# Territory in Bird Life/Chapter 4

CHAPTER IV
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 inquire 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. 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 ail 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 feelling 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 J 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 occu​pied, 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 nonbreeding 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 1 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 ReedWarbler 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 spatted 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 in​capable 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 aesthetic 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 investi​gation, 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 j 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.