Territory in Bird Life

by Henry Eliot Howard

The advantage of this territorial system is therefore apparent. Instead of this district being overcrowded and that one deserted; instead of there being too many of one sex here and too few of the other sex there; instead of a high percentage of individuals failing to procreate their kind, just because circumstances over which they have no control prevent their discovering one another at the appropriate time—each sex has its allotted part to play, each district has its allotted number of inhabitants, and the waste of energy and the loss of time incurred in the process of mating is reduced to a minimum.

Let us return again to the question of fortuitous mating, and consider the position of a male and female that have discovered one another by accident and have paired; what will be the subsequent course of their behaviour? We are assuming, of course, that a territory forms no part of their life-history. If the discharge of the sexual function takes place immediately and the ovaries of the female are in an advanced state of seasonal development, the construction of the nest will proceed without delay—and the nest will answer the same purpose as the territory in so far as it serves to restrict the movements of the birds and tends to make them remain in, or return to, its vicinity; but if not, there will be an interval during which both sexes will continue to wander as before, guided only by the scarcity or abundance of food. In the first case, there will be the attraction of the nest to prevent any untimely separation; in the second, there will be nothing in the external environment to induce them to remain in any particular spot. Now if we turn to any common species and observe the sequence of events in the life of different pairs, we shall find that pairing is seldom followed by an immediate attempt to build; that an interval of inactivity is the rule rather than the exception, and that this interval varies in different species, in different individuals, and in different seasons. Our imaginary male and female will therefore be faced with considerable difficulty; for with nothing in the external environment to attract them and with no restriction imposed upon the direction or extent of their flight, their union will continue to be, as it began by being, fortuitous. Next, let us consider their position were a disposition to establish a territory to form part of the inherited nature of the male. Each one will then be free to seek food when and where it wills and to associate with other individuals without the risk of permanent separation from its mate; and, no matter how long an interval may elapse between mating and nest-building, each one will be in a position to find the other when the appropriate moment for doing so arrives. Hence, while preserving freedom of movement for each individual, the territory will render their future, as a pair, secure.

No doubt the course of behaviour, as we observe it to-day in the lives of many species, is the outcome of, rather than the condition which has led to, the evolution of the territory. Thus, in many cases, we find that early mating is the rule rather than the exception; we find that the sexes frequently separate to seek their food, and fly away temporarily in different directions; and, under exceptional climatic conditions, we find that they even revert to their winter routine and form flocks; only, however, to return to their territories, as pairs, under more congenial conditions. Yellow Buntings, for example, pair comparatively early in the season—some in the latter part of February, others in March, and others again in April; and some build their nests in April, others in May. There is a gorse-covered common which I have in mind, a favourite breeding resort of this species. Between this common and the surrounding country, the birds constantly pass to and fro. If you watch a particular male you will observe that it sings for a while in its territory, that it then rises in the air and disappears from view, and finally that it returns to the tree, bush, or mound which constitutes its headquarters, where it again sings. Meanwhile the female, with which there is every reason to believe that this male has paired, behaves similarly; she, too, flies to the surrounding country and in time returns with equal certainty. Sometimes male and female accompany one another—that is, they leave simultaneously and likewise return; at other times, though they depart together, the male returns alone; or the male may disappear in one direction whilst the female does so in another—and, on the whole, there is a sameness in the direction of flight taken by the same pairs on different occasions. An interval of nearly two months may thus elapse between mating and nest-building, during which the sexes are not only often apart but often separated by a considerable distance.

What does this species gain by the individuals belonging to it mating so early in the season? If the appropriate condition which leads the females to seek males were to arise in each individual at a late date, the first stage in the process—mating—would not be completed before the second—the discharge of the sexual function—were due to begin. Thus, instead of having ample time, the females would have but a short period in which to discover males; and this in some cases might lead to delay, in others to failure, and in others again to needlessly severe competition, entailing physical exhaustion at a critical moment in their lives. Hence those females in which the appropriate organic condition developed early in the season would not only be more likely to find males, but would be in a position to rear more broods than those in which it developed late; and they would have a better chance of leaving offspring, which, in their turn, would reproduce the peculiarities of their parents. Moreover, within certain limitations, the more these successful females varied in the date of their development, the less severe would be the competition, and the more uniformly successful would the mating of all the individuals in a given district tend to become. But all of this renders an interval of sexual inactivity unavoidable; an interval which must constitute a danger unless there were something in the external environment to prevent the male and female from drifting apart. Inasmuch, then, as the occupation of a territory serves to remove all possibility of permanent separation, I suggest that its evolution has afforded the condition under which this beneficial procedure has developed—free to mate when they will, free to seek food where they will, free to pursue their normal routine of existence, and to meet all exigencies as they arise in their ordinary daily life—whilst free to do this, their future, as a pair, is nevertheless secure.

Thus far we have considered the territory in its relation to the discharge of the sexual function. In many of the lower forms of life, the success or the failure of reproduction, so far as the individual is concerned, may be said to end with the completion of the sexual act—the female has but to deposit her eggs in a suitable environment and then her work is done, because in due course and under normal conditions of temperature the young hatch out, and from the first are able to fend for themselves. And so, when we come to consider the question of reproduction in the higher forms of life, we are apt to focus attention too much upon the sexual function and too little upon the contributory factors, the failure of any one of which would mean failure of the whole. For a bird, success in the attainment of reproduction does not merely imply the successful discharge of the sexual function; much more is demanded; it must find somewhere to build its nest and to lay its eggs, it must shield its young from extremes of temperature and protect them from enemies, and it must be in a position to supply them with food at regular intervals. And, consequently, every situation is not equally favourable for rearing young; there must be a plentiful supply of food of the right kind in the immediate vicinity of the nest, and it must be in greatest abundance just at the moment when it is most urgently needed—that is to say, during the first few weeks after the birth of the young. Success, therefore, depends upon manifold relationships which centre in the station, and these relationships vary in intensity with the conditions of existence.

First, then, let us examine the problem from the point of view of the food-supply. There are many species whose success in rearing offspring is largely dependent upon the rapidity with which they can obtain food; and it makes but little difference which species we choose out of many—Finch, Bunting, Warbler, or Chat. I shall choose the Buntings, as their life-history in broad outline conforms to the general type, and, moreover, their behaviour is fresh in my mind. The young are born in a very helpless state; they are without covering —fragile organisms, ill-fitted, one would think, to withstand extremes of temperature, and wholly incapable of protecting themselves from enemies of any description. For the first three days after they are hatched the female spends much of her time in brooding them, and, when she is thus occupied, the male sometimes brings food to her, which she proceeds to distribute or swallows. But all the young cannot be fed, neither are they ready to be fed, at the same moment; and the parents have besides to find food for themselves, and the nest has to be cleaned—all of which necessitates the young being exposed to the elements at frequent intervals. Now it is impossible to observe the instinctive routine of the parents, when the young need attention, without being impressed with the conative aspect of their behaviour. Why, we ask, are the movements of the female so brisk; why does she seek food and clean the nest so hurriedly; why, if her instinctive routine is interrupted, do her actions and her attitude betray such bewilderment? I take it that the only answer we can give to these questions is that the part of her inherited nature which predominates just at this particular time is to brood. But why is brooding of such importance? Partly to maintain the young at the proper temperature, and thereby to induce sleep—and sleep for offspring newly hatched is as important as food—and partly to protect them from the risk of exposure to extremes of temperature. This latter danger is no imaginary one. Examine a young bird that has recently left the egg; observe its nakedness; and consider what it has to withstand—a temperature that may rise to 70° F. or may fall to 40° F., the tropical rain of a thunderstorm or the persistent drizzle of many hours' duration, the scorching effect of a summer sun or the chilling effect of a cold north-easterly wind, and, constantly, the sudden change of temperature each time that the parent leaves the nest. One marvels that it ever does survive; one marvels at the evolution of a constitution sufficiently elastic to withstand such changes. But, however much the constitution may give us cause to wonder, it is clear that much depends upon the parents. A slight inefficiency of the instinctive response which the presence of the young evokes, a little slowness in searching for food or sluggishness in returning to the nest, might lead to exposure and prove fatal. And, however much is demanded of the parents, it is clear that much also depends upon the relationships in the external environment; for no matter how sensitive or how well attuned the instinctive response of the parent may be, it will avail but little in the presence of unfavourable conditions in the environment.

Everything turns upon the question of the effect of exposure. And in order to ascertain how far extremes of temperature are injurious, I removed the nests of various species containing newly hatched young, and, placing them in surroundings that afforded the customary amount of protection from the elements, I made a note of the temperature and the atmospheric conditions and then observed the condition of the young at frequent intervals. Details of these experiments will be found at the end of the chapter.

The experiments with the Blackbirds and the Whitethroats gave the most interesting results. Both broods of each species were respectively of much the same age, yet one brood of Blackbirds survived for five, and the other only for two and a half hours, and one brood of Whitethroats lived for twelve hours whilst the other succumbed in a little over an hour. This difference is rather remarkable; and it seems clear that the power of resistance of the young diminishes rapidly when the temperature falls below 52° F. It must be borne in mind, however, that the conditions under which the experiments were made were, on the whole, favourable—the weather was dry, the temperature was not unusually low, nor was the wind exceptionally strong or cold; and even in those cases in which the young succumbed so rapidly, the atmospheric conditions could by no means be regarded as abnormal.

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 passed 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 succumbed, 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 assiduously to the sole surviving offspring, yet it too had succumbed by the following morning.