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 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.
[Illustration: Peregrine Falcon attacking a Raven
Emery Walker ph.sc.]
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 by its 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 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
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,[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 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, 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 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.
[Illustration: H. Gronvold dcl. Emery Walker ph.sc.
A battle between a pair of Green Woodpeckers and a pair of Great
Spotted Woodpeckers 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 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 not 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_) 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.