On the 6th June 1916, a nest of Linnets just hatched was removed at 6.47 A.M. The temperature was 51 F. At 7.50 A.M. the birds were cold and in a state of collapse, and only survived until 8.50 A.M.

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 compet.i.tion for stations where the egg or eggs can be deposited and incubated in safety; that, in others, there is compet.i.tion 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 compet.i.tion 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, pa.s.sing upwards and away. The attack, however, is soon repeated, and 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 hara.s.sed 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 pa.s.ses from the one to the other according to the position occupied by the birds in their respective territories.

[Ill.u.s.tration: Peregrine Falcon attacking a Raven

Emery Walker ph.sc.]

That there is constant warfare between the Green Woodp.e.c.k.e.r and the Starling is well known, the purpose of the Starling being to gain possession of the hole which the Woodp.e.c.k.e.r 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 Woodp.e.c.k.e.r 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 a.s.signed 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 d.y.k.e 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. .h.i.ther 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 t.i.t family and some of the smaller Warblers there are constant exhibitions of hostility; even the Great t.i.t is liable to be driven away, but the Blue t.i.t 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 t.i.ts invaded its ground, pa.s.sing 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 by its subsequent behaviour, for whenever a t.i.t 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 s.e.x 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 pa.s.s from one to the other according to the circ.u.mstances 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 compet.i.tion 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 att.i.tudes a.s.sumed 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 in 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-Warbler 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 c.u.mulative 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 const.i.tutions, 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 s.p.a.ce 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 t.i.ts, as a family, are notoriously pugnacious. I have seen a pair of Blue t.i.ts attack a single Long-tailed t.i.t 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,[6] "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 att.i.tude 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 pa.s.sed; 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 a.s.semble 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 a.s.sociate 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 pa.s.s 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 t.i.ts travel together in parties; hosts of Finches collect in the hollies to pa.s.s 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 a.s.sembled 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, 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 pa.s.s 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 pa.s.s 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 pa.s.sing across it, the chances are that they will find the ground occupied--and their behaviour under such circ.u.mstances is no less interesting than the behaviour of the bird upon whose ground they are trespa.s.sing. 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 circ.u.mstances 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 pa.s.sing 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 s.e.xual 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, pa.s.sed 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 s.e.x, 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 Woodp.e.c.k.e.r 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 Woodp.e.c.k.e.r. Bird for bird, the Woodp.e.c.k.e.r 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 Woodp.e.c.k.e.r shows itself, attack with determination; and not only do they do so but they are a.s.sisted, so there is reason to believe, by other individuals or pairs in the attainment of their end, so that no matter how stoutly the Woodp.e.c.k.e.r defends itself, in time it is almost certain to be deprived of its ownership.

In like manner different kinds of Woodp.e.c.k.e.rs 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 Woodp.e.c.k.e.rs endeavouring to drive away a Great Spotted Woodp.e.c.k.e.r. The excitement of all three birds was exceptional. Each of the Lesser Spotted Woodp.e.c.k.e.rs 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 Woodp.e.c.k.e.rs and a Great Spotted Woodp.e.c.k.e.r is worth mentioning. It occurred on the 24th of April.

Pa.s.sing through the middle of a wood, I noticed a Great Spotted Woodp.e.c.k.e.r fly out of a hole in an oak-tree. Shortly afterwards, a pair of Green Woodp.e.c.k.e.rs 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 Woodp.e.c.k.e.r returned to the hole and entered. Both of the Green Woodp.e.c.k.e.rs 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 Woodp.e.c.k.e.r 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 Woodp.e.c.k.e.r hurriedly left. After this, all was quiet and the Green Woodp.e.c.k.e.r eventually descended and entered the hole. The Great Spotted Woodp.e.c.k.e.r, however, returned again, but, after fluttering around the hole, disappeared, leaving the Green Woodp.e.c.k.e.rs in possession.

In this varied field of hostile behaviour which we have explored, one feature stands out prominently, namely, that the interest 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 circ.u.mstances in which it was placed. But its nature is its inborn const.i.tution, and its const.i.tution 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.

[Ill.u.s.tration: H. Gronvold dcl. Emery Walker ph.sc.

A battle between a pair of Green Woodp.e.c.k.e.rs and a pair of Great Spotted Woodp.e.c.k.e.rs for the possession of a hole in an oak tree.]

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 const.i.tution 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.