THE RELATION OF THE TERRITORY TO MIGRATION
Coincident in time with the growth of appropriate conditions in the
environment, organic changes take place rendering certain instincts
susceptible to stimulation; and the stimulus being applied, the Warbler
leaves the country wherein it had passed the winter and finds its way
back, with apparently little difficulty, to the district in which it was
reared or had previously reared offspring. What is the nature of these
changes and of the impulse which is first brought into functional
activity; whence comes the stimulus; and what directs the bird on its
journey--these are all different aspects of one great problem, the
problem of migration. I do not propose to discuss all these various
aspects, for indeed I have no suggestions to offer which are in the
least likely to be helpful, but I seek rather to ascertain whether the
phenomena which we have explored bear any relation to the problem as a
whole; whether, that is to say, the competition for territory and all
that appertains to it can have supplied the conditions under which, in
the process of time, this complex and definite mode of behaviour has
evolved.
We are sometimes told that we must seek the origin of migration in the
physical changes that have occurred in the ancient history of the
earth--in glacial conditions which gradually forced birds to the south,
or in the "stability of the water and mobility of the land" which
brought about a gradual separation of the feeding area from the breeding
area--and which continued for a sufficient length of time to lead to the
formation of an instinct, and that the instinct persists because it is
serviceable in promoting the welfare of the race. But when we consider
the lapse of time, and the changes that must have occurred in the
character of the bird population--the appearance of new forms and the
disappearance of the old, the ebb and flow of a given species in a given
area--and bear in mind that, notwithstanding this, the migratory
instinct, if not stronger, is assuredly no less strong, and the volume
of migration, if not greater, is assuredly no less; in short, that the
whole phenomenon is progressive rather than retrogressive, we shall find
the view that the instinct owes its origin to conditions which no longer
exist, receives but little encouragement.
I doubt not that, throughout the ages, geological changes have been an
important factor in directing or limiting the scope of migration, and
moreover are so still; just as climatic changes and the relative
abundance or scarcity of enemies have influenced the course of its
evolution. These are all contributory factors operating in the external
environment. But there are, besides, internal factors which form part of
the inherited constitution of the bird, and, being passed on from
generation to generation, afford the conditions under which migration is
constantly being renewed. It is, I believe, in this field of organic
change and relationship that the conditions of origin must be sought.
Just as the moth in passing from the rudimentary to the perfect
condition runs through a series of changes, each one of which is marked
by a typical behaviour response adjusted to meet some particular
circumstance in the external environment, so the annual history of a
bird displays an ordered routine, each phase of which can be observed to
correspond with one of the successive changes in the environment. In
almost every direction, we find that this routine is characterised, in
broad outline, by great uniformity; so much so that, providing we know
the history of one species, we can forecast with no small degree of
certainty the general course of behaviour of other members of the
family. But only the _general_ course. There is endless variation in
just the particular way in which the behaviour is adapted to meet the
needs of particular species--the major details may be said to be
specific, the minor details varietal.
Now it is that part of the behaviour routine which has reference to the
relationship between one bird and another upon which, for the time
being, I wish to dwell; for the interest that A displays in B is by no
means always the same--it changes according to the season, and this
change can be observed to be uniform throughout a wide range of species.
In winter, in whatsoever direction we turn, we observe not only that
different individuals but that different species also collect together
in flocks. And since food at that season is not always easy to obtain,
and, moreover, is only to be found in certain situations, which are
limited both in number and extent, it would seem that such assemblages
are in the main determined by accident. No doubt the abundance or the
scarcity of food does determine the movements of birds, and hence to
that extent may be held to account for the flocks. But we shall but
deceive ourselves if we think that it is the sole or even the principal
reason, or that the situation is in no wise affected by internal
factors. The behaviour of the individual in relation to the flock bears
ample testimony to the presence of a gregarious impulse which derives
satisfaction from the fact of close association.
As an illustration, let us take a bird whose movements are easily
watched, and in whose hereditary constitution the impulse to which I
allude seems to be strongly implanted--the Curlew. When the breeding
season is over, Curlew leave the mountain and the moor and return to
the coast or tidal estuaries for the remainder of the year. Here, at low
water, they find an abundant supply of food--crustaceans amongst the
sea-weed upon the rocks, and lobworms (_Arenicola piscatorum_) in the
mud as the tide advances or recedes. But when the tide is full, they
retire to those parts of the shore that remain uncovered--to isolated
rocks, or to sand-dunes, or it may even be to pasture-land in the
neighbourhood. During this period of repose large numbers of individuals
gather together on a comparatively small space of ground. They are not
constrained to do so by any shortage of accommodation, nor by any
question relative to food, nor, for the matter of that, by any
circumstance in the external environment; they are brought together
solely, this at least is the impression that one gains, by some
inherited impulse working towards that end. And their subsequent course
of behaviour tends to confirm that impression. For if we watch the
gathering together of the different units of which the flock is
composed, and study more particularly the emotional manifestation which
accompanies their arrival and departure, we shall find that the coming
of a companion arouses some emotion which is expressed by a vocal
outburst that sweeps through the flock.
Now each call, and the Curlew has a great variety, is not only peculiar,
generally speaking, to certain occasions, but is accompanied by a
specific type of behaviour, whence we can infer in broad outline the
type of emotion which is aroused. Thus we come to recognise fear, anger,
or sexual emotion, by just the particular sound which is emitted. But
even if we are going too far in referring particular calls to particular
emotions, we can, without a doubt, divide them into two broad
categories--those which are pleasurable and those which are the reverse.
And we need have no hesitation in placing the particular call to which I
allude in the first of these two categories, not only on account of the
nature of the sound produced, but because the activities which are
aroused are not such as normally accompany irritation. This is well seen
if the behaviour of different individuals be closely observed. After
resting on one leg for some time, first one and then another is seized
with cramp, and running a few yards in an ungainly way, bumps up against
its companions as if it had not full control over its movements. Its
behaviour produces irritation which is expressed by a vocal outburst,
and followed by actions the meaning of which is clear. Moreover, the
call is taken up by other individuals and sweeps over part of the flock
as does the greeting. But the nature of the cry is entirely different
from that which greets the arrival of a companion--humanly speaking it
is a passionate and impatient utterance, the height of displeasure. The
arrival, then, acts as a stimulus to something in the inherited
constitution which is expressed in, and presumably is satisfied by, this
vocal outburst; and, since the bird that arrives joins also in the
chorus, there is reason to think that the impulse which determines its
movements is similar to that which is temporarily aroused in the flock.
Apart, however, from the evidence derived from the affective aspect of
the operation of the instinct, the general course of behaviour lends
support to the view that the assemblies are determined by internal
factors, and are not merely the outcome of circumstances in the external
environment. Observe, for example, the manner in which the flock is
built up. Single individuals are content to rest alone so long as no
assembly is in sight, but they are drawn towards their companions
directly the opportunity arises, just as surely as the smaller
aggregation is drawn towards the flock; and so, as the flock increases,
it gradually absorbs all the lesser flocks and smaller parties, for the
greater the flock the greater the attraction seems to be; and different
individuals appear to gain some satisfaction from being in close bodily
contact with one another.
When the Curlew flies to that part of the mud-flat which is first
exposed by the receding tide, and there associates with others, it does
not then do so because it has any interest in its fellows, nor because
they serve as an attraction, but because it is constrained by hunger--in
other words, the association is determined by accident. But when, during
periods of repose, it sees a flock, flies to it, and takes up a position
in the midst of it, it does so not because suitable accommodation is
lacking--not therefore because of external constraint--but because it
derives some pleasure from satisfying something in its organic complex.
We speak of this behaviour and of the emotion which characterises it as
the _gregarious instinct_: by which we mean that the inherited nature of
the Curlew, as a tribe, is so constituted that, given the appropriate
internal conditions and adequate external stimulation, every individual
will respond in a similar manner--that is, the behaviour is primarily
determined by racial preparation. This is what we mean by the
_gregarious instinct_ biologically considered. We may resolve our own
experience in relation to the crowd into its simplest constituents,
project our own primitive feelings into the Curlew, and say that the
bird feels uneasiness in isolation and satisfaction in being one of the
flock. But in truth we know nothing, save by analogy, of the correlated
psychical state. All the knowledge we possess is derived from a study of
the objective aspect of the behaviour, which in simple terms may be
expressed thus: the individual is drawn towards its companions; there is
a relation between the size of the flock and the strength of the
attraction; and all Curlew behave similarly under similar circumstances.
This instinct controls the movements of many birds from early autumn to
the commencement of the breeding season. And so powerful is the control
that the individual is suppressed and its activities subordinated to the
welfare of the community as a whole. Flocks of Waders roam about the
tidal estuaries in search of food, and different kinds of Gulls assemble
there and preen their feathers or sleep; Warblers alter their mode of
life, and in the osier bed, or amongst the elders, seek their food
together in peace; Finches, Buntings, Pipits, and Wagtails, though food
is everywhere abundant, gather themselves together respectively into
bands which, as winter approaches, grow into flocks and even into
composite flocks; and as the Warblers leave for the south, so their
places are filled by flocks of Thrushes and Finches from the north. In
whatever direction we turn, when the days begin to shorten, it is the
community, not the individual, that thrusts itself upon our attention;
and throughout the winter continues to be the outstanding feature of
bird life.
With the approach of the breeding season we witness that remarkable
change which I have endeavoured to make clear in the previous
chapters--the disintegration of the flock and the reinstatement of the
individual. Instead of continuing with the flock, the individual now
goes forth to seek the appropriate breeding ground; and having arrived
there, is not only content to remain in isolation, but so behaves that
isolation is insured. Intolerant of the approach of a stranger,
intolerant even of the approach of the very members of the community
whose companionship was previously welcomed, it not only fights to
maintain the position it has selected, but fights indeed for the
possession of ground already occupied, and, until reproduction is
completed, asserts its individuality and exercises dominion over its
territory. What, then, is the prospective value, biologically
considered, of the changing interest that A displays in B, and to what
will such changes lead? These are the questions to which we will now
direct inquiry.
The annual life-history of a bird is in broad outline conditioned by
two powerful and at first sight opposing impulses--the one to live in
society, the other to live solitary. But, manifestly, a bird cannot be
governed by opposing impulses. It has but one character, within which,
according to the season and the circumstances, different impulses
predominate. But these impulses, no matter how different they may appear
to be, have their respective parts to play in furthering the life of the
individual. Hence they cannot oppose, though they may conflict, if the
resultant behaviour contributes towards survival.
The majority of birds live to-day in constant danger from predatory
species, and that this danger was still greater in bygone ages there can
be but little doubt. A curious mode of behaviour of the Curlew,
Whimbrel, and Godwit demonstrates this, for it must be the outcome of
the necessity for constant watchfulness. Whilst resting with its head
turned back and its beak buried in the feathers of the mantle, the bird
constantly moves the axis of its body, so that an observer, if placed in
a direct line behind it, sees at one moment the right eye and at another
the left. No movement of the feet or of the legs is perceptible, and the
shifting of the body continues whether the eyes are open or closed. This
body movement enables the bird to survey a much larger area of ground
than it would otherwise be capable of doing, and thus adds to its
security. As far as my experience goes, the movement is less evident
amongst the members of a flock than when an individual is resting alone,
or even with a few companions, which may be due to the fact that since
some members are always awake and watchful, a bird of prey would have
more difficulty in approaching a flock unawares than it would have in
approaching a single individual. With the greatest ease a Sparrow-Hawk
can pick up a Thrush as it feeds on the meadow by itself, but if it
attempts to seize one of a flock, the chances are that its approach is
signalled and that its prey escapes. And not only do the different
members give warning one to another of the approach of danger, but they
also combine to harass or even to drive away an enemy. So that there can
be no doubt that the gregarious instinct is serviceable in promoting the
welfare of the race, and has, as its end, the preservation of the
individual in order that it may take its share at the appropriate time
in procreating its kind.
In winter, then, the individual loses its individuality and is
subordinated to the welfare of the community, whilst in spring it
regains its individuality, and all its inherited instincts which then
come into operation lead to its isolation from the flock. The impulse to
seek isolation is dependent upon internal organic conditions which are
peculiar to a certain season; whereas the gregarious impulse depends
upon internal organic conditions which inhere at all times, though its
functioning is inhibited by the functioning of the former impulse. The
evidence which leads to this conclusion is to be found in the fact that
a male often deserts its territory temporarily and joins the flock,
where it remains at peace with its companions--an aspect of behaviour
which we have discussed on various occasions. The former impulse becomes
dominant in the spring owing to its innately superior strength; the
latter becomes dominant in the autumn because the organic condition
which determines the functioning of the former then subsides. The
impulse to seek the appropriate breeding ground and to dwell there would
seem to be the strongest of all the impulses save one--the sexual.
When, however, I speak of the sexual, I refer to the actual discharge of
the sexual function, which is the consummation of the whole process. But
the territory and all that appertains to it is part of that process--the
search for the breeding ground, the dwelling there, and the intolerance
of intrusion are but different stages, each one of which must have an
impulse peculiar to it; and since the completion of the sexual act can
only be successfully accomplished providing that success is attained at
every stage, the probability is that, of the impulses concerned, one is
neither more powerful nor less powerful than another.
So that we have two impulses operating at different seasons and guiding
the behaviour into widely divergent channels. But though the proximate
end to which the behaviour is directed is apparently different, there
are not two biological ends in view, but one--the attainment of
reproduction; and the changes that we witness are not contrary but
complementary, and their prospective value lies in the circumstance that
they contribute towards the preservation of the race.
If, then, every male is driven by inherited impulse to seek the
appropriate breeding ground each recurring season; if, having arrived
there, it is driven to seek a position of its own; if, in order to
secure isolation it is obliged to attack other males or to ward off the
attacks of intruders; if, in short, success can only be attained
providing that the inherited nature is so adjusted that the bird can
accomplish all that is here demanded--what will be the general result?
That the individual will rear its offspring in safety and that they will
inherit the peculiarities of their parents, enabling them, in their
turn, to procreate their kind; all this will certainly follow. We are
not concerned, however, at the moment, with the direct effect upon the
individual, but with the consequences that will accrue to the species as
a whole.
Now certain facts are presented to observation which enable us not only
to understand the nature of the change that is wrought in the history of
the species, but to foreshadow, with no small degree of certainty, the
extent of that change. I suppose that it has come within the experience
of most of us to observe, at one time or another, the ebb and flow of a
given species in a given district. Some favourite haunt is deserted for
a year, or for a term of years, and is then revisited; or, if it is
always occupied, the number of inhabitants fluctuates--plenty of pairs
in this season, only a few in that. Many intricate relationships, both
external and internal, contribute towards this state of affairs.
Fluctuation in a downward direction, or temporary extinction, is brought
about by changes in the physical world, by changes in the available
supply of food, by the increase of enemies, or by adverse climatic
conditions; whilst fluctuation in an upward direction, though due
indirectly to a combination of circumstances in the external world
favourable to the survival of large numbers of individuals, is directly
determined by the impulse to seek isolation. As individuals of
different species establish themselves, and form kingdoms and lesser
kingdoms, we can watch the gradual quickening into life of moorland and
forest and we can observe the manner in which it all comes to pass.
Males that for weeks or months have lived in society, drifting from
locality to locality according to the abundance of food or its scarcity,
now set forth alone and settle first here and then there in search of
isolation. Lapwings settle in the water meadows, and, finding themselves
forestalled, pass on in search of other ground; Blackbirds arrive in a
coppice or in a hedgerow and, meeting with opposition, disappear; and
the Curlew, wandering with no fixed abode but apparently with a fixity
of purpose, searches out the moorland where it can find the particular
environmental conditions to which its inherited nature will respond. In
fact, wherever we choose to look, we can observe in a general way the
gradual appropriation of breeding ground; and if we fix our attention
upon particular males, we can watch the method by which success or
failure is achieved.
On more than one occasion I have watched the efforts of Reed-Buntings to
appropriate territories in a marsh that was already inhabited. Sometimes
their efforts met with success, at other times with failure. In the
former case, the males, whose ground was intruded upon, were severally
forced to yield part of their holding and were thus left in possession
of a smaller area. The success of the intruder seemed to depend upon
persistent determination, rather than upon superior skill in battle.
Recently I had an opportunity of observing the intrusion of a male
Willow-Warbler upon ground already occupied. By persistent effort it
succeeded in appropriating one half of the territory of its rival. The
intruder occupied some trees on the outskirts of the territory it was
invading, and used them as a base from which it made repeated efforts to
enter the ground of its rival. These efforts were time after time
frustrated. No sooner did it leave its base than it was seen and
intercepted, or else attacked; and no matter from which direction it
attempted to effect an entrance, its efforts, for a time, were all to no
purpose. The fighting was of a determined character, and after each
attack the owner of the territory showed signs of great excitement, and,
sitting upright upon a branch, spread and waved its wings, which is the
specific emotional manifestation during the period of sexual activity.
Eventually the intruding male succeeded by persistent effort in
appropriating part of the occupied ground.
Thus we can actually witness the efforts of the individual to isolate
itself from members of its own kind, and can observe the immediate
consequences that follow from success or from failure. And from these
consequences we can infer that, within a certain range but in accordance
with the relative abundance of the species that dwell in it, every
corner of the available breeding ground will be explored and every
situation that evokes the appropriate response will be occupied.
Moreover, since the annual dispersion is not merely a repetition in this
season of that which occurred in a previous one, a progressive increase
in the area occupied will follow. Yet, if the majority of species desert
their breeding ground so soon as reproduction is ended, how can this be?
An answer to the question will be found in the fact that a bird has an
innate capacity to return to the neighbourhood of its birthplace, or to
the place wherein it had previously reared offspring--which means that
the results of prior process persist as the basis and starting-point of
subsequent process.
Bearing then in mind that the seeming peace in bird life around us in
the spring is but the expression of transitory adjustments in the
distribution of individuals and of species; bearing in mind how
widespread is the search for isolation each recurring season, how
frequently the search leads to competition and competition to failure,
and how failure implies a renewal of the search; bearing in mind that
situations, which appear to be eminently suitable for breeding purposes,
are passed by year after year and remain unoccupied, just because, for
reasons which have yet to be ascertained, the environment fails to
supply some condition which is essential if the inherited nature of the
bird is to respond--can there be any doubt that the general result of
the functioning of the disposition will be expansion; or, since no limit
is placed upon it from within but only from without--that is, by
unfavourable circumstances in the external world, that the expansion
will not merely be in one direction but in every direction?
If now, when reproduction is ended, all the impulses relating to it die
away, and the gregarious instinct again predominates, what are the
consequences to which this change will lead? Just as the consequences
which flow from the functioning of the former impulse are accessible to
observation, so likewise can we observe the change that is wrought by
the latter impulse. The process is a gradual one. Less and less
attention is paid by the individual to intruders, more and more is it
disposed to pass beyond its accustomed limits. Little by little,
accompanied by its young or without them, as the case may be, the bird
deserts its territory and wanders out into the wilderness. Here it
associates with others, and finds in them a new interest and, I doubt
not, a new enjoyment. All this we can observe as it takes place. But
just as there is an innate capacity to seek, in the spring, the place
where the pleasures of breeding had formerly been enjoyed, so we are
bound to infer the existence in the adult of an innate capacity to
revisit the former area of association; and this capacity will
strengthen and confirm the gregarious instinct and set the direction of
the general course of movement.
We have seen, then, that the interest displayed by one bird in another
changes with the seasons; we have seen that it is so modified as to be
in useful relation to different environmental circumstances; as far as
possible we have traced out the consequences, and have reached the
conclusion that the change of behaviour must, on the one hand, lead to
expansion, and on the other, to contraction; and we have seen that this
conclusion is in accord with the facts of observation--that is the
general result of our inquiry into the functioning of the two powerful
impulses, the impulse associated with the disposition to secure a
territory and the gregarious impulse.
The phenomenon of migration embraces a number of separate problems, each
one of which presents features of great interest and of still greater
difficulty. On some of these problems I do not intend to touch; I seek
only to ascertain whether the impulses that are concerned in the
securing of a territory, and in the search for society, bear any
relation to the problem as a whole. I hold that the origin of migration
is not to be found merely in conditions peculiar to a remote past, but
that the conditions inhere in the organic complex of the bird, and are
thus handed down from generation to generation. Starting with this
assumption I examined the behaviour which normally accompanies the
seasonal life-history of the individual, and found, in that behaviour,
manifestations of cyclical change leading to definite biological
consequences. I now propose to inquire whether those consequences are
such as might, in the course of time, give rise to the seasonal change
of abode.
We are apt to think of migration in terms of the Warbler that enlivens
our hedgerows in the spring after travelling hundreds of miles from the
south, or of the Redwing that comes from the far north and seeks its
food during the winter on the meadows, or perhaps of the American Golden
Plover that each year covers a vast expanse of ocean in its journey from
its breeding ground. The length of the distance strikes the imagination
and constrains us to focus attention upon the extremes.
But migration is of much wider significance than is here represented. I
sit beside the River Severn in April and watch Swallows, Tree-Pipits,
and Yellow Wagtails passing in twos and threes, in small parties, or it
may be in small flocks; and I observe that while some establish
themselves in the neighbourhood, others pass on. Or I watch
Herring-Gulls returning to the breeding station at Bolt Head, an endless
stream of individuals coming from the east as far as eye can reach;
following them for some miles inland I see them still, first as specks
upon the horizon, then passing beside me as they beat their way slowly
against the strong south-westerly winds, and finally disappearing from
view in the direction of the cliffs. Or again, I watch Buntings and
Finches deserting the flock and seeking stations in the marsh, or
amongst the furze-bushes on the common, or in the spinneys. In each case
the proximate end of the behaviour is alike--wherein then lies the
difference? Only in the distance which separates the territory from the
area in which the birds formerly associated. And intermediate between
the extremes, I doubt not, if we had a sufficient body of observations,
that we should find numerous gradations, the lesser merging step by step
into the greater. Is the Swallow a migrant and the Herring-Gull not; is
the Tree-Pipit a migrant and the Bunting not; must a bird cross many
miles of sea or of land before it can be considered a migrant; is the
length of the distance traversed a criterion of migration? Surely not.
The distance traversed is merely a collateral consequence of the process
as a whole.
The annual life-history of a bird presents, as we have seen, two
distinct phases--the one in which the individual dominates the
situation, the other in which it is subordinated to the welfare of the
community. Let us take these two phases separately and endeavour to see
how they may have influenced the seasonal movements; and first let us
take the more important of the two, namely that one which is directly
concerned in the continuance of the race.
In this phase we must consider the three factors to which allusion has
already been made:--(1) the internal impulse, (2) the innate ability to
return to the former breeding ground, (3) the conditions in the external
environment. These three work in close relation and, as I shall
endeavour to show, lead to important results.
(1) If there were nothing in the inherited nature beyond an impulse to
seek the breeding ground, if, that is to say, when the appropriate
locality were reached, the bird took no further interest in the
developing situation, the attainment of reproduction would become
largely a matter of chance. A male in a congested district, having no
incentive to seek fresh ground, would remain inactive until a female
happened to cross its path and stimulate its sexual impulse, when its
activity would take another form. Hence some districts would be
over-populated, whilst others would remain unexplored. But the system of
reproduction does not consist merely of a search for the breeding
ground, and of the discharge of the sexual function; it is a much more
complex business, yet withal more complete. Nothing is left to chance;
the end is attained step by step; and each successive stage marks the
appearance of some specific factor which contributes towards the success
of the whole. We start with the appropriate organic condition under
which, when adequate stimulation is provided, the disposition to secure
a territory comes into functional activity. Within the field of this
disposition we can distinguish certain specific impulses. In sequential
order we have the impulse to seek the breeding ground; the appropriate
situation which gives rise to an impulse to dwell in it; and the act of
establishment which supplies the condition under which the impulse to
drive away intruders is rendered susceptible to stimulation. Grouping
these impulses, for the convenience of treatment, under one general
heading, I speak of an impulse to seek isolation. It implies some kind
of action with some kind of change as its correlated effect; and from it
there flows a ceaseless energy directed towards a definite end which for
us, who can perceive its prospective value, is isolation in an
appropriate environment. The emphasis here is on "isolation," for it
involves competition, and there cannot be competition without some
change in the relative positions occupied by different individuals; so
that in each recurring season there will be not only a re-arrangement of
ground formerly occupied but an arrangement of ground formerly deserted.
(2) That the older birds return to the locality wherein they had
formerly reared offspring, and the younger to the neighbourhood of their
birthplace, was always deemed probable. But in recent years evidence
which cannot be rebutted has been supplied by the marking of birds. This
evidence, details of which can be found in the summary of results
published annually by Mr. Witherby in _British Birds_, demonstrates that
the adult frequently returns not only to the same locality in which it
formerly bred, but even to the same station; that it does so year after
year; that this mode of behaviour is not peculiar to one sex; and that
many of the young breed in the locality in which they were reared. Such
being well-established facts, we can infer the existence of an innate
ability to revisit the place wherein the enjoyment of breeding, or of
birth, had formerly been experienced. Of its nature we know little or
nothing. It would almost seem as if there must be some recollection of
past enjoyment, but all that can be definitely asserted is--that past
experience somehow becomes ingrained in the life of the individual and
determines present behaviour. What, however, is of importance to us at
the moment is not the _ad hoc_ nature of the bird, but the biological
consequences to which the behaviour leads. For if, on the average,
individuals return to their former haunts, it follows that the annual
dispersion will not be merely a repetition in this season of that which
had occurred in a previous one, but that the little added this year will
become the basis for further additions in the next. The innate ability
is handed down from generation to generation, and, in so far as it
contributes to success, is fostered and developed by selection; and the
modifications of behaviour to which it leads, since the results of prior
process in the parent persist as the basis and starting-point of
subsequent process in the offspring may in a sense also be said to be
handed down.
(3) The conditions in the external world may be organic or inorganic. By
organic I mean the conditions which depend upon the number of
competitors or enemies by which a bird is surrounded. The competitors
may include other species which require a similar environment; and the
enemies, species which prey upon it, or animals which take its young or
its eggs. They vary in different seasons, in different districts, and
in nature and extent--the success of one species leads to the failure of
another, and the multiplication of the Jay or of the Magpie robs us of
many a songster.
By inorganic I refer to the changes in the climate and in the surface of
the earth. The nourishment of the young depends upon a regular supply of
food, and the supply of food depends upon the climate which alters in
different periods; in one decade the temperature falls below, whilst in
another it rises above, the normal, and, as the insect life fluctuates,
so there is fluctuation in the bird population. The changes in the
surface of the earth are manifold. Little by little the alder (_Alnus
glutinosa_) overspreads the marsh. Young shoots spring up here and
there, in a few years grow into bushes, and in a few more years are
trees; and the dense masses of rush which seemed to choke their growth,
yielding their position of importance, slowly disappear. And where
formerly the _Orchis latifolia_, _Orchis mascula_, and _Juncus communis_
grew in mingled confusion, nothing but water, moss, and the spreading
roots of alder cover the ground. As the rush disappears, many birds that
for generations have inhabited that marsh must seek accommodation
elsewhere. Ancient breeding haunts thus disappear, new ones come into
being, and even those which appear to be permanent are almost
imperceptibly changing.
Now the bird inherits a nervous system, which works under internal
excitation and external stimulation. Given the appropriate organic
condition and adequate stimulation, and the impulse to seek isolation
comes into functional activity. What the organic condition is and how it
arises we do not exactly know; all we know is that organic changes do
take place in the breeding season, that these changes profoundly modify
character, and that they correspond with the seasonal growth of the
sexual organs. And with regard to the question of stimulation, we have
again to confess to much ignorance, although certain facts are presented
to observation which seem to indicate the direction in which the
stimulus lies. For example, it is well known that abnormal climatic
conditions influence behaviour; we see migrants retracing their flight
along the very course they travelled a short time previously--driven
headlong by the blizzard, that at least is what we say. But if the wind,
instead of being cold and from the north, is warm and from the west, do
they retrace their flight? I have not found it so. And if there be no
wind and the temperature is low, are they still affected? Again, I have
not found it so. When, as we commonly say, they fly before the storm,
some change takes place in their organic complex, some new impulse
receives stimulation or the former one lacks it. If, after Lapwings have
established themselves in their territories, the weather becomes
exceptionally severe, the birds collect together again in flocks and
revert to their winter routine; and under similar circumstances,
Buntings fail to sing and temporarily desert their territories. In such
cases it is clear that the impulse to seek isolation ceases for a time
to dominate the situation. The inference, therefore, is that atmospheric
changes bear some relation to the functioning of the instinct; but
whether it be temperature, or humidity, or the direction and velocity of
the wind, or a combination of two or more of these factors that supplies
the stimulus, we cannot tell.
The appropriate organic condition and the stimulus have then still to be
determined, and we must pursue our inquiry from the point at which the
impulse comes into functional activity. We will take a simple case, and
one free from complication.
Let us suppose that there is an area bereft of bird life, if it can be
so imagined, but in proximity to other inhabited areas. Into this area,
whilst in search of isolation, let us imagine that a Yellow Bunting
finds its way. After the manner of its race it establishes a territory
and occupies, let us say at a low computation, half an acre of ground.
It then obtains a mate, breeds, and rears offspring, two of which we
will assume are males. Reproduction ended, the birds desert the area,
and in the following spring, when the impulse again asserts itself,
parents and offspring seek again their former haunts. We now have three
males, each of which occupies half an acre, and each of which rears two
offspring--that is the position at the close of the second year. In the
third year the number will have increased to nine and the area occupied
to 4-1/2 acres; and so on in succeeding years, until by the beginning of
the eleventh year, we have 59,048 Yellow Buntings occupying 29,524-1/2
acres or 46 square miles. This, then, will be the result of the
operation of the impulse, providing that all the individuals survive and
that no complications supervene.
But of course complications are numerous, some of which retard while
others accelerate the rate of expansion. These complications arise from
various sources--in the first place from natural enemies which prey upon
the birds or upon their eggs; in the next place from climate which, if
it happens to be unfavourable, may mean that food is scarce and that
only a small percentage of the young survive; and lastly from
rivals--and by rivals I mean closely related forms that require a
similar station and similar food--which, by occupying available ground,
may check expansion, or, by forcing a continuation of the search, may
widen it.
Now when individuals fail as many do fail in their initial attempt to
secure territory, the activity of the impulse still persists, and there
is no control over the direction in which the bird continues to wander
whilst in search of its end. Some therefore seek in this direction,
others in that; some wander inwards into inhabited areas and fail to
find accommodation, or, according to the relative strength of their
impulse, perhaps succeed and so set free a new competitor, others wander
outwards into country uninhabited by the species. These latter we will
call "pioneers." They may find accommodation within a comparatively
short distance of their base, or they may come into competition with
rivals and fail, not necessarily on account of any congenital weakness
of ability, but because being warned by an alien song, they may be
precluded from coming into contact with just the environing conditions
which can supply the stimulus and allow behaviour to run its further
course--and so be obliged to extend their search into remoter districts.
But it must not be overlooked that they will be placed in a most
advantageous position so far as the attainment of reproduction is
concerned. In their search for territory they will meet with little
opposition and will be free to select whatsoever ground they will; and
be free also from intrusion by neighbouring males, which is so frequent
in occurrence and continues for so long in congested areas. Moreover, in
thinly populated districts, the pressure upon the available means of
support will not be so great, neither will natural enemies be so
plentiful; and since the offspring, guided by prior experience, return
to the neighbourhood of their birthplace, the advantages thus gained
will be shared by the succeeding generation. It follows, then, that the
range of a species will not always be continuous, will not, that is to
say, proceed by a series of successive steps, but that sometimes in this
direction and at other times in that, the chain of territories will be
interrupted and different individuals separated by distances of greater
or lesser extent. New colonies will thus come into being; and as the
unlimited increase of the population over limited areas gradually
reintroduces into them the struggle for territory, new centres of
distribution, where the process will repeat itself and from which
expansion will proceed afresh, will be formed. Hence, though it is
clearly impossible for the progeny of one pair of Yellow Buntings to
overspread the whole of the 46 square miles, it is by no means
impossible for the limits of their range to exceed even those limits
within the eleven years.
To sum up our knowledge regarding this phase. Of the organic condition
which renders the impulse responsive to stimulation we know very little;
and though certain facts of observation seem to indicate the direction
in which the stimulus is to be found, we must here again confess to much
ignorance. So far as can be seen, however, the impulse to seek isolation
with its correlative territory, leads to constant modification in the
breeding range of most species. The occupation of the small space of
ground which each individual requires, the extent of which has been
gradually adjusted to suit the needs of different species, results in
expansion not only in one direction but in every direction, and not only
in one season but in every season. And if there were no complications in
the external world this expansion would proceed, as we have seen, with
astonishing rapidity. But complications, some of which are favourable
and others unfavourable, are numerous, and it is difficult to estimate
their importance or to indicate their precise effect; the former,
however, accelerate the rate of expansion, whilst the latter retard it.
Those individuals that wander outwards and seek territory on the
outskirts of the range we have called "pioneers." They will have
advantages over others that, wandering inwards, seek isolation in
congested districts, and will succeed where the latter fail; and since
there is in the young an innate ability to return to the district
wherein they were reared, the advantages so gained may be said to be
handed on from generation to generation.
Let us now turn to the contra-phase, and endeavour to ascertain whether
the gregarious instinct bears any relation to the seasonal desertion of
the breeding ground. The conclusion at which we have already arrived
regarding this instinct is that it forms part of the inherited nature of
most species; that its functioning is suppressed when a bird is actually
in occupation of a territory; and that it is serviceable in promoting
the welfare of the individual. We cannot of course observe the instinct.
What we observe, when reproduction is ended, is a change in the
relations of different individuals; instead of arousing mutual
hostility, they attract one another, from which we infer the existence
of something which determines their conduct, and this "something" we
speak of as an instinct.
To what does this change lead? Let us suppose that there is an area
inhabited by one species; that the number of inhabitants has reached the
maximum that the means of sustenance will allow; and that the season of
reproduction is drawing to a close. The position will then be as
follows. All the available breeding ground is divided into territories;
each territory is occupied by one unit, the family, and each individual
is able to fend for itself; changes both internal and external begin to
take place, the gregarious instinct comes into functional operation, and
the supply of food diminishes--that roughly is the position. The
internal factor operates so that the sight of this individual or the
call of that, instead of evoking hostility as heretofore and keeping
different units apart, proves now an irresistible attraction; so that in
place of a number of individuals evenly dispersed over the whole of this
area, a small number of flocks of various dimensions are stationed at
certain points, which points are determined partly by experience, partly
by the supply of food, and partly by accident. This implies for each
individual some movement in some direction. But since the population of
this imaginary area has reached the maximum, and the supply of food,
though limited in distribution, is nevertheless plentiful, such
movements will be irregular and will proceed in no definite direction.
Now let us suppose that the breeding range extends and that fresh ground
is occupied by pioneers. When reproduction and the rearing of broods are
ended and the gregarious instinct becomes dominant, these pioneers, or
at least some of them, will revisit the area wherein formerly they
associated with companions. Their offspring, however, though they will
have the inherited impulse and the innate tendency, will not have the
experience; how then will they behave? There can be no doubt that some
will accompany the older birds, and, being led by them, will share the
experience of a former generation; nor any question that others will
collect together in the neighbourhood of their birthplace and, if their
impulse is satisfied, will remain there so long as food is to be found.
Thus the gregarious instinct, working in close relation with acquired
experience, will on the one hand lead to the formation of organised
movements in certain directions, whilst on the other it will lead to the
formation of new areas of association which will follow in the wake of
the expansion.
We have assumed, in the imaginary case which we have just taken, that
the conditions in the external world are such as enable the birds to
endure throughout the year--in short, that there are no complications
regarding the supply of food. But we must bear in mind that so long as
conditions are favourable during the period of reproduction, which is of
short duration, the breeding range can continue to expand, and that
therefore, in the course of centuries, regions will come to be occupied
wherein, owing to alternations of climate or physical changes in the
surface of the earth, food will be impossible, or at any rate difficult
to obtain at certain seasons. Hence there will come a time when the area
of association ceases to follow in the wake of the expansion, and the
breeding area begins to diverge from the subsistence area.
How, then, is the gulf between these two areas to be bridged? We can of
course say that those individuals which, in virtue of some slight
variation of hereditary tendency, return to regions where food is
plentiful will survive; whilst others, less well endowed, will perish.
We can state the position in some such general terms, and doubtless
there would be truth in the statement, but it does not carry us far; we
wish to know more of the nature of the tendency, and of the manner in
which it has evolved. Well now, in this new situation which arises, two
things are apparent--that the struggle for existence becomes a struggle
for the means of subsistence, and that anything in the inherited
constitution of the bird which can be organised to subserve the
biological end in view becomes of selection value. So long as food can
always be procured in the new areas of association, the individuals that
behave in accordance with ancestral routine gain thereby no particular
advantage; but directly the breeding range extends into regions where
the supply fluctuates, traditional experience becomes a factor in
survival, and those individuals that come under its influence will, on
the average, be more likely to endure and so to procreate their kind and
maintain the tradition. Let it once be granted that there is an innate
capacity to retain in later phases of routine the experience gained in
earlier phases, and it is difficult to see how traditional guidance can
be refused recognition as a factor in the developing situation. But only
_a_ factor, and by no means the most important one; for observation has
shown that the young are capable of performing the return journey
without guidance. Something therefore _is_ inherited, some impulse
which comes into functional activity at a specified time, and leads the
bird to set forth in a given direction.
There are no grounds for supposing that the experience of one generation
forms any part of the hereditary equipment of subsequent generations. In
what direction then are we to look for the congenital factor? What is
given is an inherited tendency to co-operation and mutual help, and an
innate capacity to make use of the results of experience. The inherited
tendency, as we have seen, leads on the one hand to the formation of new
areas of association, whilst on the other, since it is the means of
bringing isolated individuals into contact, it leads to experience being
handed on from generation to generation, which, in its turn, results in
a certain amount of backward movement along the line of expansion. It
forms part of the hereditary equipment of many species, and is
serviceable in promoting the welfare of the individual. Moreover, there
is reason to believe that its origin dates back to an early period in
the evolution of the higher forms of life; and if in the subsequent
course of evolution it could have been so organised as to serve a double
purpose, so much the more reason would there have been for its survival.
In what does the instinct consist? Is it merely that the sight of this
individual or the call of that proves at some particular moment an
irresistible attraction, or does the appropriate organic condition give
rise, as is generally supposed, to some preceding state of uneasiness?
In the former case, the temporarily isolated individual or colony would
have but little chance of sharing in the benefits which mutual
association confers upon the associates; in the latter, the feeling of
discomfort would lead to restlessness, and would thus bring the bird
into touch with the environing circumstances under which instinctive
behaviour could run its further course. So that it is probable that the
movements of each individual, prior to its becoming a unit in the flock,
are not accidental but are determined in some measure by racial
preparation.
Now if the fundamental assumption of the doctrine of the struggle for
existence be true, the gregarious instinct will not be quite alike in
all the members of different broods, nor even in each member of the same
brood; that is, variation will occur in all possible directions. And we
shall not, I think, exceed the limits of probability if we assume that
different individuals vary in the persistency with which they strive to
attain their unknown end, and in the direction in which they travel in
pursuit of it. So that in each generation they will fall into three
classes: (1) those which are inert, (2) those which wander along the
line of expansion, (3) those which wander in other directions. If then
the struggle for life at this particular juncture in the evolution of
the breeding range is a struggle for the means of subsistence, the
members of these three classes will not be in a like satisfactory
position so far as the competition for food is concerned. Those in the
first class--_i.e._, those in which the activity feelings are weak--will
neither gain the benefits which arise from mutual help, nor will they
have much prospect of enduring through the season of scarcity. Those in
the third class will, it is true, derive some assistance one from
another, and so be in a better position to discover what food may be
available; but inasmuch as they will remain in regions where the climate
alternates and the supply of food is liable to fall below the minimum
required, the chances are that a high percentage will fail in the
struggle for existence. We come now to those in the second class, and it
is upon them that I wish more particularly to focus attention. The
initial movement in their case will be in the direction from which
outward expansion has all along taken place. Within a comparatively
short distance they will reach districts where the species is plentiful,
and here, associating with others that have some traditional experience,
they will be guided by them and will find themselves in regions where
food is plentiful. Hence in each generation those will survive that,
owing to some congenital variation of their instinct, seek satisfaction
for their impulse in a direction which brings them under the influence
of tradition. And though at first but slight and not in themselves of
survival value, such variations, since they coincide with modifications
of behaviour due to acquired experience, will be preserved and in the
process of time so accumulated as to be capable of determining the
direction and extent of the movement.
But the young Cuckoo deserts this country many weeks after its parents,
and there is no reason to suppose that it lives in society when
eventually its destination is reached; and the young Falcon passes to
the south, and is certainly not gregarious--how then can we explain
their behaviour in terms of something which they show no signs of
possessing? I do not wish to make light of a difficulty which
admittedly, at first sight, is a grave objection to the view that the
gregarious instinct has been operative in the manner here claimed for
it. It must, however, be borne in mind that this instinct, though
originally developed to serve the purpose of mutual protection, supplies
the material upon which evolution works when the extension of the
breeding range creates a situation requiring readjustment on the part of
the organism to new conditions of life; and that those variations which
can be so modified as to be in useful relation to the new environmental
circumstances are seized upon by natural selection and, being
transmitted, form the foundation of a specific inherited response, no
longer dependent upon, though operating in close relation with the
primitive response whence originally it sprang. Thus the primordial
instinct becomes so organised as to serve a secondary purpose, that of
rendering secure a means of access to a certain food supply. In the
course of evolution species were bound to arise which, owing to some
peculiar conditions, derived greater advantage from living solitary than
from living in society. Does it then follow, because such species
manifest no inclination to live in society, that the instinct never has
played any part in their lives? Or because the primary purpose has
lapsed, does it follow that the secondary no longer exists?
Let me recapitulate the principal considerations which I have discussed
in this chapter.
Though I have been advancing a theory, and though I have taken much for
granted, yet it will, I think, be admitted that both the theory and what
has been taken for granted rest on observational grounds. As our
starting-point we have a bird whose inherited nature alternates
according to the season, and in whose nature we can distinguish two
contra-phases--the one to live in society, the other to live solitary.
While both have their part to play in furthering the life of the
individual, for biological interpretation there is only one end, the
prospective value of which is the continuance of the race. We may say
that the latter phase is the more important of the two because it is
directly concerned with reproduction. But we shall make a great mistake
if we attach peculiar importance to one phase, or to one mode of
behaviour within that phase, or to one action within that mode of
behaviour; for if there is one thing certain it is that the whole is an
inter-related whole in which each part depends for its success upon that
which precedes it.
In that phase in which the territory is the central feature of the
situation, the struggle for existence is in operation in its acutest
form; all the congenital and acquired capacities of the bird--pugnacity,
song, capacity to utilise in later phases the experience gained in prior
phases, all these are organised to subserve an end--a proximate
end--which in its simplest terms may be described as "isolation."
Isolation is then the first step in the process of reproduction, and any
individual that fails to make it good, fails to procreate its kind. But
isolation implies separation, and the degree of separation varies in
different species, from the few square feet of cliff required by the
Guillemot to the few square miles of barren moor over which the
Peregrine exercises dominion. One species must occupy sufficient ground
to enable it to secure food for its young; another requires sufficient,
but no more, upon which to deposit its egg; and a third must secure a
position for its nest within the community. Hence it follows that the
degree of separation varies with the conditions of existence. Since,
however, the conditions in the external world are constantly changing
according to the relative abundance or scarcity of enemies, the rise or
fall of rivals, the physical changes in the earth's surface, and the
alterations of climate, it is clear that isolation can only be obtained
with difficulty, and that the competition for it must be severe. Some
individuals therefore fail to breed, whilst others, perhaps because
their impulse is stronger, persevere and seek stations elsewhere. What
are their prospects of finding them? By extending the field of their
activities, they will wander into districts remote from the scene of
competition, districts where not only food is plentiful but where
enemies and rivals are scarce; and to these pioneers, if to any, success
in reproduction will most certainly be assured. But not only is it they
who will benefit; their offspring also, when the time comes for them to
take their part in the maintenance of the race, will share in the
success of their parents, for even though they may not escape
competition from individuals of closely related forms, they will meet
with but little from those of their own kind. Now species which live
throughout the year in the vicinity of their territory are comparatively
few, the majority are obliged to wander in search of food so soon as
reproduction is ended, and their behaviour is determined not only by its
abundance or scarcity, but also by the powerful gregarious impulse which
waxes in proportion as the instincts connected with reproduction wane.
If, then, when the sexual instinct again becomes predominant, the
experience of the former season nowise affects their movements, little
or no progress will be made in the expansion of the range. But just as a
certain entrance into the bush and pathway through it, when once made
use of in the process of building, becomes so firmly established as to
form the sole highway to and from the nest, so likewise, when the
impulse to seek isolation repeats itself, the bird is constrained to
seek the neighbourhood wherein it had experienced the enjoyment of
breeding or of birth. Thus the little that is added one year becomes the
basis for further additions in the next, and new centres of distribution
are continually being formed from which expansion proceeds anew.
Now as the range gradually extends into regions where the climate
alternates and food at certain seasons is consequently scarce, the
distance between the customary area of association and that of
reproduction must perforce widen. The question then arises: How will the
young that have no experience find their way to regions wherein they can
endure? The forces which may have been organised to subserve the end in
view are three: (1) Acquired experience, (2) tradition, (3) the
gregarious instinct. The pioneer that carries the range a little further
forward starts from a base where it has associated with companions and
found food plentiful; and when the impulse to live in society again
asserts itself, it not only repeats its former experience but hands on
the habit thus acquired to those of the next generation that happen to
accompany it. Granting, however, that by successive increments in the
distance traversed, traditional guidance may in time accomplish much, it
cannot account for all the known facts, it cannot at any rate explain
the fact that in some cases the inexperienced offspring finds its way to
the food area without guidance. Something, therefore, _is_ inherited.
And my suggestion is this: That the gregarious instinct, the ancient
origin of which we can infer from its manifestation in so many and
diverse forms of life, supplies the material upon which evolution works;
that variations of the initial impulse, at first slight and not in
themselves of selection value, in so far as they coincide in direction
with modifications of procedure due to experience or tradition, are
preserved; and that, in the process of time, they are so accumulated as
to form a specific congenital endowment determining a definite mode of
behaviour.
FOOTNOTES:
[1] June 1915, R. M. Barrington.
[2] _Dictionary of Birds_, p. 556.
[3] _Social Psychology._
[4] _Manual of Psychology._
[5] _Ibis_, April 1918.
[6] _Zoologist_, 1912, p. 327.
INDEX
Acquired experience, 300
Adjustments, transitory, of distribution, 275
Alarm notes, 119
Arrival, advantages and disadvantages of late, 33-44
Assemblies in winter, 262, 263
Assembly grounds, 173
Attainment of reproduction, 171
Barrington, R. M., on the sex of migrants, 25
Battle between two male Cuckoos, 82
---- between two Moor-Hens, 86, 92, 93, 94
---- ---- Pied Wagtails, 86
---- ---- Raven and Buzzard, 217
---- ---- Raven and Peregrine, 216
law of, 13,19
Behaviour routine, 262
---- sexual, 3
Bickerings, 96
Birthplace, return to, 43, 50
Blackbird, 87, 182, 222, 244
Blackcap, 81, 156, 224, 230
Black Grouse, 63
Black-tailed Godwit, 53
Boundaries, 1, 5
---- conflicts for retention of, 7, 62
---- disputes as to, 1
Brambling, 124
Breeding ground, search for appropriate, 270, 271
---- range, extension of, 291-92
---- site, acquirement of, 3
---- stations, evolution of, 15-19
---- ---- repeatedly visited long before nesting-time, 64
---- territory, 2, 3, 7
---- ---- evolution of, 18
---- ---- foundation of, 7
---- ---- innate capacity to return to former, 279-81
Bridled Guillemot, 64
Brooding, 180
---- impulse, 191
Bunting, Cirl, 28, 140
---- Corn, 28
---- Reed, 28, 68, 69, 85, 104, 132, 158, 160, 244
---- Yellow, 28, 30, 47, 64, 140, 159, 162, 183, 187, 188, 189,
235, 236, 286
Buzzard, 217
Capacity, innate, to return to former breeding territory, 279-81
Carrion Crow, 226
Chaffinch, 28, 31, 32, 33, 45, 87, 103, 156, 159, 235, 236, 244
---- Donegal, 160
Change of breeding quarters owing to unsuitableness, 50
Chiffchaff, 49, 51, 80, 139, 140, 221, 224, 244
Cirl Bunting, 28, 140
Clarke, W. Eagle, _Studies in Bird Migration_, 24
Cleanliness of nest, 180
Cliff-breeding species, 63
Climatic changes, alteration of routine, due to, 284
---- changes, food dependent on, 283
---- conditions, influence of, 20
Communities, 202
---- birds after breeding-season remain in, 265-67
Competition, female, for males, 13
Complexity of strife, 84-85
Conflicts between males during the mating period, 74, 86
---- between males during the nesting period, 87
---- for areas, 10, 11, 13, 62
---- of Ruff, 54
---- sexual, 10
Congenital disposition, 135
Contests between males for possession of females, 80
Coot, 61
Corn-Bunting, 28
Corncrake, 39
Crow, Carrion, 226, 227
Crow, Hooded, 202
Cuckoo, 52, 82, 144, 296
---- restricted breeding area, 52
Curlew, 119, 138, 140, 250, 262, 263, 265, 273
Danger warnings, 269
Darwin, C., _Descent of Man_, 35
---- on the arrival of males before females, 35
Defence of territory, 6
Development, sexual, 6
Disposition, congenital, 135
---- functioning of, 74
---- inherited, 5
---- to defend the territory, 73-118
---- to mate, 27
---- to remain in a particular place in a particular environment, 6
---- to secure a territory, 6, 20-72
Distribution, adjustment of, 275
Dove, Turtle, 126, 232
Dunlin, 250
Emotional behaviour, 53, 82, 114
---- manifestation, 90, 283
---- response, 26
Enemies, 282
Energy, waste of, 219
Environment, 6
---- and food, 56
---- changes of, 283
---- external, conditions in, 279, 282
Equipment, hereditary, 6
Evolution of breeding stations or territory, 15, 19
---- of the territory, 176
---- of the voice, 163
Existence, struggle for, 294
Experience, acquired, 300
Experiments, removal of nests for, 181, 185, 190, 213, 214
Exposure, its effect on nestlings, 180
External environment, conditions in, 279, 282
Falcon, 48, 71
Feeding grounds, neutral, 125 in communities, 70
Females, fighting amongst, 109-118
---- sexual impulse of, 13
Fieldfare, 124
Fighting instinct, 79, 82
Flight, emotional behaviour of Godwit during, 53
Flocks, in winter, birds collect together in, 262
Flycatcher, 244
Food, procuring of, 5
---- abundance, or scarcity of, its relation to prosperity of young,
15, 16
---- its bearing, on the movement of flocks, 262
---- rearing of young dependent on rapid and regular, 179, 195
---- supply, proximity to, necessary for rearing young, 179, 195
Fortuitous mating, 174
Fowler, Ward, on the value of communities, Rooks, 202
Fulmar, 121, 247
Functional activity, 259
---- instinct of Reeve, 173
Functioning of the disposition, 275
---- of the primary dispositions, 100
Garden Warbler, 223, 225, 230
Gätke, H., _Birds of Heligoland_, 24
---- on the absence of song in birds on Heligoland, 124
---- on the early arrival of Guillemots on Heligoland, 64
Godwit, emotional behaviour of, during flight, 53
---- Black-tailed, 53
Grasshopper Warbler, 39, 131, 139, 153, 155, 187, 244
Greenfinch, 28, 33, 140, 235, 236
Gregarious instinct, 20, 61, 141, 265-66, 269, 276, 289, 290, 291,
296, 300
Grouse, Black, 63
Guillemot, Bridled, 64
---- Common, 63, 64, 121, 192, 195, 206, 211, 247
---- Ringed, 64
Gull, Common, 119
---- Herring, 210, 278
Habit formation, law of, 8, 62, 65, 66, 67, 205
Hawfinch, 28
Headquarters, 176, 206, 207, 274
---- restricted, 8, 9, 30, 50, 58, 64, 127
Hedge-Sparrow, 213, 221, 244
Hereditary equipment, 6
Herring-Gull, 210, 278
Hooded-Crow, 202
Hostility and territory, relationship between, 242
House-Sparrow, 218
Imitation, vocal, powers of, 156, 157, 161
Impulse, internal, 279
---- to brood, 191
Inherited disposition, 5
Instinct, fighting, 79-82
---- gregarious, 20, 61, 141, 265, 266, 269, 276, 289, 290, 291, 296,
300
---- migratory, 37
---- of song related to establishment of territory, 125
---- sexual, reawakening of, 4, 18
Instinctive response, 180
Instincts susceptible to stimulation, 259
Internal impulse, 279
Internal stimulation, 62, 123
Interpretation of battles, 75
Intolerance of other birds, 218, 219
Intrusion resented, 274
Isolation, impulse to seek, 288
---- of male, 12, 62, 65, 73, 81
---- of male during breeding season, 267, 272, 273, 275, 281
Jay, 87, 156, 283
Kestrel, 228
Kittiwake, 116, 200, 247
Lapwing, 58, 59, 61, 62, 64, 84, 103, 104, 126, 189, 190, 220, 251, 284
Lapwing, life-history of, 58-61
Late arrival, advantages and disadvantages of, 33-44
Law of battle, 74, 75, 86
Lesser Whitethroat, 230, 244
Linnet, 156
M'Dougall, Dr, _Social Psychology_, 77
Magpie, 219, 283
Males arrive before advent of females, 24
Marsh-Warbler, 39, 40, 52, 81, 132, 140, 153, 155, 156, 165, 225
Martin, 201, 218
Mating, difficulties of, 172
---- fortuitous, 174
Maximum number supportable in a given locality, 49
Meadow-Pipit, 188
Meeting places for antics, 54, 63
Mental Image, 77
Merlin, 227
Migration, 3-4
---- distance no criterion, 279
---- its relation to territory, 259
Migration, origin of, 260, 277
---- phenomenon of, 277
Migratory instinct, 37
---- species more highly specialised than resident species, 56
Missel-Thrush, 21
Mobility of the land and stability of the water, 260
Moor-hen, 61, 85, 103, 218, 250, 251
Morgan, Professor Lloyd, on instinctive behaviour, 74
---- on emotional behaviour, 114
Nest, cleaning of, 180
---- construction of, 3
Nests, removal of, for experiments, 181, 185, 190, 213, 214
Neutral feeding grounds, 62, 125
---- ground, 98
Newton, E., on the arrival at breeding stations of males before female,
24, 35
Nightingale, 39, 156
Notes of alarm, 119
---- of anger, 119
---- of recognition, 139
---- of warning, 119, 139, 141, 145, 151, 153
Offspring, rearing of, 3, 4
Organic change, sexual, 92, 123
---- changes, 65
---- condition of Reeve, 173
Owl, Wood, 156
Paired for life, 55-56
Parental instinctive response, failure of, 185
Partridge, 87, 218
Persecution, Carrion Crow and Magpie, 226
Persecution, Carrion Crow and Rook, 227
---- House Sparrow and Martin, 218
---- Lapwing and Snipe, 220
---- Raven and Buzzard, 217
---- ---- and Peregrine, 216
---- Starling and Woodpecker, 218, 237
Persistency to remain in territory, 68
Pied-Wagtail, 86, 155
Pigeon, Wood, 219
Pipit, Meadow, 188
---- Tree, 51, 188, 189, 222, 244, 278
Polyandrous females, 144
Predatory species, 268
Promiscuous pairing of Ruffs, 172
Proximity to food-supply necessary for rearing young, 179, 195
_Psychology, Manual of_, 1
Puffin, 63, 116, 200
Pugnacious instinct, 87-109
Pugnacity, 11, 62
---- of females to obtain mates, 109-118
---- of males, prior to mating-season, 77-81
---- of Moor-Hen, 218
Racial preparation, 41, 43, 46, 67, 205, 206, 266
Rail, Water, 218
Raven, 48, 202, 216
Razor-bill, 63, 64, 200, 247
Readjustment of territory, 146
Rearing of offspring, 3, 4
Red-backed Shrike, 39, 50, 51, 156
Redbreast, 47
Redshank, 139
Redwing, 124
Redstart, 230, 244
Reed-Bunting, 28, 68, 69, 85, 104, 132, 156, 158, 160, 244, 246, 273
Reed-Warbler, 49, 51, 68, 81, 132, 140, 152, 153, 211, 225
Reeve, 171
Relation of song to the territory, 119-68
---- of territory to migration, 259
---- of territory to the system of reproduction, 169-214
Relationship to a territory, 169
Reproduction, 14, 15
---- and territory, 169-214
---- attainment of, 2, 6, 37
---- goal of, 6
Ringed Guillemot, 64
Robbery of territory, 104-107
Rock-formation, suitability for Guillemots nesting on, 196
Rook, 202, 227
Routine behaviour, 262
Ruff, 54, 63, 172
---- meeting places for conflicts, 54
Ruffs, promiscuous pairing of, 172
Savi's Warbler, 139
Sedge-Warbler, 25, 44, 152, 226, 244
Selous, E., on the life-history of Ruffs and Reeves, 172
---- on meeting places for conflicts and antics, 54
---- on the meeting places of Black Grouse, 63
Service, Robert, on flocks of unmated Sedge-Warblers, 44, 45
Sexual behaviour, 3
---- conflicts, 10
---- development, 6
---- function, discharge of, 2, 3, 26
---- impulse of females, 13
Sexual instinct in the migratory male, 26
---- of Reeve, 173
---- ---- reawakening of, 4, 18
---- life of birds, 1
---- maturity, males arrive at, before females, 36
---- organic change, 92, 123
---- selection, 166
Shag, 121
Shrike, Red-backed, 39, 50, 51, 156
Skylark, 188, 236, 244
Snipe, 153, 156, 219, 220
Sociability when not paired, 125, 126
Song, as an aid in searching for a mate, 12
---- its influence on mating, 167
---- origin of, 138
---- relation to reproduction, 123
---- relation to territory, 119-168
---- volume of, influenced by age, seasonal sexual development,
or isolation, 166
Song-Thrush, 222, 244
Sparrow, House, 218
Sparrow-Hawk, 269
Spring, at approach of, birds lose their shyness, 138
Stability of the water and mobility of the land, 260
Starling, 217, 218, 237, 251
Stimulation, internal, 62, 123
---- question of, 284
Stonechat, 87 187, 188, 189, 222, 233, 234
Stout, Dr, _Manual of Psychology_, 1, 77
Struggle for existence, 294
Susceptibility to position, 96
Swallow, 21, 156, 278
Territory, 1, 5
---- adjustment of, 10
Territory and reproduction, 169-214
---- breeding, 2, 3, 7
---- dates of acquisition of, 33
---- defence of, 6
---- desertion of, after rearing young, 276
---- disposition to defend, 73-118
---- disposition to secure, 6, 20-72
---- establishment of, 74, 285
---- evolution of, 176
---- failure to secure, 286
---- fights for, 10, 11, 13, 62
---- ownership of, 189
---- possession of, a stimulus to song, 136
---- its relation to migration, 259
---- its relation to reproduction, 169-214
---- readjustment of, 147
---- restriction of, advantageous for mating, 172
---- restricted, 8, 9, 30, 50, 58, 64
---- separate for male and female Cuckoo, 144
---- song, its relation to the, 119-68
---- temporary desertion of, 28, 35, 58, 59
---- and hostility, relationship between, 242
Thrush, Song, 222, 244
Tit, Blue, 221, 226
---- Great, 221
---- Long-tailed, 226
Tradition, 300
Tree-Pipit, 51, 188, 189, 222, 232, 244, 278
Turtle-Dove, 126, 232
Union of sexes, 12
Ussher, H. B., on the hostility between Choughs and Hooded Crows
and Choughs and Ravens, 227
Vocal Imitation, 156, 157, 161
Voice calls of Curlew, 263
Wagtail, Pied, 86, 155
---- Yellow, 278
Wanderings from land, Guillemots, 193
Warbler, Garden, 223, 225, 230
---- Grasshopper, 39, 131, 139, 155, 187, 244
---- Marsh, 39, 40, 52, 81, 132, 140, 155, 156, 225
---- Reed, 49, 51, 68, 81, 132, 140, 152, 153, 211, 225
---- Savi's, 139
---- Sedge, 25, 44, 152, 226, 244
---- Willow, 25, 47, 50, 51, 80, 91, 140, 187, 211, 232, 244, 273
---- Wood, 50, 51, 132, 221
Warfare between different species and its relation to the territory,
215-58
Warning notes, 119
---- of danger, 269
Water Rail, 218
Wheatear, 25, 51
Whimbrel, 140
Whinchat, 39, 50, 51, 81, 222, 232, 233, 234, 244
Whitethroat, 25, 50, 68, 69, 124, 140, 182, 187, 189, 190, 213, 230,
244
---- Lesser, 230, 244
Wild Duck, 250
Will, the, to fight, 102
Willow-Warbler, 25, 47, 50, 51, 80, 91, 140, 187, 211, 232, 244, 273
Winter assemblies, 262-63
Witherby, H. F., in _British Birds_, on the return to former
breeding-ground, 281
Wood-Owl, 156
Wood-Pigeon, 219
Wood-Warbler, 50, 51, 132, 221
Woodpecker, Lesser Spotted, 237
---- Great Spotted, 237, 238
---- Green, 20, 71, 156, 208, 218, 237
Wren, 244
Yellow Bunting, 28, 30, 47, 64, 140, 159, 162, 183, 187, 188, 189,
235, 236, 286
Young die in nest from exposure, 184, 185
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End of Project Gutenberg's Territory in Bird Life, by H. Eliot Howard