Territory in Bird Life

by Henry Eliot Howard

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 assemblages 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 ownership over individual ownership. A rookery is an example of a true community. Neither shortage of nesting accommodation nor scarcity of food can account for Rooks assembling 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 ownership; 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 space of a few hours, it will acquire sufficient strength for its continuance; but continued repetition 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 assume 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, asserts 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 repetition 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 sex 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 competition 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 space because it not only builds a nest but rears four instead of a single offspring, and it can be allowed more space 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 space 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 circumstances in which the individual finds itself; and we came to the conclusion that the movements of the bird, subsequent to the initial act of establishing 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.