Territory in Bird Life Part 6

What, then, would happen in an unusually wet or cold breeding season?

For how long would the young then survive? In the spring and early summer of the year 1916, I was fortunate in observing the effect of exposure under natural but inclement conditions. I happened to be watching the Yellow Buntings on Hartlebury Common--200 acres of Upper Soft Red Sandstone, profusely overgrown with cross-leaved heath (_Erica tetralix_), ling (_Calluna vulgaris_), and furze (_Ulex_)--in one corner of which eight males had established adjoining territories covering some fifteen acres of ground. The males obtained mates towards the end of March or at the beginning of April; nests were built in the middle of May, and the successful pairs hatched out their young in June. On the 10th June the weather became exceptionally cold, and during the next ten days the temperature fell at times to 40 F. during the daytime. Slight frosts were registered at night in the district, and the young bracken, which covered the Common in places, had the appearance of having been scorched and eventually withered away. At the coldest period of this cold spell the young were hatched in two of the nests--in the first one on the 10th June, and in the second a day or so later; and on the morning of the 10th June, having found a suitable position near the first nest, I began to watch the movements of the parents, with the intention of keeping some record of their behaviour each day so long as the young needed attention. An hour pa.s.sed without their appearing, and on examining the young I found that they were cold, feeble, and unresponsive, but the female presently arrived and went to the nest.

Later in the day the young were lively and responded freely when the nest was approached, but nevertheless I was impressed with the length of time during which the parents were absent; for, judging by the experience of previous experiment, there seemed to be every likelihood of their losing their offspring in such abnormally cold weather, unless they brooded them more persistently. On the 11th June at 5.50 A.M.

neither parent was to be seen and the young could scarcely be made to respond; but shortly afterwards both male and female appeared, and, after remaining a few minutes, again disappeared without even approaching the nest. At 6.45 A.M. no attempt had been made to brood and the young were then so feeble that they were scarcely able to open their mouths, and at 6 P.M. one was still alive but the remaining three were dead. Yet the parents returned and the female went to the nest; and, from a distance of a few feet, I watched her brooding the living and the dead. At 5.45 A.M. the following day the remaining young bird had succ.u.mbed, the temperature then being 49 F.

At the second nest, I was unable to watch the behaviour of the parents so closely. On the 15th June the nest contained three young from three to four days old, and during the morning of that and the succeeding day nothing unusual occurred, with the exception that the period of exposure seemed, as in the former case, to be too long. On the 17th June at 3.10 A.M. the young had collapsed and were stiff, but the parents were in their territory and anxious apparently to attend to their brood. At 9.15 A.M. only two of the young were left in the nest, and though I searched amongst the undergrowth and in the gorse bush in which the nest was placed, no trace of the third bird was to be found. Of the two remaining young, one was alive and responsive but the other was dead, and though the female attended a.s.siduously to the sole surviving offspring, yet it too had succ.u.mbed by the following morning.

In a third territory, there was a nest containing four eggs. These eggs were due to hatch at much the same time as those in the two nests just referred to, but they failed to do so, and an examination showed that they contained well developed but dead chicks.

To what can the death of the young and of the chicks in the eggs be attributed? Not to any failure in the instinctive response of the females, for they fed their young, they brooded them, they even brooded the dead as well as the living, and probably did all that racial preparation had fitted them to do. Yet the fact that the young in the second nest were lifeless and exposed at 3 A.M. seems to betoken absence on the part of the parents during the night, and may be interpreted as a failure of the parental instinctive response. Let us return for a moment to the experiments. These showed, it will be remembered, that a rise or fall in the temperature of but a few degrees was sufficient to make an astonis.h.i.+ng difference in the length of time that the young were able to survive without their parents; that when the temperature reached 58 F.

the bodies of the young retained their warmth, and that under such conditions even a night's exposure had little, if any, effect; so that even supposing that the parents were absent during the night, the death of the young cannot be said to have been due to a failure of the parental instinct, because under normal conditions--and under such has their instinctive routine been evolved--their absence would not have prejudiced the existence of the offspring. I attribute the collapse of the young solely to the exceptional cold that prevailed at just the most critical time, and I base this conclusion partly on the experience gained from experiment, but mainly on their condition observed at different intervals; for during exposure they collapsed rapidly, their flesh became cold and their movements sluggish, their response grew weak, and gradually they became more and more feeble until they could scarcely close their bills after the mandibles had been forced asunder.

Yet, even after having reached so acute a stage of collapse, the warmth from the body of the brooding bird was sufficient to restore them temporarily; once more they would become lively and responsive, only, however, to revert to the previous condition soon after the parent had again abandoned them. Doubtless their power of resistance grew less and less during each successive period of exposure.

If the nestling Bunting is to be freed from the risk of exposure, it is evident that there must be, in the vicinity of the nest, an adequate supply of food upon which the parents can draw liberally. Hence those pairs that exercise dominion over the few acres surrounding the nest, and are thus able to obtain food rapidly, will stand a better chance of rearing their offspring than others which have no certain supply to draw upon--and this, I believe, is one of the biological ends for which the territory has been evolved. But it must not be supposed that each pair finds, or even attempts to find, the whole of the food within its territory, or that it is necessary for the theory that it should do so; all that is required is that such overcrowding as might lead to prolonged absence on the part of the parents and inordinate exposure of the young shall be avoided. So that the problem has to be considered not merely from the point of view of the individual, but from the larger point of view of all the pairs inhabiting a given area.

Now there were eight pairs of Yellow Buntings occupying the one corner of Hartlebury Common, and their territories in the aggregate covered some fifteen acres. The birds obtained part of their food-supply amongst the gorse and in some young scattered oak-trees, and part in an adjoining coppice and on the surrounding arable land. But they were not the sole occupants of this corner of the Common; other insectivorous species had territories there also--amongst which were Whitethroats, Gra.s.shopper-Warblers, Willow-Warblers, Whinchats, Stonechats, Meadow-Pipits, Tree-Pipits, and Skylarks. Suppose then that there had been sixteen pairs of Yellow Buntings instead of eight; that there had been other pairs, which a.s.suredly there were, inhabiting the locality; that they had also resorted, which a.s.suredly they did, to the coppice and arable ground for the purpose of securing food; and that their numbers had also been increased in a similar ratio--would a supply of food for all have been forthcoming with the necessary regularity and prompt.i.tude? Well, the parents might have had to travel a little farther; but even if they had been compelled to do so, their absence would only have been prolonged by so many minutes the more, and under normal conditions what harmful result to the offspring could possibly have followed? The question for us, however, is not what might have occurred under normal conditions, but whether the life behaviour is so adjusted as to meet the exigencies of diverse, and in this case of abnormal, circ.u.mstances. Now the capacity of the young to resist exposure diminishes very rapidly when the temperature falls below the normal--the danger zone seems to be reached at approximately 52 F., and the length of time during which they survive then becomes astonis.h.i.+ngly short--and moreover the fall in the temperature would tend to decrease the supply of insect life upon which they depend, so that if the size of the territories had been reduced by one half, and the parents in consequence had been compelled to seek their food at a greater distance, can it be doubted that the c.u.mulative effect of even a few minutes of additional exposure would have been detrimental, if not disastrous, to the offspring?

We speak, however, of the parents extending their journeys a little farther in this direction or a little farther in that, as though they could do so with impunity except in so far as it affected themselves, or their offspring, or the other Yellow Buntings inhabiting that particular area. But, most certainly, any extension would have meant so much encroachment upon the available means of support of other members of the species inhabiting adjoining areas, whose young in turn would have been liable to have been affected; and, with even greater certainty, the Whitethroats, the Stonechats, the Tree-Pipits, and the Willow-Warblers that had also established themselves in that one corner of the Common would have been hard pressed to find sufficient food with sufficient rapidity.

Let me give another ill.u.s.tration of a somewhat different kind. Lapwings, as we saw in the previous chapters, establish territories and guard them from intrusion with scrupulous care. The young are able to leave the nest soon after they are hatched, and consequently the parents are not necessarily obliged to bring food _to_ them--they can, if they so choose, lead them _to_ the food. Whether each pair limits its search for food to its territory, I do not know. But even supposing that all owners.h.i.+p of territory were to lapse directly the young were hatched, that the boundaries were to cease to exist, and that the birds were free to wander at will without fear of molestation, the end for which the territory had been evolved would none the less have been obtained; for inasmuch as the parents are accompanied by their young, it matters not in what part of the meadow they seek their food; all that matters is that the number of families shall not exceed the available supply of food. So far, then, as the Lapwing is concerned, the territory fulfils its purpose when once it limits the number of males, since, by doing so, it limits the number of families and prevents undue pressure upon the means of support.

Nevertheless, there are many birds that seem to rely entirely upon the territory to supply them with all that is necessary. Each Warbler seeks its food within the precincts of its own particular domain, and, except in occasional instances, neither resorts to neutral ground nor makes excursions into the locality immediately surrounding the territory, as does the Bunting. Probably it would be disastrous if it attempted to do so, for since its young at birth are so delicate and so susceptible to changes of temperature, it cannot afford to be absent from them for long. Of the two experiments made with young Whitethroats, one was made under favourable and the other under unfavourable conditions. In this latter case the temperature was 50 F., and the young, it may be remembered, only survived for a little over one hour. Now exposure at that temperature is evidently dangerous, but it would be still more dangerous if the weather were wet instead of dry, and the temperature 46 F. instead of 50 F.; and it is, I imagine, on this account that the impulse to brood is so strongly implanted in the female. No sooner, it seems, does she depart than she returns with a small quant.i.ty of food which she hurriedly distributes and immediately settles down to brood; and if forcibly prevented from returning, her att.i.tude betrays symptoms of what, humanly speaking, we should term great distress. If, then, the conditions in the external environment were such as would make it difficult for the female to obtain food rapidly, what advantage would she derive from so strongly developed an impulse? Might it not be a disadvantage? Might it not mean that she would abandon the search too readily and be content to return with an insufficient supply, and might not that be as injurious to the young as prolonged exposure? Manifestly the impulse to brood could only have developed strength in so far as it fitted in with all the other factors that make for survival; and the princ.i.p.al factor in the external environment seems to be the territory.

How could the young have been freed from the risk of exposure if the impulse to brood had not been so strongly implanted in the parent? How could the impulse to brood have been free to develop if a supply of food had not been first insured? How could the supply of food have been insured if numbers of the same species had been allowed to breed in close proximity?

From the foregoing facts it is clear that the young of many species are at birth susceptible to cold and unable to withstand prolonged exposure.

The parents must therefore be in a position to obtain food rapidly, and consequently it is important that there should be an ample supply in the vicinity of the nest. This end the territory certainly serves to promote; it roughly insures that the bird population of a given area is in proportion to the available means of subsistence, and it thus reduces the risk of prolonged exposure to which the young are always liable.

This leads on to a consideration of those cases in which the question of securing food is subordinate to the question of securing a station suitable for reproduction.

I take the Guillemot as an example. In principle its behaviour is similar to that of the Bunting; the male repairs to a definite place, isolates itself, and becomes pugnacious. But the Guillemot is generally surrounded by other Guillemots, and the birds are often so densely packed along the ledges that there is scarcely standing room, so it seems, for all of them. Nevertheless the isolation of the individual is, in a sense, just as complete as that of the individual Bunting, for each one is just as vigilant in resisting intrusion upon its few square feet as the Bunting is in guarding its many square yards, so that the evidence seems to show that that part of the inherited nature which is the basis of the territory is much the same in both species. What we have then to consider is, What is the biological value to the Guillemot of an inherited nature which, for the Bunting, has utility in relation to the supply of food for the young? Up to a point, the act of securing a territory has like value for each respective species, whether the area occupied be large or small--that is to say, it enables the one s.e.x to discover the other with reasonable prompt.i.tude.

For the greater part of the year, Guillemots live at sea; singly, in twos or threes, or in small parties, they move upon the face of the waters, extending their wanderings far away from land, out into the broad ocean, where for weeks together they face the gales and heavy seas of the Atlantic. But in due course and in response to internal organic changes, they return, like the Warbler, to their breeding grounds--rocky headlands or islands appropriately situated and affording the appropriate rock formation. During all these months of wandering, the majority seem to ignore the land, to pa.s.s away from it altogether, and to spread themselves over the surface of the ocean regardless of mainland or island. Some useful observations, which throw some light on the distance that Guillemots are accustomed to wander from land, were made by Lieut. B. R. Stewart during a number of voyages between various ports in Great Britain and Ireland and ports in North America, princ.i.p.ally New York and Quebec. Thus, on the 24th March, large numbers were seen in lat. 55 N., long. 24 W., five hundred miles approximately from land, though on the following day--four hundred miles off Tory Island--they were not so plentiful. Again, on the 1st October, in lat.

53 N., long. 27 W., seven hundred miles or so from land, one bird was seen, whilst on the following day, in lat. 52 N., long. 21 W., a single individual was washed on board by the heavy seas and seemed little the worse for the adventure. Within two hundred miles of the west coast of Ireland, he found them plentiful on various occasions. From this it is clear that the circ.u.mstances under which the bird lives for many months in succession must impose a considerable strain upon its const.i.tution; and how it is able to withstand the buffeting of wind and water, to secure its food, and to endure, is a mystery. It is important, therefore, that the young bird should be properly nourished and protected from anything that might harm its const.i.tution, and important, too, that the parents should be freed from any undue strain during the course of reproduction.

The conditions which the breeding station has to fulfil are threefold: in the first place, it must be in proximity to the food-supply; secondly, it must provide the necessary shelter for the egg and for the helpless offspring; and, in the third place, it must be so situated that the young can reach the water in safety. We will examine these conditions one by one.

The proximity to the food-supply is a consideration of some importance.

The life of the Guillemot during the winter is a strenuous one; we know that large numbers succ.u.mb in stormy weather, and we can infer that slight const.i.tutional defects might make all the difference between failure and success; and, therefore, the less severely the const.i.tution of the parent is taxed during reproduction, and the more securely the const.i.tution of the offspring is built up, the greater prospect will both have of resisting the hards.h.i.+ps of the winter successfully. Much, then, will depend upon the distance the parents have to travel in order to obtain food. The farther the breeding station is removed from the feeding ground the greater the physical strain which will be imposed upon the birds, and the greater the chance will there be of the offspring being improperly nourished. Now the food consists of small fish, largely of sand-eels, which are secured in deep water, and the abundance of which varies, possibly according to the nature of the currents. Hence cliffs which are situated away from the water, or from which the water recedes at low tide, or which are surrounded by an area of shallow water, and are thus not in proximity to the feeding ground, even though they may fulfil the second and third condition, will not answer the requirements of a breeding station.

Of no less importance is the type of rock-formation. Not every formation affords the necessary ledges upon which the egg can be deposited with safety--the face of the cliff may be too smooth, or too jagged, or the shelves may run at too acute an angle. Many of the large a.s.semblages of Guillemots in the British Islands are found where the rock is quartzite, mica-schist, limestone, or chalk. The reason of this is that such rocks are weathered along the planes of stratification, of jointing, of cleavage, or of foliation--the strata being probably of unequal durability--with the result that innumerable shelves, ledges, and caverns, which are taken advantage of by the birds, form a network over the face of the cliff. But only those ledges can be made use of which are placed at a considerable height above the water, because, when the cliff faces the open sea, the lower ones are liable to be washed in stormy weather by the incoming swell and thus become untenable. There is a small cove in the midst of the most precipitous part of the breeding station at Horn Head, wherein the s.h.i.+ngly sh.o.r.e shelves rapidly to the Atlantic and faces to the west. Here, towards the end of July, young Kittiwake Gulls can sometimes be found washed up on the beach--some living, but in every stage of exhaustion, others dead, and in every stage of decomposition; here is the young bird, recently caught by the swell and thrown upon the sh.o.r.e, lying side by side with the remains of others that had previously succ.u.mbed to starvation--on every side evidence of the devastation wrought by the Atlantic. May not some of this destruction have been brought about by the nests having been placed upon the lower ledges within reach of an exceptionally heavy sea? Hence much depends upon the nature of the rock-formation, and many a mighty precipice, even though it may fulfil the first and third condition, is nevertheless valueless as a breeding station.

Finally, the young bird must occupy a ledge from which it can reach the water in safety. There is much difference of opinion as to the manner in which it leaves the ledge, but all agree that it does so before it is capable of sustained flight. If, then, the face of the cliffs were made up of a series of broken precipices, or if the rocks at the base projected out into the water, or if detached rocks abounded in the waters beneath, the mortality amongst the chicks would no doubt be considerable.

The coast-line of Co. Donegal will ill.u.s.trate the foregoing remarks. On the southern and western side of the Slieve League promontory there is no real Guillemot station; only on the northern side--the quartzite in the vicinity of Tormore--are the birds to be found in large numbers.

Northwards from here, a wild and rugged coast is pa.s.sed over before other stations are reached--at the eastern end of Tory Island and on Horn Head; and beyond this, to the east, there are none, not even on the old rocks that form the promontory of Inishowen. Why, we ask, do countless numbers crowd the ledges of Horn Head, whilst they are absent from the precipices of Slieve League; why, too, are they absent from the granite cliffs of Owey? The reason is not far to seek. Either the face of the cliff is made up of a series of broken precipices, or the face of the precipices is too smooth, or the otherwise suitable ledges are situated too near the water, or the water recedes from the base of the cliff at low tide. Many miles of rock-bound coast are thus useless for the purpose of reproduction.

Now when we bear in mind how large an expanse of coast is formed of blown sand or of rocks of low alt.i.tude, and how many miles of cliff fail to supply the three essential conditions that we have been considering, we can see that suitable breeding stations must be limited both in number and extent. From a wide expanse of ocean hosts of individuals are therefore obliged to converge at certain definite points; and hence, each recurring season, there must arise a compet.i.tion for positions at the station, just as there is compet.i.tion between individual Buntings for positions in the marsh. And the ability to obtain a position upon a suitable ledge involves, in the first place, an impulse to search for it; in the second place, an impulse to dwell in it; and in the third place, an impulse to resist intrusion upon it. It would be useless for an individual to be pugnacious if it had no fixed abode; equally useless for it to establish itself on a particular ledge if it had no power to defend it--all of which implies an inherited nature similar to that of the Bunting. But the proximate end to which the compet.i.tion is directed is not alike in the case of both species. In the case of the Guillemot it has reference solely to the piece of rock whereon the egg is laid; in the case of the Bunting to a piece of ground capable of furnis.h.i.+ng an adequate supply of food for the young; and the reason for the difference is this, that there is always an abundance of food in the water beneath the cliff, but breeding stations are scarce, whereas there is always an abundance of situations in the marsh in which the Bunting can place its nest, but the supply of food varies and at times can only be obtained with difficulty.

If then the Guillemot were to behave after the manner of the Bunting and a.s.sign to itself a portion of the face of the cliff, or if it were only to occupy a few ledges, or an even lesser area--a single ledge--what would be the result? That it would attain to reproduction is beyond question; that the egg would be safely deposited there can be no manner of doubt; neither is there any reason to suppose that the offspring would not be successfully reared. But, indirectly, its behaviour would affect the Guillemot race. For if it be true, as the crowded ledges certainly seem to show, that there is a dearth of suitable breeding ground, no greater calamity could befall the species than that some members should exercise dominion over too large an area of the habitable part of the cliff and thus prevent others from breeding. Under such conditions the race could not endure, since in this, as in every case, its survival must depend upon a close correspondence between the behaviour of the individual and the circ.u.mstances in the external environment.

Scarcity of suitable cliffs is the princ.i.p.al reason of the ledges being so closely packed with Guillemots, just as it accounts for this part of the precipice being crowded with Kittiwake Gulls, that part with Herring-Gulls, and that part again with Razorbills and Puffins. Yet each individual preserves its few square feet of rock or soil from molestation, and the area each one occupies varies according to the conditions of existence of the species. Thus the Herring-Gull occupies a comparatively small area, although one many times larger than that of the Guillemot. It requires more s.p.a.ce than the latter, owing to the fact that it not only builds a nest but rears four instead of a single offspring, and it can be allowed this, because, since its young remain in the nest until they are capable of sustained flight, it can make use of many miles of cliff from which the tide recedes at the base, or which have, at their base, rocks jutting out into the sea; but manifestly it cannot be allowed so much s.p.a.ce as the Bunting.

Martins build in close proximity to one another, owing probably to shortage of accommodation, and, in their case, the nests have to be so situated as to be sheltered from the wet. If water drips upon them for any length of time, the mud, of which they are composed, crumbles and large pieces fall away, with the result that the eggs or the young are precipitated to the ground. Consequently, not every house or perpendicular cliff will answer the purpose of a breeding station. A few pairs build their nests beneath the eaves close against the walls of my house, and year after year the result is much the same; after every downfall of rain, the water collects into rivulets, trickles down over the eaves, is absorbed by the mud and destroys the nests. Thereupon, the birds set to work and rebuild; but again the nest is destroyed, and again they rebuild, and so on throughout the summer, and only on rare occasions do they succeed in rearing offspring at the proper season.

Similar conditions must prevail in many situations; but, clearly, the more binding and plastic the building material, the longer the nest will withstand the action of the dripping water and the greater chance will there be of the young being reared in safety. Observe, therefore, how far-reaching an effect so small a detail as the nature of the mud can have upon the status of the species in any given locality. Where the conditions are favourable, there the birds must congregate to breed, and, like the Guillemot, if each individual exercised dominion over too large an area, the species as a whole would suffer.

In all these examples, the fact of different individuals being in such close proximity may afford some protection from enemies both as regards the egg and the offspring, and in so far as there is a mutual advantage such a.s.semblages may be spoken of as communities. A community, however, in the true sense of the word, is a collection of individuals brought together, not primarily as a result of shortage of breeding ground, but in consequence of advantages of communal owners.h.i.+p over individual owners.h.i.+p. A rookery is an example of a true community. Neither shortage of nesting accommodation nor scarcity of food can account for Rooks a.s.sembling together to breed; for if the different pairs which go to make up the rookery were to scatter throughout the surrounding neighbourhood, they would, as a rule, find plenty of trees in which to build their nests, and plenty of food.

How, then, can the theory apply to a species that breeds under such conditions? What part can the territory play in furthering the life of the individual when large numbers of nests are built closely together in the same tree? There is much evidence to show that mutual protection is a necessary condition of the Rook's existence; many cases are on record of rookeries being destroyed by Carrion-Crows, Hooded Crows, and Ravens.

For instance, Mr. Ward Fowler records a case in which a pair of Crows attacked a small rookery, ransacked the nests, and destroyed the eggs, with the result that not a single pair of Rooks was left in the settlement. Each Rook must therefore secure a position within the precincts of the community if it is to have a chance of success in the attainment of reproduction. But every locality cannot supply sufficient trees of the right kind, appropriately situated and in suitable relation to the food supply, in which numbers of nests can be built in close proximity; so that if more than one community were to attempt to establish itself in a limited area, the supply of food or the supply of trees might become a pressing problem. Each community must therefore be prepared to defend its own interests, and each must be regarded as one unit and the area occupied as one territory within which are included a number of lesser territories. The individual may fail to establish itself within a community, but, even if it succeeds, the community may fail to establish the rights of communal owners.h.i.+p; hence it has to face a twofold possibility of failure, and if it lacked the inherited nature which leads the Guillemot to secure a position upon the ledge, or the Bunting to obtain a position in the marsh, the chances are that it would fail in the attainment of reproduction.

The question now arises as to how it comes about that the area occupied by each individual conforms in broad outline to that which has proved beneficial for the welfare of the species as a whole. We shall find that up to a point the answer is a simple one. No one could study the behaviour of animals without observing the important part that habit plays in the life of the individual; an action performed to-day is liable to be repeated to-morrow and the following day until it becomes ingrained in the life of the individual. This must not be taken to mean, however, that a particular action has to be performed for many days in succession before it becomes definitely fixed; if only it is repeated a number of times, even within the s.p.a.ce of a few hours, it will acquire sufficient strength for its continuance; but continued repet.i.tion gives increased fixity, and, as time goes by, it becomes increasingly difficult for the creature to make a change unless the character of the situation necessitates readjustment.

For example, when the organic condition which leads to nest-building becomes active, the bird tentatively collects some of the necessary material in its bill, flies round with it, and then drops it. After a while it collects some more, and this time leaves it perhaps in a bush.

Later on it makes another attempt, and, meeting with a situation which calls forth the appropriate response, it thereupon lays the foundation of the structure. We will a.s.sume that the nest is placed in the midst of a tangled bush. Well, the bird lays the first strands of the foundation and then goes in search of more material. The next time it approaches the nest from the opposite side of the bush, and presently it finds yet a third entrance. But each entrance is not made use of in turn: one is employed more frequently than the other two, and in the course of time becomes the sole highway to and from the nest. Suppose now that, when the young are hatched, I cut away the foliage from the bush on the opposite side from that on which the bird customarily enters, and by so doing leave the nest exposed, what is the result? The female arrives with food, threads her way through the bush, and, when beside the nest, pauses as if aware that some change had taken place, and then flies away through the new opening. In a short time she returns, flits from twig to twig on the outskirts of the bush, and comes upon the new opening--there she hesitates. But though the nest is in full view and within a few inches of her perch, and though the young stretch out their necks, yet so strong is the former habit that she is compelled to return to the opposite side and approach the nest by the usual circuitous route before she distributes the food amongst her offspring.

Let us see how far this law of habit formation may have been effective in defining the extent of the area occupied. When a male Warbler arrives at its destination in the spring it seeks out a suitable environment, and, having found a place unoccupied by any other male, settles in it and remains there--its behaviour up to this point being determined by racial preparation. After the fatigue of the journey its movements are at first sluggish; hunger, however, a.s.serts itself and a search is made for food; wandering away from the position in which it first settled and which acts as a headquarters, it hunts through certain trees here or certain bushes there and returns, and presently it wanders away again, perhaps in another direction, but, as before, works its way back again to the headquarters. The journeys thus radiate outwards from the headquarters, and according to the success with which the bird meets, so, probably, it happens that some trees are searched more often than others and certain directions are taken more frequently than others, and by constant repet.i.tion a routine is established which limits the direction and scope of its wanderings.

But in the case of the Guillemot the conditions of existence are reversed: food can be had in abundance but suitable breeding stations are scarce. The few square feet of ledge correspond to the tree or clump of bushes which acts as a headquarters for the Warbler, and the occupation of them is determined, as it is in the case of the Warbler, by racial preparation. Since, however, the ledge is only made use of for the immediate purpose of incubation and is in no way affected by questions relating to food, there is no occasion for the bird to wander along the ledge nor to encroach upon those adjoining. Hunger stimulates the Warbler to search the surrounding trees, and so to extend its area; but hunger takes the Guillemot down to the water, and hence the area which it primarily occupied remains unmodified.

To sum up: the territory is useful in various ways, but not necessarily in the same way for every species. Reproduction would always have remained fortuitous, and the number of individuals that attained to it would seldom have reached the possible maximum unless some provision had been included in its system for insuring that the males and females could meet at the proper moment and afterwards remain in touch with one another, and that the number of pairs inhabiting a given area did not exceed the available means of support. I have tried to show that the inclusion of a disposition to secure a territory tends to remove these difficulties. In the first place, the disposition which leads to its occupation comes into functional activity (in the male) early in the season; and so, by the time that the appropriate pairing condition arises in the females, the process of acquiring territories is well advanced, and the males being regularly distributed, each in its respective position, are readily found by their prospective mates. The behaviour of each s.e.x is thus adjusted to further the end of mutual discovery. Next, after mating has taken place, the position occupied by the male acts as a headquarters to which the birds can always repair, and becomes a bond of union which is serviceable in that it prevents any possibility of their drifting apart. And in the third place, the males become pugnacious and in this way secure for themselves areas which vary in size according to the conditions of existence of the species, so that there is no possibility of too many congregating in this locality, and all the less likelihood of too few finding their way to that; and hence, on the average, different pairs are distributed throughout all suitable localities. Furthermore, owing to the fact of their having a headquarters, the male and female are allowed a freedom of movement which otherwise they would only possess when the construction of the nest had actually begun; they can seek their food independently, and, even though paired, they can if necessary continue their winter routine without risk of separation. This means that the organic condition which leads to pairing, is free to develop in the female earlier than would be the case if there were nothing in the external environment to attract the pair to a particular spot; and the longer the period over which the process of pairing can be spread, the greater chance will females have of discovering mates, the less severe will the compet.i.tion tend to become, and, consequently, the smaller the percentage of individuals that fail to obtain suitable partners.

In these ways the territory has been serviceable alike to a number of species. But much as the questions of mutual discovery and regular distribution may have influenced the course of its development, there can, I think, be little doubt that, on the one hand, the supply of the necessary accommodation for rearing offspring, and on the other, the necessity for an adequate supply of food in close proximity to the nest, have been the main determining factors, and have led to a wide divergence in its function. At the one extreme the function is to insure a plentiful supply of food for the young; at the other, to insure a station suitable for rearing offspring. I took the Bunting and the Guillemot as types of the two extremes. The young of the former species are born in a very helpless state. They are susceptible to cold and unable to withstand prolonged exposure, and therefore it is essential that there should be an ample supply of food, upon which the parents can draw liberally, in the vicinity of the nest. But the nest is placed in a variety of situations, and accommodation in this respect may be said to be unlimited. The young of the latter species are not so susceptible to exposure, and moreover there is always an abundance of food in the waters beneath the cliff; but ledges of rock, upon which the egg can be securely deposited and the young successfully reared, are limited both in number and extent. The position then is as follows: there are situations in plenty in which hosts of Buntings can build their nests but the supply of food is a difficulty, and if the respective areas of different individuals were insufficient to supply them with the necessary food with the necessary rapidity, they would run the risk of losing their offspring and the species would not endure; on the other hand, cliffs upon which the Guillemot can rear its young are limited, but the supply of food presents no difficulty, and consequently the smaller the area over which each individual exercises dominion, the greater the number that will attain to reproduction and the greater prospect the species will have of survival. The emphasis in the one case lies on the fact that the area occupied must be sufficiently large; on the other, on its being just sufficient and no more to accommodate the egg. Hence the difference in the function at the opposite extremes is brought about, not by modifications of the instinctive behaviour which leads to the establishment and defence of the territory, but solely by modifications in the size of the area occupied, in accordance with the conditions prevailing in the external environment. No doubt, if we had the life-histories of a sufficient number of species worked out, we should find that the gradations were complete from the one extreme to the other. We are justified in thinking that this must be so because in many directions we can not only observe differences in the size of the area occupied, but can recognise a close correspondence between those differences and the conditions of life of the species. Thus the Herring-Gull occupies a comparatively small area, though one which is many times larger than that of the Guillemot. It requires more s.p.a.ce because it not only builds a nest but rears four instead of a single offspring, and it can be allowed more s.p.a.ce because the young remain in the nest until they are capable of sustained flight, and consequently it can make use of many miles of cliff from which the tide recedes at the base, and which on this account are denied to the Guillemot, but manifestly it cannot be allowed so much s.p.a.ce as the Bunting, for then comparatively few individuals would attain to reproduction.

Again, the Reed-Warbler inhabits swamps overgrown with the common reed, and in such places insect life is abundant just at the time when the young are hatched. But these swamps cover a comparatively small acreage in the breeding range of the bird, and if each pair were to attempt to establish dominion over an area equal, let us say, to that of the Willow-Warbler, the species would have but a poor chance in the struggle for existence. So that, in a case of this description, the supply of food and the comparative scarcity of breeding stations have been factors of like importance in the evolution of the territory.

Finally we were led to inquire as to how it comes about that the extent of the area occupied by each individual is adapted to the circ.u.mstances in which the individual finds itself; and we came to the conclusion that the movements of the bird, subsequent to the initial act of establis.h.i.+ng itself in a position, are regulated and defined by the law of habit formation. For example, the Warbler, in response to its inherited nature, takes up a position in an appropriate situation. It then proceeds to search for food; it makes short journeys first in this direction and then in that; it repeats these journeys, and gradually forms a habit which compels it to remain within more or less well-defined boundaries. But the actual distance that it traverses on the occasion of its first attempt must be determined by the relative abundance or scarcity of the particular kind of insect life which it requires. So that, although habit defines and in some measure helps to determine the boundaries of the territory, it is clear that in the last resort they must depend upon the nature of the conditions in the external environment.

We have, then, the congenital basis which leads to the occupation of a position, and to the enmity shown by the owner of the position towards other individuals; and this congenital basis is found alike in many widely divergent forms, living under equally widely divergent conditions; we have acquired accommodation; and we have relations.h.i.+ps in the organic and inorganic world--and the outcome of it all is a system of behaviour which we, who can perceive the end to which such behaviour is tending, are justified in speaking of as "a disposition to secure a territory." In the development of this system a primary value must be ascribed to the conditions in the external environment, for they determine the direction of the variations of instinctive procedure and of acquired habit which work towards the same goal--that of adjustment to the conditions of life.

NOTE

The following are the experiments referred to on page 181:--

On the 14th May 1915, a nest of Blackbirds approximately four days old was removed at 6.45 A.M. The temperature was considerably below the normal, and snow lay on all the high ground in the neighbourhood. In a short time the birds collapsed, and at 9.15 A.M. were dead. On the 29th May, at 6 A.M., a second nest was removed, containing young of approximately the same age, and although the conditions were more normal, the temperature being 50 F., the birds collapsed at 8 A.M., and an hour later one of the brood showed little signs of life. The wind, however, then changed to the west, and the temperature rose one degree, with the result that they were still living at 11 A.M. A further experiment was made with Song-Thrushes on the 5th June. The wind was in the south and the temperature 63 F. The young, approximately four days old, were removed at 7.25 A.M., but as they showed no signs of collapse at 1 P.M. I replaced the nest in the original site.

On the 30th May, a nest of Whitethroats three days old was removed at 7.15 A.M. The wind was northerly and the weather fine, but the temperature low--50 F. At 8.15 A.M. the birds showed no sign of life. A second experiment with this species was made on the 10th June under more favourable circ.u.mstances, for although the sky was overcast and the wind northerly, the temperature was 59 F. In this case the young survived from 6.55 A.M. to 7 P.M.

On the 27th May 1915, a nest of Hedge-Sparrows hatched the previous day was removed at 7 A.M. The temperature was below the normal, being 49 F.

At 8 A.M. the young were cold and in a state of collapse, but they survived nevertheless until 3.20 P.M.

On the 7th June 1915, a nest of young Skylarks three days old was removed at 7.15 A.M. The temperature was 62 F., and the birds survived until 4 A.M. the next day.