept swooping in turn at their rival, sometimes in the air and sometimes when it was settled on the topmost branches of a dead tree, and the sounds produced reminded one of the piping of a flock of Oyster-Catchers in flight.
A battle between a pair of Green Woodpeckers and a Great Spotted Woodpecker is worth mentioning. It occurred on the 24th of April. Passing through the middle of a wood. I noticed a Great Spotted Woodpecker fly out of a hole in an oak-tree. Shortly afterwards, a pair of Green Woodpeckers settled near the hole and then flew to some oak-trees close at hand, where they were joined by their rival and signs of hostility were soon apparent. Presently the Great Spotted Woodpecker returned to the hole and entered. Both of the Green Woodpeckers then flew into the tree; and one of them, settling upon the trunk, climbed up to the level of the hole and, when it became aware of the Great Spotted Woodpecker within, extended its wings fully and proceeded to peck viciously at its opponent. Whereupon there was a scuffle at the mouth of the hole and the Great Spotted Woodpecker hurriedly left. After this, all was quiet and the Green Woodpecker eventually descended and entered the hole. The Great Spotted Woodpecker, however, returned again, but, after fluttering around the hole, disappeared, leaving the Green Woodpeckers in possession.
In this varied field of hostile behaviour which we have explored, one feature stands out prominently, namely, that the interest 
A battle between a pair of Green Woodpeckers
and a Great Spotted Woodpecker for the
possession of a hole in a tree[3]
which a bird displays in other species varies not only at different seasons but even from hour to hour. I have used the word "nature" as equivalent to "interest," and I have spoken of the bird's nature changing or altering according to the circumstances in which it was placed. But its nature is its inborn constitution, and its constitution cannot change from day to day, still less from hour to hour. So that, in a sense, and having regard to strict scientific accuracy, it is misleading in this particular connotation to say that the bird's nature changes.
What then does happen? The instinct of pugnacity must form just as much a part of the hereditary make-up of the migrant, when on migration, as when finally it reaches its destination; still more must it form part of the constitution of the Bunting when it leaves its headquarters temporarily and joins the flock. And, if it is there, the question arises as to why it does not respond. Now every instinct requires for its response a stimulus of an appropriate kind, and, therefore, a reasonable view to take would be that the necessary stimulus was lacking. But this is a view which we cannot uphold, because on all these occasions an opposing male was present-and, so far as it is possible to judge by observation, that is the stimulus which in the main evokes a hostile response. We must therefore look elsewhere than in its direction for a reason which will adequately explain the behaviour.
Though it be true that every instinct requires for its functioning a stimulus of an appropriate kind, yet it is also true that the condition which will render it responsive must be present. What we have then to consider is whether the phenomena which we have explored give us any clue as to the particular nature of that condition. In the first place, we have the general fact that the hostility is not confined to a few species belonging to a few families, but that it is of wide application—birds of prey. Warblers. Woodpeckers, all supply us with evidence which serves to show, in greater or less degree, its nature and extent. Next, we found that the hostility was peculiar to a certain season—and that one the season of reproduction. And if the question were asked: What condition would then be most likely to render the instinct susceptible, the answer that would most certainly be given would be—^the presence of a female. And in reply to a further question as to the particular nature of the stimulus to which the instinct would respond, we should be told—the presence of another male of the same species. Now the possible influence of the female on the course of the male's behaviour was the subject of inquiry in the second chapter, wherein we endeavoured to explain the hostility between males of the same species, and we came to the conclusion that it was not alone sufficient to account for the facts disclosed. Still less likely, therefore, is it that her presence can bear any direct relation to the hostility between different species, the more so since the biological end of securing a mate is definitely excluded. And we have something in the nature of proof of the correctness of this view in the fact that she accompanies her mate when he joins the flock, and that there his instinct is not susceptible to stimulation. We then proceeded to examine certain cases in which all the indications pointed to the fact that the "will to fight" was present in only one of the opponents; and we attached considerable importance to this circumstance, because we knew from experience that the same bird which seemed to lack courage, could at other times and in other situations be most aggressive. If then we ask what condition was present on the one occasion that was absent on the other, we have no difficulty in finding a reply—on every occasion on which the opponents appeared to be unevenly matched, one was in occupation of a territory and the other was not. And if we inquire further as to which of the two was the aggressor, the answer is again clear, namely, the bird that occupied a territory. Finally we considered some particular instances in which the "will to fight" was present alike in both opponents, and in which the battles were protracted and severe.
But the fact that a bird has established a territory is riot in itself sufficient to render its hostile nature susceptible; it must be actually in occupation if a response is to be elicited. We reach this conclusion step by step: the behaviour of the migrant, that lacks the "will to fight" when on migration but is pugnacious when it has secured a territory, shows it; the behaviour of the resident, which temporarily joins the flock and is there sociable, shows it; and it is shown also by the determination with which both opponents fight when the question of ownership of a station is in dispute. And of all the facts we have reviewed, this is perhaps the most important in relation to our present subject, for it demonstrates that the change from sociability to hostility is not merely an incident of the sexual season, not merely an indirect result of the functioning of the general disposition which leads to the establishment of a territory, but that it is intimately associated with the whole process, and that the particular part of the bird's nature which is concerned is so nicely balanced that it will respond under one condition and one only.
Thus we are led to the only conclusion which seems consistent with the facts, namely that there is a relationship between the "territory" and the hostility.
If we are satisfied that all this warfare is not merely an expression of an instinct which is serviceable in another direction, what part does it play in the whole scheme of reproduction? The young of many birds are delicate at birth and unable to withstand exposure to cold, and in the previous chapter we came to the conclusion that the territory was serviceable in that it provided an adequate supply of food in the vicinity of the nest, and thus obviated the necessity of the parents being absent from them for long. But manifestly no matter how active a male may be in driving away members of its own sex and kind, it will neither make its position secure, nor insure a supply of food for its young, so long as any number of individuals of different kinds are allowed to establish themselves in the same space of ground. On the one hand, then, we have the fact that there is constant strife between males of close affinity, whilst on the other, we know that many species require like conditions of existence and are bound to assemble wherever these conditions are suitable; and we can infer that the territory would fail to serve its purpose if no restriction were imposed upon the measure of such assemblies.
The question then arises: Does all this warfare contribute towards the attainment of reproduction? Not far from my house there is a small water meadow, three acres in extent, which for some years has been derelict and is now overgrown with the common rush (Juncus communis[4]) and small alder trees. For three successive seasons I watched the bird life of this meadow, and more especially the Reed-Buntings whose behaviour I was studying at the time. In every respect the meadow was suitable for this bird; there was an abundance of food and numberless situations in which nests could be placed. Each year all the pairs were successful in rearing one, if not two broods, yet the number of pairs never exceeded five—the first year there were three; the second year five; and the third year four. In addition to the four pairs of Reed-Buntings, there were in the spring of 1915, six pairs of Whitethroats, one pair of Lesser Whitethroats, four pairs of Willow-Warblers, one pair of Sedge-Warblers, two pairs of Grasshopper-Warblers, one pair of Chiffchaffs, three pairs of Hedge-Sparrows, two pairs of Tree-Pipits, one pair of Skylarks, one pair of Whinchats, one pair of Flycatchers, two pairs of Song-Thrushes, one pair of Blackbirds, one pair of Redstarts, three pairs of Chaffinches, and one pair of Wrens—in all, thirty-five pairs, whose young were mainly dependent for their living upon the insect life of that meadow and the ground immediately surrounding it. If we allow three young to each pair—and this would take no account of second broods—we arrive at the following result, namely, that one hundred and five young and seventy adults had to be supplied with food from that locality, which would mean, if the search for food were strictly limited to that meadow, that 83 square yards would be allotted to each individual.
Suppose now that the four male Reed-Buntings had each admitted one other male, and that they had secured mates, what would have been the effect upon the whole community? The four additional pairs with their young would have represented twenty individuals, which would have represented a decrease of 8.5 square yards in the space allotted to each individual. The pressure of the bird population upon the means of support would then have been materially increased; and not only the Buntings, but the Warblers. Pipits, and all the rest would have suffered. But the result would have been the same if, instead of the four additional male Reed-Buntings, four males of other kinds had been allowed to enter the marsh, and we can multiply the number four until we arrive at a point when the means of subsistence would no longer have been adequate for the adults, still less for the young. If, then, there were nothing to prevent this happening, many of the birds in that marsh would have no chance of rearing their young successfully. Hence, if the territory is adequately to serve the purpose for which we believe it has been evolved, some provision must have been included in the system to meet the difficulty.
There are three ways by which this may have been accomplished—indirectly, by increasing the size of the area occupied by each individual, and thereby reducing the relative number of each species; or directly, by rendering the fighting instinct of the bird susceptible to stimulation by individuals of other species; or, possibly, by a combination of the two. There were four pairs of Reed-Buntings in the marsh, and their territories covered the whole of it. But inasmuch as other insectivorous birds were established there also, and found sufficient food to maintain both themselves and their families, it is clear that the area these Reed-Buntings occupied was in excess of that which they would have required if they had been the sole inhabitants. And such often appears to be the case. Many a Warbler allocates to itself a space of ground more than sufficient to supply it with all that it needs; so, too, does the Finch, or the Pipit, or the Falcon—if we take no account of kindred species. Thus there is reason to believe that, by limiting the number of individuals in a given locality, this apparently wasteful expanse of territory is serviceable in that it provides against the pressure of the bird population upon the available means of support becoming too great. But though a reduction in the numerical standing of the different species would certainly follow from any increase in the area occupied by the respective individuals, and with even greater certainty would place them in a more secure position as regards their supply of food, yet, when we remember how large a number are dependent upon a supply of insect life for their young, we can understand that it would not alone be a sufficient safeguard against the dangers attendant upon overcrowding. It is here. I believe, that we shall find the true explanation of the hostility; it roughly insures that the number of pairs in any given area does not exceed the available means of support, and indeed it is difficult to imagine how such uniformity of distribution as would free the young from the risk of exposure could be obtained without some such control.
Some birds, however, have no difficulty in finding the necessary food for their young, yet have great difficulty in finding a station where they can rear their young in safety; and the area each one occupies has been reduced to the smallest proportions in order that the maximum number can be accommodated. Here, any increase in the size of the territory would inevitably lead to the extinction of the race, so that nothing stands between failure and success except the ability of the bird to defend its territory. If we study the bird population at one of the breeding stations on the coast, we find, generally speaking, that each kind of bird inhabits a particular portion of the cliff; on the lower ledges are the Guillemots and Kittiwake Gulls; higher up are Razorbills and Fulmars, and at the top, where the cliff is broken and the, face of the rock covered with turf and soil, the Puffin finds shelter for its egg. At the same time there is much overlapping; the kind of ledge that suits a Razorbill is equally suitable for a Guillemot or a Fulmar, and so, no matter how successful the Razorbill may be in establishing a territory and preventing intrusion upon it by other Razorbills, it will be all to no purpose if it allows itself to be jostled out of its position by a Fulmar. Hence, inasmuch as breeding stations are limited and competition for territory so severe, only those forms in which the fighting instinct responds freely to a wide range of stimuli will be in a position to maintain a footing upon the cliff.
In trying to estimate the importance of the hostility in its relation to the territory, we must bear in mind that competition varies in different seasons and in different localities. The surface of the land is constantly undergoing modification, partly owing to human and partly to physical agency—forests are cleared; marshes are drained; the face of the sea-cliffs is altered by the erosion of the waves; here the coast may be locally elevated, there locally depressed; and so forth. Many of these changes are slow and imperceptible, many can be observed in our own lifetime. The timber is felled and the undergrowth cleared in some wood, and the following spring we notice a change in the character of the bird population. Migrants which formerly found in it no suitable accommodation now begin to appear, and as the seasons pass by and the undergrowth affords more and more shelter for the nests and an increasing supply of insect life, so their numbers increase until the wood becomes an important breeding station, resonant with the song of many individuals. But slowly the growth increases; the bushes pass into saplings and the saplings into trees, and the undergrowth then disappears just as surely as do the migrants which can no longer find there the conditions which they require.
Or, as an illustration of the effect produced by natural agency, let me describe a change which has taken place in a corner of Co. Donegal. The promontory of Horn Head is bounded on the west by extensive sand-hills, 100 ft, or more in height. On the southern side it is divided from the mainland by a channel, which narrows down to 100 yards or so in width where it fringes the sand-hills, and then widens out again, covering an area of approximately 270 acres. As far as is known in the memory of man, this area has always been tidal. But in recent years a change has taken place, and the blown sand has silted up the channel, with the result that this tidal area has been transformed into a brackish lake. What has brought about the change is not easy to determine. There is evidence, however, of a slow alteration of the level of the shore-line; for in the midst of the sand-hills, situated 150 yards or so from the present sea-margin, and running parallel with it, there is an accumulation of pebbles some 3 feet high by 4 feet deep. This raised beach is now separated from the Atlantic by sand-drifts of considerable height, and consequently there are some grounds for believing that secular elevation is taking place, which, if it be the case, will account for the change in progress. Now the effect on the bird population can be seen even now, and will doubtless become more apparent as the years pass by. Sand-Martins used to find plenty of places to breed amongst the sand-drifts, and moreover do so still. But their nesting sites are constantly changing and disappearing, and the breeding-place of one colony, that was situated in the bank of a stream twelve years ago, is now buried 10 feet or more below the surface of the sand. The area that was once tidal, but is now a brackish lake, is fed by mountain streams, and as the fresh water predominates, so in course of time will it become fringed with vegetation; and instead of the flocks of Curlew. Dunlin, and other waders that, at low water, resorted there to feed. Coots will fight with one another for the possession of territories, and the Wild Duck will teach her young to seek their food.
In whatever direction we turn, we find that many breeding grounds are subject to incessant change. Ancient haunts disappear, new ones come into being, a change which makes life impossible for this bird, as likely as not benefits that one, and so on. There is no stability. Hence in any given district each recurring season there must needs be a large number of individuals which are obliged to seek new stations, and if there were no control over their distribution, if each one were free to establish itself wherever it chanced to alight, this locality might be overcrowded and that one deserted; and, bearing in mind how many species there are that require similar conditions of existence, we can infer that the successful attainment of reproduction would become impossible for many of those individuals so long as each species was indifferent to the presence of the others. On the other hand, if there were no control over the range of the intolerance, the smaller bird would have no chance in competition with the larger, and it is doubtful whether the larger would gain an advantage commensurate with the energy it would expend in ridding its area of the smaller. I have described battles in which the opponents were only distantly related; for instance, the Moor-Hen will attack almost any bird—Partridge, Lapwing, or Starling—that approaches its territory even temporarily. Nevertheless the antagonism between kindred forms is more prevalent, and, as a rule, characterised by more persistent effort; and thus it seems as if the susceptibility of the fighting instinct has its limitations, the degree of the responsiveness being dependent upon the affinity of the opponents.
Suppose now that we take an area inhabited by a number of different species requiring like conditions of existence, divide it into three sections, and imagine that in one they were all sociable, that in another they were all hostile, and that in a third those which were closely related were intolerant of one another. Let us suppose further that each one of them was represented by the full number of individuals that the law of territory would allow. In the first section an individual would establish itself, and, becoming intolerant of its own kind, would exercise dominion over an area roughly sufficient, providing conditions were normal, to insure an adequate supply of food for its young. But it would take no account of other species, and since any number might occupy the same ground, the fact of its having established a territory would not alone suffice to render its supply of food secure. Success in the attainment of reproduction would then become largely a matter of chance, depending upon the number of individuals that happened to settle in this place or in that. In the second section there would be perpetual warfare; for whereas the appropriate organic condition which leads to pairing arises in different species at different times, fresh claimants to occupied ground would constantly be appearing, and the efforts of the inhabitants to preserve their boundaries intact would have to be maintained throughout the whole period of reproduction; and while the stronger or more persistent forms would be more likely to breed, they would do so at the expense of their young, to which they would be unable to devote proper attention, and with an expenditure of energy that would reflect itself upon the future of the race. But the conditions of life in the third section would be such as would be more likely to yield good results. The relations of the different members of the community would be more evenly balanced, for a male would only be called upon to compete with those of its own size and strength. Thus, on the one hand, accommodation would be so divided as to secure the breeding of the maximum number of individuals with the minimum of expenditure of energy, whilst on the other, any undue pressure upon the available means of subsistence would be prevented.
There can be no question that in the latter section a higher percentage of individuals would succeed in rearing offspring. And so, by reason of the fighting instinct being more susceptible or less susceptible according to the affinity of the opponents, a control is established which, while preventing unnecessary extension of warfare, allows for sufficient extension to render the biological end secure.
These, then, are the facts—this the conclusion which can be drawn from them. It may, however, be said of these facts, as it has been said, with even less justification, of the battles between individuals of the same species, that they do not afford evidence of genuine hostility. No doubt there are many naturalists who could supplement these facts with others in which the conflicts resulted in bodily injury, or terminated fatally, or at least were of a more determined kind. But I have already drawn attention to the fact that, so long as a definite result is attained, the severity of the struggle and the amount of injury inflicted are matters of small moment. Let us, however, run over the substance of the argument, and then briefly refer again to this point of view.
After enumerating instances of hostility, sufficient in number, so it seemed, to constitute reasonable ground for the belief that they had a part to play in the life-history of the individual, the two questions we set ourselves to examine in this chapter were: Is there any circumstance in the life behaviour of the individual with which the hostility can be definitely related; and, will the hostility lead to the securing of a greater measure of success in the attainment of reproduction?
Many different species assemble together in winter and roam from place to place in search of food. But in spring their behaviour undergoes a remarkable transformation; they avoid one another and become quarrelsome, so much so that whereas the outstanding feature of the winter is sociability, that of the spring is hostility. With this general fact before us, we proceeded to investigate this change of behaviour still further. First of all we took the case of a migrant, and, comparing its behaviour, as it journeyed, with that when finally it reached its destination, we found that the bird which was notoriously pugnacious when in occupation of a territory betrayed no interest in other species as it travelled to the accustomed breeding ground. Not only so, but even though it was attacked, we found that its pugnacious instinct still failed to respond. Here, however, it may be contended, and with reasonable justification, that in the interval which elapses before the ultimate destination is reached, some change in the organic condition of the bird may occur which will account for its altered behaviour; or, it may be urged, with no less justification, that whereas on migration the bird is unpaired, when the destination is reached it is probably in possession of a mate and is therefore quarrelsome. Now, at the most, the interval can only be a matter of a few days, and it is unlikely that organic changes sufficient to bring about so important an alteration of behaviour could occur in so short a time, still less likely that they could be timed to come into functional activity just at the moment when the bird reaches its breeding ground. And with regard to the suggestion that the change can be accounted for by the presence of a mate, we shall do well to remember not only that males as a rule precede the females by some days, but that a male may even remain in its territory, mateless, for some weeks, and yet display hostility.
Nevertheless the case of the migrant did not, by itself, afford sufficient evidence upon which to base any conclusion. We therefore inquired into the behaviour of some of the residents at a corresponding period. The Bunting served as an illustration. Early in the season it establishes a territory, and because food is then scarce it is forced to seek it elsewhere than on the small plot of ground which it has acquired; and so it makes its way to some spot where the supply is abundant, and there, meeting with other species bent on a similar errand, forms with them a flock. Part of its time is then spent in the territory and part on the .feeding ground, and between these two points a highway is formed by the bird passing constantly to and fro. But the attention which it pays to other species is very different on these two occasions—when in the territory it is intolerant of strangers, but when it accompanies the flock it displays no interest in their movements. From hour to hour its nature seems to change. But, as we saw, the inborn constitution of the bird cannot change, and therefore we came to the conclusion that an explanation of the altered behaviour was to be found in the fact that the pugnacious instinct is only rendered susceptible under a certain condition. So that all the evidence tended to confirm the impression which we had gained from the course of events in the life of the migrant, namely, that the hostility bears a direct relation to the occupation of a territory.
Finally we were led to inquire whether the hostility was serviceable in promoting the welfare of the individuals. We saw that many different species require similar conditions of existence, that ancient breeding haunts disappear and that new ones come into being, and that in the ordinary course of events such species must often assemble in the same area for the purpose of reproduction. So that even though a male might be successful in protecting its ground from intruders of its own kind, yet it might still fail to rear offspring, just because it happened to choose a position in which other kindred forms had gathered. Hence if the territory is adequately to serve its purpose, some control over the local distribution of species is of paramount importance. Nevertheless, if all the different forms that require similar conditions of existence were intolerant of one another in a like degree, the smaller bird would have no chance in competition with the larger. This, however, is not the case. Some, as we saw, arouse little or no animosity in others, in fact the more closely related the rivals, the more responsive their pugnacious nature seems to become.
To return now to the view that the fighting is not really serious, but, on the contrary, that it is either vestigial and has no longer any part to play in furthering the life of the individual, or that it is a by-product of the seasonal sexual condition to which no meaning can be attached. First, there is the relationship with the territory, and this, it seems to me, is a fact of some importance; for if the fighting were merely an exuberant manifestation of sexual emotion, one would expect to find it occurring under all conditions, and not merely under one particular condition in the life of the bird. The hostility is too widespread, however, and too uniform in occurrence for us to suppose that it has no root in the inherited constitution of the bird; and if it served some useful purpose in the past, the instinct might still persist, so long as it were not harmful. Thus the view that the behaviour is vestigial is not perhaps unreasonable. But manifestly it makes no difference whether it be vestigial or a by-product of sexual emotion. whether the battle be fierce or so trivial as to appear to us to be more in the nature of "play," so long as some change in the relative prospects of the opponents is the result.
For us, then, the main consideration lies in the question: Is the behaviour serviceable now in furthering the life of the individual? Whether the evidence which we have examined affords sufficient ground for the belief that the hostility is genuine and has a part to play in the whole scheme of reproduction, each must judge for himself.
- ↑ Zoologist, 1912, p. 327. [Scan at [1] (Wikisource contributor note)]
- ↑ The most northerly lock on the River Severn, about 1 mile (1.6 km) downstream from Stourport. (Wikisource contributor note)
- ↑ Erratum applied here (Wikisource contributor note)
- ↑ The 'common rush' is now known as Juncus effusus. (Wikisource contributor note)