THE RELATION OF SONG TO THE TERRITORY
If we listen to the voices of the Waders as, in search of food, they
follow the slowly ebbing tide, we shall notice that each species has a
number of different cries, some of which are uttered frequently and
others only occasionally. Not only so, but if we study the circumstances
under which they are uttered, we shall in time learn to associate
certain specific notes with certain definite situations.
The Curlew, when surprised, utters a cry with which most of us, I
suppose, are familiar; but when with lowered head it drives away another
individual from the feeding ground, it gives expression to its feelings
by a low, raucous sound, which again is different from its cry when a
Common Gull steals the _arenicola_ that has been drawn out of the mud
with such labour.
Thus we come to speak of "alarm notes," "notes of anger," "warning
notes"--naming each according to the situations which normally accompany
their utterance. And so, all species, or at least a large majority of
them, have, in greater or lesser variety, cries and calls which are
peculiar to certain seasons and certain situations; and since on many
occasions we have indisputable evidence of the utility of the sound
produced--as when, upon the alarm being given by one individual, the
flock of Lapwing rises, or when, in response to a particular note of the
parent, the nestling Blackcap ceases to call--so are we bound to infer
that all the cries are, in one way or another, serviceable in furthering
the life of the individual.
But besides these call-notes, birds produce special sounds during the
season of reproduction--some by instrument, others by voice, others
again by the aid of mechanical device. And not only is this the case,
but many accompany their songs with peculiar flights, such as soaring to
a great height, or circling, or floating in the air upon outstretched
wings. These special sounds and special flights are those with which I
now propose to deal, including under the heading "song" all sounds
whether harsh or monotonous or beautiful, and whether vocally or
otherwise produced; and I shall endeavour to show not only that they are
related to the "territory," but that they contribute not a little to the
successful attainment of reproduction.
The vocal productions are infinite in variety and combination. At the
one extreme we have songs composed of a single note repeated slowly or
rapidly as the case may be, whilst at the other we have the complex
productions of the Warblers; and between these two extremes, notes and
phrases are combined and recombined in ways innumerable. And just as
there is a rich variety of combination, so there is a very wide
variation in the purity and character of the notes--some are harsh,
others melodious, some flute-like, others more of a whistle, and others
again such as can only be likened to the notes of a stringed instrument.
Hence in variety of phrase combination added to variety in the character
of the note, there is a possibility of infinite modes of expression.
If, in the latter part of May, we take up a position at dawn in some
osier bed, we listen to songs which have reached a high degree of
specialisation, songs, moreover, which appeal to us on account of their
beauty; if, on the other hand, we climb down the face of the sea cliff,
we hear an entirely different class of songs--harsh, guttural, weird,
monotonous sounds, which, appeal to us though they may, lack the music
of the voices in the osier bed. And just as, in the osier bed, we can
recognise each species by its voice, so we can distinguish the "cackle"
of the Fulmar, the "croak" of the Guillemot, or the "grunt" of the Shag.
In the osier bed, however, there is considerable variation in the song
of different individuals of the same species, so much so that we can
recognise this one from that; whereas on the cliff we cannot distinguish
between the voices of different individuals. And the more highly
developed the song, the greater the range of variation appears to be;
but notwithstanding this--notwithstanding the fact that the pitch may
differ, the phrase combination may differ, and the timbre may
differ--the song remains nevertheless specific. So that the two
principal features of "song," broadly speaking, are "diversity" and
"specific character."
In contrast with the call-notes, the majority of which can be heard at
all times of the year, the song is restricted as a rule to one season,
and that one the season of reproduction. It is true, of course, that
some birds sing during the autumn, and, if the climatic conditions are
favourable, in the winter also, just as others betray, in the autumn,
symptoms of emotional manifestation peculiar to the spring; but just as
the manifestation of the latter is feeble and vestigial, so, too, does
the song of the former lack the vigour and persistency which is
characteristic of the spring. Again, in contrast with the call-notes,
which are common alike to both sexes, song is confined to one sex--a
peculiar property of the males.
Now all, I think, will agree that it must serve some biological
purpose--this at least seems to be the conclusion to be drawn from the
two outstanding features of "diversity" and "specific character"; and
since the voices of different individuals of the same species vary, it
has been suggested that, by creating a more effective pairing situation,
it is serviceable in furthering the life of the individual. I do not
propose at the moment to enquire whether this doctrine be true, but
rather to direct attention to other ways in which the song may be
useful.
Is the instinct susceptible to stimulation under all conditions during
the season of reproduction, or only under some well-defined condition?
This is the question to which we will first direct inquiry.
Song in its full development belongs, as we have seen, to the season of
reproduction; it is heard at the dawn of the seasonal sexual process,
and is the most conspicuous outward manifestation of the internal
organic changes which ultimately lead to reproduction. These changes
would appear, at first sight, to be the primary condition which renders
the instinct susceptible to appropriate stimulation. But while this is
true up to a point, in so far, that is to say, as organic changes are a
necessary antecedent of all behaviour connected with the attainment of
reproduction, closer acquaintance with the circumstances under which the
instinct is allowed full play leads to the belief that they are not
alone sufficient to account for the facts as observed. In order to
arrive at a decision we must seek out the specific factors in the
external environment with which "song" is definitely related.
Some birds cross whole continents on their way to the breeding grounds,
others travel many miles, others again find suitable accommodation in a
neighbouring parish--nearly all have a journey to perform, it may be
short or it may be long. The flocks of Finches gradually decrease and we
observe the males scattering in different directions in search of
territories; we watch the summer migrants on their way--small parties
halting for a few hours in the hedgerows and then continuing their
journey, single individuals alighting on trees and bushes and resting
there for a few minutes, and the constant passage of flocks of various
dimensions at various altitudes; and we see Fieldfares, Redwings, and
Bramblings slowly making their way from the south and the west to their
homes in the far north. Occasionally we hear their song, not the
emotional outburst customary at this season, but, except in isolated
cases, a weak and tentative performance. Gätke speaks of the absence of
song on the Island of Heligoland, and refers to the Whitethroat as one
of the few migrants that enliven that desolate rock with their melody.
On the other hand, many migrants that rest temporarily on the Isle of
May sing vigorously.[5] But on the whole there is, I think, no question
that the male whilst travelling to its breeding grounds, and, even after
its arrival, whilst in search of a territory, sings but little--and that
little lacks the persistency characteristic of the period of sexual
activity. Yet, when a suitable territory is eventually secured, the
nature of the bird seems to change; for, instead of being silent and
retiring, as if aware of some end not fully attained, it not only makes
itself conspicuous but advertises its presence by a song uttered with
such perseverance as to suggest that that end is at length attained.
Hence, in a general way, the instinct of song seems to be related to the
establishment of a territory.
Now the subsequent course of behaviour tends to confirm this view. We
have already had occasion to refer to the fact that the males of some
species desert their territories temporarily and join together on ground
which is regarded by the birds that associate there as neutral, and that
they do so not merely for the purpose of securing food but because they
derive some special pleasure from the act of association, and we shall
find that the altered behaviour of the male when it leaves its territory
to seek food or to join the flock is an important point for us just now.
Buntings desert their territories temporarily and collect in flocks on
the newly sown fields of grain. Some of the males are single, others are
paired, and accompanied, it may be, by their mates; they wander over the
ground in search of food, uttering their call-notes from time to time,
or, settling upon the hedges and trees surrounding the field, rest there
and preen their feathers. But even though a male may be surrounded by
other males, even though it may occupy a position where it is
conspicuous to all around, even though, that is to say, it is
apparently in contact with just those stimulating circumstances which
will evoke a response when it returns to its territory, yet it makes no
attempt to sing.
Lapwings, when they resort to the neutral ground, run this way and that
in full enjoyment of one another's companionship, behaving as they do
when they flock in autumn and winter. Specific emotional manifestation
is, however, absent, and their actions seem to be in nowise affected by
the powerful impulse which only a few minutes previously determined
their conduct, for of the characteristic flight with its accompanying
cry there is no sign.
Early in the season Turtle Doves often collect from the surrounding
country at certain spots where their favourite food is abundant. The
croak of this Dove--its true song--is a familiar sound during the
summer, but in addition the bird has a sexual note characteristic of the
race. I watched a flock of upwards of one hundred on some derelict
ground approximately eight acres in extent. Here, in May, the birds were
attracted by the seeds of _Stellaria media_ which was growing in
profusion. After 5 A.M. there was continuous traffic between this piece
of ground and the surrounding neighbourhood, a constant arrival and
departure of single individuals or pairs; and, as they fed, the sexual
note could be heard in all directions. Now some of the males occupied
territories close at hand, and one could watch their passage to and
fro; yet in no single instance did I hear the true song uttered on the
feeding ground, although the moment a male returned to its territory its
monotonous croak could be heard, uttered moreover with that persistence
which is so marked a feature of all song or of the sounds that
correspond to it.
Thus it will be seen that, even after the internal organic changes have
taken place, the instinct of song is not susceptible to stimulation at
all times and under all circumstances, but only at certain specified
times and under special circumstances which can be observed to
correspond with the occupation of the territory.
In many species each male singles out within its territory some
prominent position to which it resorts with growing frequency. This
position is an important feature of the territory, and exercises a
dominating influence on the life of the bird. I have referred to it as
the "headquarters," and it may be a solitary tree or bush, an
outstanding mound or mole hillock, a gatepost or a railing--anything in
fact that supplies a convenient resting place so long as it fulfils one
condition, namely that the bird when it is there is conspicuous. It need
not, however, be a tree or a mound or indeed anything upon which the
bird can perch, for there is reason to think that the soaring flight
undertaken at this season by so many males, since it is generally
accompanied by the specific sexual sound, answers the same purpose as
the topmost branch of a tree.
Now there is nothing in the external environment to which the song is
more definitely related than to the "headquarters"--this at least is the
conclusion to be drawn from the behaviour, and I will indicate the sort
of evidence upon which such conclusion is based. There is, first of all,
the persistency with which the male resorts to the same tree, even to
the same branch, and, as it seems, solely for the purpose of
advertisement. We know by experience the approximate routine of the
male's behaviour; we know where to seek it, where to hear it, and when
once we have discovered its headquarters, we know that there it will
sing day after day for weeks or it may be for months together--perhaps
the most striking feature of its behaviour at this season. Next, we find
that other trees, though made use of, are not made use of to a similar
extent for the purpose of song. The area occupied varies much according
to the nature of the environment; it is sometimes extensive, and seldom
less than half an acre or so in extent; but in most instances it
contains plenty of trees and bushes which could, one would imagine,
serve the purpose of a "headquarters" just as well as the particular one
selected, and yet the bird, when there, betrays no inclination to sing
at all comparable with that which can be observed when it occupies its
accustomed perch. Further evidence is afforded in the behaviour of those
species that make temporary excursions from their territories. The male,
on its return, flies as a rule direct to its special tree and sings.
Sometimes, however, it settles upon the ground, not unfrequently
accompanied by the female, and while there remains silent; but presently
rising from the ground and deserting its mate, it flies to the
headquarters and sings. Again, nearly every male at one time or another
in the course of the season is aroused to action by the intrusion of a
rival. The emotional tone of the owner of the territory is then raised,
and the intruder is pursued and attacked; but this alone is not
sufficient, it seems as if the chain of instinctive activities, when
once aroused by appropriate stimulation, must pursue its course to the
end--and the end in such a case is only reached and complete
satisfaction only gained when the bird has not merely returned to his
"headquarters" but has given vocal expression to his emotion. Finally,
we must bear in mind these two facts, that the "headquarters" is
occupied solely by the male--it forms no part of the life of the
female--and that it is the male only that sings.
Many such subtle incidents of behaviour as the foregoing can be
perceived but not readily described, and trifling though they may seem
to be in themselves, yet in the aggregate they yield full assurance of a
close relationship.
The distant song of a male, or the presence of an intruding male, have
also stimulating effects, though in somewhat different ways. The former
evokes the normal reply, that is to say the bird, if silent, is liable
to utter a corresponding reply; the latter arouses hostility into which
is infused much feeling tone, the bird sings hurriedly while in pursuit
of its rival, and, which is more remarkable still, even in the midst of
an encounter. Both the normal reply and the emotional song must be
similar in origin--different aspects of the same situation--and both are
clearly related to the other male.
The arrival of a female may also be followed by an emotional outburst
which can be heard at intervals for some days; on the other hand, the
song may continue as before or, for a time, entirely cease.
To take the emotional outburst first. This would appear to be
susceptible of explanation on the hypothesis that the voice contributes
to a more effective pairing situation; an hypothesis which admittedly,
at first sight, gains some support from the fact that a second or a
third male is frequently present. But, in truth, the presence of a
second male makes the situation, so far as the relationship between the
song and the female is concerned, all the more perplexing; for, as we
have already seen, the instinct of pugnacity, when aroused by the
appearance of an intruder, is also liable to be accompanied by a
similarly extravagant song. On each occasion the vocal effort is infused
with much feeling tone, and it would be impossible to point to any one
feature which is peculiar to only one occasion. The question therefore
arises as to whether the emotional outburst which we are attributing to
the arrival of a female may not after all be due to the presence of an
intruding male. It may be so. But although I can recall no single
instance in which the presence of an intruder could be definitely
excluded, yet I should hesitate to base upon this any broad
generalisation.
When the normal course of the song is not interrupted by the arrival of
a female, when, that is to say, the male still pursues the routine to
which he has all along been accustomed, and still sings at stated
intervals in stated places with a voice that betrays no heightened
emotional tone, even though the song may convey some meaning to the
delicate perceptual powers of the female, we have nothing to lay hold
upon which can be construed as an indication of direct relationship
between the song and the presence of the female.
The partial or complete suspension of the song after pairing has taken
place is the most interesting, as it is the most noticeable, feature.
Not that it is by any means universal--if it were so, some of the
difficulties that beset the path of interpretation would be removed, but
it is sufficiently widespread to demand explanation. In nearly every
case it is, however, only temporary, the period during which the male is
silent varying from a few days to a few weeks. The male
Grasshopper-Warbler, when it first reaches us, sings persistently, but
when it is joined by a female a change becomes apparent; instead of the
incessant trill, there are spasmodic outbursts of short duration, and in
the course of a few days the bird lapses into a silence which may be
broken for a short while at dawn, or late in the evening, but is often
complete. More striking still is the change in the case of the
Marsh-Warbler, and the sudden deterioration, or even suspension, of
strains so beautiful and so varied, at a moment, too, when it might
least be expected, at once arrests the attention. The Reed-Warbler that
had its headquarters in a willow sang vigorously from the middle of May
until a female arrived on the 20th June, when its voice was hushed,
except for occasional outbursts which lacked force and were of short
duration. When the Wood-Warbler secures a territory it repeats its
sibilant trill with unwearying zeal, yet no sooner does a mate appear
than its emotion is manifested in other directions. The Reed-Bunting is
vociferous during February and March; but when a female arrives, periods
of silence are frequent and the instinct of the bird becomes
progressively less susceptible to stimulation. After the manner of the
race the male makes temporary excursions from its territory accompanied
by his mate, and it is noteworthy that when he returns and she is absent
he sings, but that the moment she joins him, or even comes into sight,
he is silent. In fact, in greater or less degree, a change is noticeable
in the song of many resident and migratory species under similar
circumstances, a deterioration so marked that we learn by experience to
regard it as a certain indication of the arrival of a mate.
Thus it becomes clear that there are certain specific factors in the
external environment with which the instinct can be definitely related,
and in the order of their importance they are (1) the territory as a
whole; (2) the headquarters; (3) an intruding male; (4) the female.
To what extent are these relationships interrelated? Are they all
mutually dependent upon one another, or is there one which conditions
the remainder?
In the first place it is evident that if a male were not to establish a
territory, no opportunity would be afforded for making use of any
special post or for acquiring a habit in relation to it, and so without
further consideration we may say that the connection between the song
and the headquarters, whatever it may be, is primarily dependent upon
the establishment of a territory.
Next, we have the fact that the distant voice, or still more so the
presence, of another male has an exciting influence and evokes a
corresponding reply. Here we have a direct relationship, and one which
at first sight appears to be exclusive of cross-correlation. But is it
really so; does no circumstance arise under which even the proximity of
a rival fails to evoke response? The reply is not doubtful. Such a
circumstance _does_ arise--when a male for one reason or another passes
outside the limits of its accustomed area. This aspect of behaviour has
already been fully discussed in connection with the question of
hostility, and everyone, I imagine, must by now be pretty well familiar
with the facts. However, it does not often happen that we are given
such an aid to interpretation as is vouchsafed to us in the altered
behaviour of the male when it joins the flock, and if, as I believe,
song and hostility are intimately associated, forming part of an
inter-related whole which, for biological interpretation, has, as its
end, the attainment of reproduction, it is not surprising that
circumstances which lead to the modification of the one should likewise
affect the other; I offer no apology, therefore, for adverting to this
aspect of behaviour once again.
Now a male may leave its territory for three reasons--to pursue an
intruder, to join the flock on neutral ground, or to find the necessary
means of subsistence on other feeding grounds. On each of these
occasions it hears the song of, and is in close contact with, other
males; and if the relationship of which we are speaking be really
exclusive of cross-correlation, its instinct ought to respond with the
customary freedom. But what happens? A male pursues its rival, betraying
much emotion and singing extravagantly, until the boundary is passed,
when emotion subsides and it is silent; or, it flies to the flock on
neutral ground, and, although surrounded by the very males that a short
time previously evoked response, is there unresponsive; or again, it
goes in search of food and collects with other males bent on a similar
errand, and in presence of what we know would be an exciting influence
under other circumstances, it nevertheless remains silent. Hence the
relationship between the song and a male rival seems, as in the case of
the headquarters, to depend in the first instance upon the occupation
of a territory.
So that the relationship between the song and the territory as a whole
is clearly of a different order from that which obtains between the song
and the headquarters, or the song and a male rival; for the first, as
far as can be judged by observation, is exclusive of, whilst the second
and the third involve, cross-correlation. How are these facts to be
explained? We have already seen that it belongs to the nature of the
male during the season of reproduction to establish itself in a definite
place, and this action is just as much a part of its hereditary nature
as the building of the nest is of that of the female, and it is just as
necessary for the successful attainment of reproduction. What exactly
the stimulus is to this mode of behaviour we do not know; we can go no
further back than the internal organic changes which are known to occur
and which we assume, not without some reason, are responsible for its
initiation. Granting, then, that there is this congenital disposition,
what relation does it bear to the song? Without a doubt the song is
likewise founded upon a congenital basis; it is truly instinctive, and
as such requires appropriate stimulation; furthermore the male sings
only when in occupation of its territory. Having regard to these two
facts we might say that the territory is the stimulus to the song. But
this can scarcely be a true interpretation, for inasmuch as the stimulus
would be relatively constant, a relatively constant response ought to
follow, and even a slight acquaintance with the daily round of
behaviour will furnish plenty of evidence to the contrary, seeing that
the song, though persistent, is never continuous--in fact there are long
periods of silence during the daytime, and only in the morning and the
evening does the male become really vociferous. What then is the
stimulus? Through awareness of something in the environment the male
responds to stimulation, and the only reply we can give is that the
headquarters, or a distant song, or the proximity of another male--with
all of which, as we have seen, the instinct is definitely related--are
the specific factors which normally evoke response--and experience
teaches us that the periods of quiescence are just those when life is at
its lowest ebb and these stimulating factors less in evidence. Bearing
this in mind, bearing in mind the fact that when a male joins the flock
or crosses the boundary its instinct ceases to respond, bearing in mind,
that is to say, that there is evidence of relationship between these
specific factors and the song only when the territory is actually
occupied, the conclusion seems inevitable that we have here the
determining condition which renders the instinct susceptible to
appropriate stimulation.
There remains the female. I place her last in order of importance, not
because I regard her influence as of small consequence, but because the
evidence is of a varied and complex kind, so much so that it is
difficult to ascertain by observation just how far she is a situational
item. It will be remembered that the only direct evidence we had of such
influence was a deterioration or, in some instances, a complete
cessation of vocal manifestation. Clearly then we are confronted with a
relationship of a different kind from that which we have been
discussing; for not only is anything in the nature of stimulation
absent, but, and this is a remarkable fact, the other items in the
environment which formerly evoked response no longer do so in quite the
same way. Is there any awareness on the part of the male of the relation
between his voice and the mate that is to be, or is it merely that as
the sexual situation increases in complexity some inhibiting influence
comes into play? These are questions which lead up to difficult
problems. But it is no part of my task to discuss the psychological
aspect of the behaviour; my purpose is merely to show that the situation
on the arrival of a female undergoes marked modification, that the
instinct of the male is then less susceptible to stimulation, and that
the factors in the external environment which formerly elicited response
become relatively neutral.
Hence the appearance of the female on the scene marks the opening of a
new stage in the life-history of the male, and, to judge by the course
of events, it would seem as if the song with its network of
relationships had now served its main biological purpose.
And now, what is the purpose, and what the origin, of song? Is it, as
some naturalists have conceived, a means of raising the emotional tone
of the female, of creating a more effective pairing situation, and so
of removing a barrier to the successful discharge of the sexual
function; or, is the emphasis here too much upon the emotional, too
little upon the strictly utilitarian, aspect? All, I think, will agree
that it must serve some biological purpose, and the position we have so
far reached is that the determining condition of its manifestation is
not merely the establishment, but the actual occupation of a territory,
and that there are no factors in the external environment which can
evoke response in the absence of such condition. This being so, the
further questions arise as to whether it contributes towards the
attainment of the end for which the whole territorial system has been
built up, and what precisely is the way in which it does so.
Everyone knows that in the spring the shyest of birds no longer practise
the art of concealment. The Curlew soars to a great height, and upon
outstretched wings hovers in the air whilst uttering its plaintive wail;
the cock Grouse, as if dissatisfied with its "crowing," springs into the
air and becomes a conspicuous object of the moor; the wary Redshank,
poised on flickering wings, forgets its mournful alarm cry, and finds
again its melodious song; and even the secretive Grasshopper-Warbler
crawls out of the midst of the thicket in order to "reel," just as, for
a similar reason, Savi's Warbler climbs to the top of a tall reed. In
fact the males of most species, when they are finally established on the
breeding grounds, make themselves as conspicuous as possible by sight
and by sound. And since the sounds produced by no two species are
exactly alike, the females are able to recognise their prospective
mates, and the males that are still in search of ground have ample
warning if that upon which they are treading is already occupied. So
that you see, from the remarkable development of the vocal powers in the
male, there follow two important results--"recognition" and "warning."
We here turn from song as the expression of an instinctive disposition,
and the question of what calls forth this expression, to the impression
produced by the song on the hearer.
Most birds have a call-note or a number of call-notes, which, generally
speaking, are specifically distinct. But to the human ear they are not
always so, perhaps because our power of hearing is less sensitive than
that of a bird, and unable to appreciate delicate differences of tone.
Be this as it may, however, the fact remains that we often find it
difficult, and in not a few cases impossible, to recognise a bird merely
by its call. The plaintive notes of the Willow-Warbler and of the
Chiffchaff are to our ears very closely akin, so, too, are those of the
Marsh-Warbler and of the Reed-Warbler, and there is a great resemblance
between the hissing sound produced by the two Whitethroats. In Co.
Donegal I have been deceived by the spring-call of the Chaffinch which,
owing possibly to the humidity of the atmosphere, is, there, almost
indistinguishable from the corresponding note of the Greenfinch. The
Yellow Bunting and the Cirl Bunting frequently make use of a similar
note, so do the Curlew and the Whimbrel. In fact, numberless instances
could be quoted in which notes appear to us identical, and, as a rule,
the more closely related the species, the more difficult it becomes to
distinguish the sounds--alike in plumage, alike in behaviour, alike in
emotional manifestation, it would be surprising if they were not alike
in voice. But the moment we pass from the call-notes to a consideration
of the songs we are faced with a very remarkable fact, for not only are
these readily distinguished, but in many cases they bear no resemblance
in any single characteristic. What could be more unlike than the songs
of the Willow-Warbler and of the Chiffchaff, of the Marsh-Warbler and
the Reed-Warbler, or of the Yellow Bunting and the Cirl Bunting?
Now when different individuals collect in flocks at certain seasons,
they assist one another in finding food, and afford mutual protection by
giving timely warning of the approach of a common enemy, and the
gregarious instinct is thus of great advantage to the species; but no
matter how powerful the impulse to flock might be, if there were no
adequate means of communication, the different units would frequently
fail to discover their neighbours. Here the specific cries and calls
come into play, enabling them as they move about in search of food, or
change their feeding grounds, or whilst they are on migration, to keep
constantly in touch with one another; and hence one purpose that these
call-notes serve is that of recognition. Moreover, they convey their
meaning to individuals of other species and are acted upon, and are thus
in every sense socially serviceable; but on the other hand, whilst
there is much evidence to show that the song is of great individual
value, there is none to show that it is in any like manner of direct
advantage to the community.
If, then, there is in the call-notes an adequate means of communication
and of recognition, why do I suggest that the song has also been evolved
primarily for the purpose of recognition?
What, first of all, are the conditions in the life behaviour during the
season of reproduction that make the intervention of the voice a
consideration of such importance? The general result of our
investigation might be summed up thus: we found that the male inherits a
disposition to secure a territory, that at the proper season this
disposition comes into functional activity and leads to its
establishment in a definite place, and that it cannot search for a mate
because its freedom of action in this respect is forbidden by law; that
the female inherits no such disposition, that she is free to move from
place to place, free to satisfy her predominant inclination, and to seek
a mate where she wills; and, since the appropriate organic condition
which leads to pairing must coincide with appropriate conditions in the
environment, that the union of the sexes must be accomplished without
undue delay. Furthermore we found that a territory is essential if the
offspring are to be successfully reared; that, since the available
breeding ground is limited, competition for it is severe, and that the
male is precluded from leaving the ground which he has selected, and is
obliged, in order to secure a mate, to make himself conspicuous. That
was our general result. Now there are two ways by which the male can
make himself conspicuous--by occupying such a position that he can be
readily seen, or by producing some special sound which will be audible
to the female and direct her to the spot. The former, by itself, is
insufficient; in the dim light of the early dawn, when life is at its
highest, and mating proceeds apace, what aid would it be for a male to
perch on the topmost branch of a tree, how slender a guide in the depth
of the forest? But whether in the twilight or in the dark, in the
thicket or the jungle, on the mountain or on the moor, the voice can
always be heard--and the voice is the principal medium through which the
sexes are brought into contact.
Well now, we come back to the question, why, if all species have a
serviceable recognition call, that call should not be sufficient for the
purpose, just as, without a doubt, it is adequate for all purposes at
other seasons? The answer is, I think, clear. The recognition call is
not confined to one sex, nor only to breeding birds; it is the common
property of all the individuals of the species, and if the female were
to rely upon it as a guide she might at one moment pursue another
female, at another a non-breeding male; she might even be guided to a
paired female or to a paired male, and time would be wasted and much
confusion arise. So that no matter how much a male might advertise
himself by cries and calls which were common alike to all the
individuals of the species, it would not assist the biological end which
we have in view. Something else is therefore required to meet the
peculiar circumstances, some special sound bearing a definite meaning by
which the female can recognise, amongst the host of individuals of no
consequence to her, just those particular males in a position to breed
and ready to receive mates. Hence the vocal powers, the power of
producing sounds instrumentally, and the power of flight, have been
organised to subserve the biological end of "recognition."
And this view is strengthened, it seems to me, by the erratic behaviour
of certain species, more particularly by one remarkable case, the case
of the Cuckoo. The male, after having established himself, utters his
call persistently from the day of arrival until approximately the middle
of June; but, in contrast with the large majority of species, the female
has a characteristic call which she, too, utters at frequent intervals.
The female is polyandrous and has a sphere of influence embracing the
territories of a number of males; she wanders from place to place, is
often silent, and not unfrequently is engaged in dealing with her egg or
in searching for a nest in which to deposit it, and therefore she is not
always in touch with a male, still less with any particular one. Now
there is much evidence to show that the discharge of the sexual function
amongst birds is subject to control, and that this control operates
through the female--through her physiological state becoming
susceptible to stimulation only at certain periods. So that we have
these considerations, that the female is polyandrous, that she has a
territory distinct from that of the male, and that her sexual impulse is
periodical; and the further consideration that the impulse, since it is
periodical, is of limited duration and must receive immediate
satisfaction. Such being the circumstances of the case, would the voice
of the male serve to insure the union of the sexes at the appropriate
moment? Well, the fact that she is polyandrous implies that every male
in her sphere of influence is not always capable of satisfying her
sexual instinct. Is, then, the male's call an indication of his
readiness to yield to stimulation? Without a doubt it is an index of the
general physiological state which generates the sexual impulse, without
a doubt it denotes a general preparedness to breed, but there is no
evidence to show that it denotes the degree of ardour of the male at any
particular moment, and much that proves the contrary. So that only by
the female producing some special sound which will attract the males
that are eager and bring them rapidly to the spot where she happens to
be, only thus is it possible to insure the consummation of the sexual
act. This, it seems to me, is the purpose of the peculiar call of the
female--a call which, so far as biological interpretation is concerned,
is just as much a song as the melody of the Marsh-Warbler--and its
interest for us just now lies in this, that here we have a special case
in which the sexes have separate territories, the female is polyandrous,
and the voice of the male is not sufficient by itself to bring to pass
the union of the sexes; and in which, consequently, if the purpose of
song be that of recognition, we should expect to find, as we do find,
that the female had a distinct and penetrating call.
We now come to the question of "warning," by no means the least
important purpose of song. I pointed out that one of the chief
differences between the call-notes and the song was that the former were
socially serviceable, whereas the latter was only serviceable to certain
individuals; and in making this statement, I had in mind the direct
benefits to the community which proceeded from an appreciation of sounds
having a mutually beneficial meaning, not the indirect, though none the
less beneficial, consequences to the species as a whole. Biologically
considered, song, if it acts as a warning and thereby leads in one way
or another to more complete success in the rearing of offspring, may be
spoken of as socially serviceable; but it is legitimate to draw a
distinction between the prospective value of remote relationships which
we can foresee, and the mutual assistance which the individuals of a
community derive from their close association.
If there were always sufficient breeding ground to support the offspring
of all the individuals of each species, if the individuals were always
so distributed that there was no possibility of overcrowding in any
particular area, and if the conditions of existence of different
species were so widely divergent that the presence of this one in no way
affected the interests of that, no opportunity would be afforded for the
development of so complex a system as is involved in the "territory" and
all that appertains to it. But the available breeding ground is by no
means unlimited. The supply of food, which is a determining factor in
the environment, is always fluctuating according to the climate and
according to the changes in the earth's surface; and so the distribution
of the bird population in any given area, though it may be suitably
adjusted for one year or even for a period of years, is bound in the
course of time to require readjustment. Now there cannot be readjustment
without competition, nor competition without combat. But the appeal to
physical force is only a means to an end, and, since no male can endure
incessant warfare and the perpetual strain of always being on the alert,
without experiencing such physical exhaustion as might affect his power
of reproduction, its direct effect upon the combatants cannot be
otherwise than harmful--in fact it is a necessary evil which for the
good of the species must be kept strictly within bounds. Bearing in
mind, then, these two facts, namely that the distribution of the males
is never stable and that overmuch fighting may defeat the end in view,
we can appreciate the importance of any factor which will lead to a more
uniform distribution and at the same time insure security by peaceable
means.
The proximate end of the male's behaviour is isolation--how is it to be
obtained? If, after having occupied a territory, the bird were to remain
silent, it would run the risk of being approached by rivals; if, on the
other hand, it were merely to utter the recognition call of the species,
it would but attract them. In neither case would the end in view be
furthered, and isolation would solely depend upon alertness and the
capacity to eject intruders. Supposing, however, that the song, just as
it serves to attract the females, serves to repel other males, a new
element is introduced deserving of recognition; for those males that had
established themselves would not only be spared the necessity of many a
conflict, but they would be spared also the necessity of constant
watchfulness, and so, being free to pursue their normal routine--to seek
food, to rest, and, if migrants, to recover from the fatigue of the
journey, they would be better fitted to withstand the strain of
reproduction; and those that were still seeking isolation in an
appropriate environment, instead of settling first here and then there
only to find themselves forestalled, would avoid and pass by positions
that were occupied, establishing themselves without loss of time in
those that were vacant. Without the aid of something beyond mere
physical encounter to regulate dispersal, it is difficult to imagine how
in the short time at disposal anything approaching uniformity of
distribution could be obtained. Hence, both in the direction of limiting
combat, of insuring accommodation for the maximum number of pairs in the
minimum area, and of conserving energy, the song, by conveying a
warning, plays an important part in the whole scheme.
And if this be so, if the song repels instead of attracting, it follows
that the more distinct the sounds, the less likelihood will there be of
confusion; for supposing that different species were to develop similar
songs, whole areas might be left without their complement of pairs just
because this male mistook the voice of that, and avoided it when there
was no necessity for doing so. So that just as from the point of view of
"recognition" each female must be able to distinguish the voice of its
own kind, so likewise the warning can only be adequate providing that
the sounds are specifically distinct. A point, however, arises here in
regard to closely related forms. Some species require similar food and
live under similar conditions of existence; they meet in competition and
fight with one another; and, if they did not do so, the food-supply of a
given area would be inadequate to support the offspring of all the pairs
inhabiting that area. Generally speaking, the more closely related the
forms happen to be, the more severe the competition tends to become; and
it may be argued that in such cases a similar song would contribute to
more effective distribution and in some measure provide against the
necessity of physical encounter; that, in fact, it would stand in like
relation to the success of all the individuals concerned, as does the
song to the individuals of the same species. But we must bear in mind
that the primary purpose of song is to direct the females to those males
that are in a position to breed; and to risk the possibility of prompt
recognition in order that the males of closely related species should
fight the less, would be to sacrifice that which is indispensable for a
more remote and less important advantage.
What meaning does the song convey to a male that is unestablished? Does
the bird recognise that it is forestalled; does it foresee and fear the
possibility of a conflict, and conclude that the attempt to settle is
not worth while? I do not imagine that it thinks about it at all. How
then does the warning warn? We will endeavour to answer this question,
but, in order to do so, we must review the stages by which a territory
is secured.
We take as our starting point the internal organic changes which are
known to occur. These changes are correlated with other changes,
manifested by a conspicuous alteration in behaviour--to wit, the
disappearance of sociability and its replacement by isolation. Having
found a station which meets the requirements of its racial
characteristics, the male establishes itself for a season, becomes
vociferous, displays hostility towards others of its kind, and in due
course is discovered by a female. The whole is thus an inter-related
whole, a chain of activities which follow one another in ordered
sequence. Now we have seen that it is neither pugnacious nor vociferous
until the territory is actually occupied; we have seen that the fact of
occupation is the condition under which the instincts of pugnacity and
of song are rendered susceptible to appropriate stimulation; we have
discussed the nature of the stimulus in each case, and we wish to know
the sort of meaning that the song conveys to an individual which is
still in the preliminary stage of seeking a station. In sequential order
we have the following: (1) internal organic changes which lead to
isolation, (2) the appropriate environment which gives rise to an
impulse to remain in it, (3) the occupation of a territory which is the
condition under which the instincts are rendered susceptible to
stimulation, (4) the various stimuli. Each is dependent upon that which
precedes it, and no part can be subtracted without failure of the
biological end in view, neither can the different stages be combined in
different order. So that, in considering the significance of song to an
unestablished male, we are dealing with the situation at a point at
which all the latent activities have not been fully felt, for all that
so far has occurred is the change from sociability to isolation
determined by internal organic changes. The bird has not established a
territory because it has not come into contact with the appropriate
environment, and it is not pugnacious because the condition which
renders its instinct susceptible is absent; and so, as it wanders from
place to place and hears the voices of males here or males there, it
merely behaves in accordance with that part of its nature which
predominates just at that particular moment--the impulse to avoid them.
But given the appropriate environment, given, that is to say, just that
combination of circumstances which might bring into functional activity
all the latent instincts of the intruder, and no matter how vociferous
the occupant of a territory might be, it would not be preserved from
molestation. The advantage of the song, biologically considered, is then
this, that it will often prove just sufficient to preclude males in
search of isolation from coming into contact with the environmental
conditions adequate to supply the stimulus to their latent activities
and to convert them into rivals.
If this interpretation be correct, if we are right in attributing the
withdrawal solely to the fact that the first stage only in the
relational series has been reached, it follows that the effect of song
upon males that have reached subsequent stages in that series must be of
a very different kind. We have dealt with the male when in the
preliminary stage of seeking isolation, we must deal with it now when
eventually it occupies a territory. How does it behave when it hears, as
it is bound to do, the voices of rivals in its neighbourhood? You may
remember that some allusion was made to the fact that an outburst of
song from one individual was followed, not unfrequently, by a similar
outburst on the part of other individuals in the immediate locality. For
example, silence may reign in the reed-bed except for an occasional note
of the Reed-Warbler or Sedge-Warbler. Suddenly, however, a dispute
arises between two individuals, accompanied by a violent outburst of
song, and forthwith other males in the vicinity begin to sing excitedly
and continue doing so for some minutes in a strangely vigorous manner,
the tumult of voices affording a striking contrast to the previous
silence. Spasmodic outbursts of this kind, stimulated by an isolated
utterance, are by no means uncommon. But not only does song stimulate
song; under certain conditions it has the still more remarkable effect
of arousing hostility. The boundary that separates two adjoining
territories is by no means a definite line, but rather a fluid area
wandered over by this owner at one moment, by that at another. Now so
long as the bird is silent while in this area, the probability is that
it will escape detection and remain unmolested; let it however sing--it
often does so--and it will not merely be approached but attacked, and
consequently this area is the scene of much strife. The point to be
noticed here is that the song brings about no withdrawal; it elicits a
response, attracts instead of repelling, and, in short, arouses the
impulse that is always predominant in the nature of the male when
eventually it occupies a territory--the impulse of self-assertiveness.
Therefore it seems clear that the different stages in the process of
reproduction mark the appearance of different conditions, each of which
renders some new impulse susceptible to stimulation, and that the
significance of song depends upon the stage which happens to have been
reached. Hence when we speak of song acting as a "warning," we do not
mean that it arouses any sensation of fear; it is but a stimulus to
that part of the inherited nature of the hearer which predominates at
the moment.
Are we then justified in the use of such terms as "warning,"
"significance," or even "meaning," when it is but a matter of stimulus
and response? In what does the impulse to avoid other males consist?
There is no reason to suppose that there is any sensation of fear in the
first stage, and the course of behaviour demonstrates that there is none
in the later stages. But it is difficult to conceive of an impulse which
has, as its end, the isolation of the individual from members of its own
sex and kind, without some feeling-tone, the reverse of pleasurable,
entering into the situation; just as it is difficult to believe that the
female experiences no pleasurable sensation when she hears the voice of
the male that directs her search. So that the song may be actually
repellent in the one case and attractive in the other; and it is none
the less repellent when, as in the later stages, it attracts a
neighbouring male, for the attraction is then of a different order,
determined by the presence of the condition which renders the pugnacious
nature susceptible and leads to attack. In a sense, therefore, we can
speak of "meaning"--though not perhaps of "significance"--and of
"warning," when we refer to the prospective value of the behaviour.
So much for the purpose of "song"; there still remains the more
difficult question--the question of origin. Let me make clear what I
mean by origin. As we have already seen, there is infinite diversity in
the sexual voice of different species; some are harsh and others
monotonous, and some strike the imagination by their novelty whilst
others are melodious; and to the naturalist each, in its particular way
and in a particular degree, probably makes some appeal according to the
associations that it arouses. But just why a Marsh-Warbler is gifted
with a voice that is so beautiful and varied, whilst the
Grasshopper-Warbler must perforce remain content with a monotonous
trill; just why the tail feathers of the Snipe have developed into an
instrument, whilst the Pied Woodpecker has developed muscles which
enable it to make use of a decayed branch as an instrument--we know no
more than we do of the nature of the forces which lead the Reed-Warbler
to weave its nest to reeds, or the caterpillar of the Elephant Hawk Moth
to assume so peculiar an attitude when disturbed. When therefore I speak
of the origin, I do not refer to the mode of origin of variation; I take
for granted that variations somehow arise, and I seek to ascertain
whether there is anything in the phenomena which we have explored which
might reasonably be held to determine the survival of this one in
preference to that.
When we reflect upon the problem of song and consider the numerous and
diverse forms in which it is manifested, we are apt to draw a
comparison between the sounds we hear and those produced by musical
instruments, and hence to conclude that each bird is gifted with a
special instrument in virtue of which it produces its characteristic
melody. But there is a very remarkable phenomenon connected with the
singing of birds which shows that this is really not the case--I mean
the phenomenon of imitation. There are plenty of good imitators amongst
our native species, and the power of imitation is not the exclusive
property of those which have reached a high degree of vocal development,
nor, for the matter of that, of song-birds at all. Even the Jay, than
which few birds have a more raucous voice, that "hoots" like the
Wood-Owl, or copies the sounds produced by the tail feathers of the
Snipe, will occasionally imitate the most melodious strains of some
other species; and the Red-backed Shrike, whose sexual call is
principally a few harsh notes rapidly repeated, bursts at times into
perfect imitations of the song of the Swallow, Linnet, or Chaffinch.
Nevertheless it is amongst such typical songsters as the Warblers that
we find the greatest volume of imitation, and no limit seems to be
placed upon their capacity. The Marsh-Warbler can utter the call of the
Green Woodpecker, or sing as the Nightingale does, with as much facility
as it sings its own song; and the Blackcap is well-nigh as proficient in
copying the cries and melodies of surrounding species--and so, if it
were necessary, we might proceed to add to the list.
These examples demonstrate that different songs are not represented by
a corresponding number of different physiological contrivances; for if
the difference were really attributable to some structural peculiarity,
then the range of sounds embraced in the call-notes and the sexual call
of any given species, must be the measure of the capacity of its
instrument; and no matter how great its power of imitation may be, it
follows that it will only be capable of copying those sounds which fall
within that range. There is plenty of evidence to show that the power of
imitation is almost unlimited, at all events that it is not confined
within such narrow limits as are here demanded. Hence it seems clear
that the diversity of song is not to be sought in structure, but in some
innate capacity to play one tune in preference to another; and if this
be so, and if out of the same instrument, which has been primarily
evolved to further the biological end of intercommunication, all manner
of diverse sounds can be made to proceed, the problem of the origin of
song is to that extent simplified.
We must next inquire into the nature of song, and endeavour to ascertain
whether all the individuals of a species are alike proficient, or,
failing this, whether there is any quality which can be observed to be
constant under all conditions. I watch the Reed-Buntings in a marsh and
find that there are three males occupying adjoining territories. Two of
them are fully mature and their plumage is bright: that is to say the
crown is black, the collar and breast are white, the flanks are dull
white spotted with black, and the mantle is reddish-brown. The third is
immature: the crown, instead of being black, is suffused with brown; the
collar, instead of being white, is mottled with brown; and the flanks
are more heavily streaked with brown. These three birds take up their
positions in February, and, as is their wont, sing incessantly each day
at daybreak. The song of the first two is normal, including the usual
number of phrases which flow in no definite sequence, but are combined
and recombined in different order, and the tone is pure; that of the
third, the immature bird, is, however, very different; for just as in
comparison its plumage is dull, so the phrases of its song are limited
and reiterated with great monotony, the tone is impure, and the whole
performance is dull and to our ears unmusical. I watch them from
February to June, and observe the order in which they are mated--first a
mature male; next, after a short interval, the immature male; and
finally, after a still longer interval, the remaining bird gets a mate.
As the season advances, still keeping watch on the development of the
plumage and of the voice of the immature male, I observe that no very
definite change takes place--that the colours remain dull, that there is
a conspicuous absence in the song of certain phrases, and that the notes
lack purity of tone.
If now, instead of Reed-Buntings in a marsh, I watch Yellow Buntings on
a furze-covered common, I find that, establishing themselves early in
February, they sing persistently, and in a few weeks are paired. But
what arrests my attention more particularly is the quality of the song;
for although the voice is unmistakably the voice of the Yellow Bunting,
yet it is incomplete and lacks the variety of phrases and musical
notation which we customarily associate with the bird. Nevertheless, as
the season advances, there is a progressive development in both these
directions, and by the end of March or the beginning of April the song
possesses all those qualities which appeal to us so forcibly.
There is one other fact to which attention must be drawn--the variation
in the song of the same species in different districts. As an
illustration let us take the case of the Chaffinch. In Worcestershire
the bird sings what I imagine to be a normal song--the notes are clear
and the phrases are distinct and combined in numerous ways. With the
notes fresh in mind I leave them and go to the west of Donegal, where I
am at once conscious of a difference; not a subtle difference that
perplexes the mind and is difficult to trace, but a change so remarkable
that one is conscious of a passing doubt as to whether after all the
voice is the voice of the Chaffinch; the song is pitched in a lower key,
certain phrases are absent, the notes lack tone and are sometimes even
harsh, and the bird seems wholly incapable of reaching the higher notes
to which I am accustomed.
Now the immature Reed-Bunting, though to our ears its song is but a poor
representation of that of the adult, gains a mate; the Yellow Bunting
pairs, and the discharge of the sexual function may even have taken
place before its voice attains what we judge to be its full development;
and there are no grounds for supposing that the Donegal Chaffinch, with
its less musical notes, has on that account any the less chance of
procreating its kind--facts which demonstrate that the biological value
of song is neither to be sought in the purity of tone, nor in the
variety and combination of phrases, nor, indeed, in any of those
qualities by which the human voice gains or loses merit, and which leave
us with no alternative but to dismiss from our minds all æsthetic
considerations in the attempt to estimate its true significance.
What, then, determines its value? Are there any qualities which, whether
the bird is mature or immature, whether it is untrained or has acquired
fuller expression by practice, whether it inhabits this district or
that, are alike constant? Well, no matter how great the variation, no
matter how much this voice falls below or exceeds the standard, judged
from the human standpoint, attained by that, even we, with our duller
perception, have no difficulty in recognising the species to which the
owner of the voice belongs; in other words, the song is always specific,
and this is the most noticeable, as it is the most remarkable,
characteristic.
There is still, however, another quality to which I would draw
attention--that of loudness. The sounds produced are on the whole alike
penetrative, and the individuals of any given district, even though the
climate by affecting their vocal muscles may have modified the character
of the song, are at no disadvantage in this respect; neither are the
females on the same account the less likely to hear the undeveloped
voice of the immature male.
We have then the following considerations: firstly, there is the
widespread and remarkable phenomenon of imitation, from which we can
infer that the diversity of song is not due to structural differences
but must be sought in some innate capacity to play one tune in
preference to another; secondly, not all the individuals of the same
species play a similar tune--we find that there is in certain directions
a noticeable variation which nevertheless does not seem to affect the
question of success or failure in the attainment of reproduction; in the
third place, in contrast with this variation, we can observe a striking
uniformity in two important particulars, namely in the specific
character and penetrative power of the song--qualities which we know are
essential for the purposes of "recognition" and "warning"; and finally,
from the general course of our investigation, we can infer that if a
male had no certain means of advertising its position, the territory
would not be brought into useful relation in its life. Have we here
sufficient ground on which to construct a theory of origin; in other
words, has the evolution of song been incidental to, and contributory
to, the evolution of the territory?
We have all along spoken of the song and of the call-notes as if they
were manifestations of separate emotional states having their respective
and well-defined spheres of usefulness; and while, speaking generally,
this is a true statement of the case, there is much evidence to show
that the relationship between them is nevertheless very close. There
are, for example, quite a number of cases in which a particular
call-note is uttered with unusual energy during sexual emotion, and is
attached to the song, of which it may be said to form a part; but a
still closer connection can be traced in many simple melodies which are
merely compositions of social and family calls repeated many times in
succession, and even in some of the more complex productions there will
be found indications of a similar construction. And since this is so,
since moreover, in the seasonal vocal development of such a bird as the
Yellow Bunting, we can observe the gradual elaboration from simple to
complex--from the repetition of single notes to phrases and from phrases
to the complete melody--we have every reason to suppose that it is along
these lines that the evolution of the voice has proceeded.
In all probability there was a time when vocal expression was limited
to primitive social and family cries which would be called into play
with special force during times of excitement, more particularly during
the sexual season which is the period of maximum emotional excitement.
But the excitement would express itself in all the congenital modes of
behaviour peculiar to the season, and thus the repetition of these cries
would become associated with combat, with extravagant feats of flight,
and with other forms of motor response. Now the more emotional
individuals would be the more pugnacious, and all the more likely
therefore to secure territory and so to procreate their kind; and, being
of an excitable disposition, they would at the same time be the more
vociferous. Hence variations of the hereditary tendency to vocal
expression, even though in themselves they were not of survival value,
would be fostered and preserved, so long as they were not harmful, in
virtue of their association with pugnacity. But if, instead of being
neutral, they helped to further the biological end of combat, the
relationship between the voice and pugnacity would be of a mutually
beneficial kind; and those individuals in which variation in both
directions happened to coincide, would have a better chance of success
in the attainment of reproduction.
A territorial system, closely corresponding to that which we have
discussed, forms part of the life behaviour of certain mammals, and of
its existence much lower in the scale of life evidence is not wanting;
from which we can infer that it is not of recent origin, but that the
conditions in the external environment demanded such a system at a
remote period of avian development. Now even in its incipient stages the
system must have involved a separation of the sexes, and howsoever
slight the degree of separation may have been in comparison with that
which can be observed to-day, inasmuch as the power of locomotion was
then less highly developed, mating could only have proceeded
satisfactorily providing that males fit to breed had some adequate means
of disclosing their positions. Thus there is reason to think that from
the very commencement of the process variations of emotional disposition
expressed through the voice would have been of survival value.
But expressed in what direction, in loudness and persistency of
utterance, these are the qualities which, I imagine, would have been
more likely to have facilitated the search of the female? Yet if she
were uncertain as to the owner of the voice, neither loudness nor
persistent repetition would avail much; and as species multiplied and
the competition for the means of living became increasingly severe, so
the necessity of a territory would have become intensified, and so, too,
with the extension of range, would the separation of the sexes have been
an ever-widening one; and as with their multiplication, irregularities
and delays in mating, arising from the similarity of the calls, would
have increased in frequency, so a distinctive call, which would have
tended to minimise these risks, would have come to possess biological
value.
Here we have a theory of origin, but origin of what? Of certain
characteristics of song--nothing more; and therefore to suppose that it
furnishes a complete explanation, which satisfies all the requirements
of scientific logic, of so wonderful an intonation as that, for example,
of the Marsh-Warbler, or that no other relationships, except that of the
territory, enter into the total emotional complex, simplifying here or
elaborating there to meet the exigencies of diverse circumstances--to
suppose this would be foolish. That there are many relationships which
even to-day are leading to modifications in important particulars, but
which at the present time are beyond our cognisance, of this there can
be no doubt.
There is one process by which song may have attained a fuller
development, and which would account in some measure for the
elaboration, inexplicable merely in terms of "recognition." It is this:
the effect of the sexual call upon the female cannot well be neutral, it
must be either pleasurable or the reverse--it must, that is to say, be
accompanied by some suggestiveness, and by suggestion I mean the
arousing of some emotion akin to that of the male; and if there are
degrees of suggestiveness, which well there may be, some males will mate
sooner than others and some will remain mateless--this is the theory of
sexual selection. The question to be decided here is whether the
biological emphasis is on loudness, or specific distinctness, or pitch,
or modulation, or the manner in which the phrases are combined--that is,
on some qualities in preference to others--or whether the emphasis is on
the whole. We have already seen, and it is well known, that there is
much variation in the voices of different individuals of the same
species, and thus the first condition of the theory is fulfilled. Now
the conditions which lead to variation are threefold--immaturity,
seasonal sexual development, and isolation. Of the three, the variation
in the case of the immature bird is the most instructive; the tone is
not so pure, the combination of phrases is incomplete, and elaboration
is imperfect, and yet, notwithstanding all these imperfections, we can
observe that the bird pairs as readily as does the adult. But even if we
lacked this demonstrative evidence, we should still be justified in
assuming that such must be the case, for we know from experience in the
preservation of game, where there is no surer way of reducing the stock
than by leaving too high a percentage of old cocks, that for the young
bird to be at any disadvantage in competition with the adult is
detrimental, if not disastrous, to the species. So that while there is
plenty of evidence of variation in those particular qualities which
appeal to our æsthetic faculties, there is at the same time evidence
which demonstrates that such variations exercise no influence on the
course of mating; and inasmuch as it is difficult to conceive of any
voice departing more from the normal type in these particular qualities
than the immature does from the adult, if there be degrees of
suggestive influence, we must seek it in some other direction. There
remain the two other characteristics which we found to be constant under
all circumstances, namely, loudness and specific distinctness; and if,
in addition to serving the purpose of disclosing the positions of the
males, they serve to evoke some emotion in the female, which helps to
further the biological end of mating, so much the more reason is there
for their survival.
There can be no question that this ingenious and attractive theory, if
it were true in its special application to song, would immensely
simplify interpretation, and moreover that preferential mating would
contribute not a little to the success of the whole territorial system.
No one can deny the strength of the argument: that the sexual instinct,
like all other instincts, must require a stimulus of an appropriate
kind; that the effect of the sexual call upon the female cannot be
neutral; and hence the probability that stimulation varies too; no one,
I say, can question the strength of this evidence, and, one might add,
of the evidence derived from the analogy of the human voice. But when we
have said this, we have said all; and our acceptance of the hypothesis,
so far as song is concerned, must remain provisional so long as the
evidence remains but secondary evidence.