Territory in Bird Life Part 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 circ.u.mstances under which they are uttered, we shall in time learn to a.s.sociate 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 cla.s.s 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 s.h.a.g.

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 princ.i.p.al 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 s.e.xes, song is confined to one s.e.x--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 s.e.xual 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 circ.u.mstances 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 pa.s.sage of flocks of various dimensions at various alt.i.tudes; 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. Gatke 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 s.e.xual 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 a.s.sociate 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 a.s.sociation, 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 circ.u.mstances 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 s.e.xual 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 s.e.xual note could be heard in all directions. Now some of the males occupied territories close at hand, and one could watch their pa.s.sage 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 circ.u.mstances, but only at certain specified times and under special circ.u.mstances 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 s.e.xual 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 advertis.e.m.e.nt. 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 a.s.surance 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 Gra.s.shopper-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 circ.u.mstances, 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 circ.u.mstance arise under which even the proximity of a rival fails to evoke response? The reply is not doubtful. Such a circ.u.mstance _does_ arise--when a male for one reason or another pa.s.ses 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 a.s.sociated, forming part of an inter-related whole which, for biological interpretation, has, as its end, the attainment of reproduction, it is not surprising that circ.u.mstances 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 pa.s.sed, 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 circ.u.mstances, 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 a.s.sume, 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 s.e.xual 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 s.e.xual 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 c.o.c.k 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 Gra.s.shopper-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 pa.s.s 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 a.s.sist 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 s.e.xes 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, compet.i.tion 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 princ.i.p.al medium through which the s.e.xes 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 s.e.x, 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 a.s.sist the biological end which we have in view. Something else is therefore required to meet the peculiar circ.u.mstances, 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 s.e.xual 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 s.e.xual 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 circ.u.mstances of the case, would the voice of the male serve to insure the union of the s.e.xes 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 s.e.xual 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 s.e.xual 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 s.e.xual 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 s.e.xes have separate territories, the female is polyandrous, and the voice of the male is not sufficient by itself to bring to pa.s.s the union of the s.e.xes; 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 a.s.sistance which the individuals of a community derive from their close a.s.sociation.