REPRODUCTIVE HABITS Any treatment of the problems connected with territory in bird life must begin by pointing out that relevant data are still meagre, and-that theories put f orward remain to be adequately tested by criticism and by work in the field. The far-reaching nature of these problems has only very lately been appreciated, and attempts to deal with them are in an elementary and almost chaotic state. To analyse in detail the conflicting views would be a protracted task, and would throw little light upon the subject, since these are often hasty statements which cannot hold their ground for long. The method here adopted, therefore, is first to give a tentative sketch in the light of what is at present known, and then to summarize briefly the data on certain relevant points.
Some birds are solitary in their mode of life, others tolerant, and others more or less sociable. With many species sociability is at its lowest ebb during the breeding season, when the flocks or parties break up and settle in pairs, dispersed over the country. But with others, particularly sea-birds, the re verse holds good; their scattered members rally from all direc tions to particular points where they breed in massed colonies, often so dense that they hardly leave room to tread. These are not simply manifestations of a communist or an individualist temperament ; they are solutions of the economic problem which, in one form or another, every species has to meet. This is well illustrated by the case of finches which live on seeds themselves, but feed their young entirely upon insects. In winter they are sociable, living in mixed flocks; in spring a revolution takes place, and they are found everywhere in pairs, occupying compact ter ritories from which all others of their kind are jealously excluded. Their character has not changed its mould, but the new economic problem of rearing a delicate brood consuming its own weight in small insects every day has temporarily imposed upon the species a different manner of life.
Once this pressure is relaxed the social instincts reassert them selves, and the family parties, renouncing a fixed territory, begin to drift together into ficicks again. Where the supply of f ood in the breeding season is more concentrated, or the foraging range more ample, obstacles to breeding in colonies may disappear.
The fundamental law involved may be stated thus: the breeding organization of a given species in a given locality varies in direct ratio to the abundance of accessible food-supply. To take the simplest possible case, a hypothetical falcon feeds entirely upon birds. Each pair has an effective foraging range of x square miles around its eyrie and k victims are required per season to enable a brood to be reared. If the bird population of x sq.m. were as low as k it might theoreti cally be possible for the falcons to make a clean sweep, rearing a brood at the cost of depopulating the area. Obviously that could only be done once, and would be no more to the ultimate advan tage of the falcons than of their prey. In order to maintain them selves indefinitely the falcons must confine their demands to the annual surplus of potential victims. In practice the relationship between the falcons and their prey is stabilized within that margin of safety. This is due partly to the fact that an increased toll leads to increased wariness and more difficult hunting, but above all to the territorial system, which operates to "safeguard the standard of living." There is therefore, a third factor to be con sidered—y, the number of victims per season which the population
of x sq.m. will yield over an unlimited period. This naturally
varies in proportion as the population itself varies. Clearly, if y is smaller than k the requirements of the falcons cannot be met, and no breeding pair will be sustained. Once y=k a suitable territory for a pair of falcons is available (the breeding site and freedom from human persecution being assumed) . This territory must be kept inviolate ; should a second pair begin to poach upon it, or to usurp a part, the food resources will prove incapable of standing the strain. If y=4k there will be room for 4 pairs of falcons each with a territory of +x sq.m.; if y= r ok for i o pairs each occupying only
sq.m., and so on, for x sq.m. is the maxi mum area that the species is physically capable of hunting over, and an ideal territory will be well within that margin. But nest ing accommodation is limited, and as the increasing abundance of prey gradually relaxes the struggle for existence the native fierce ness of the falcon ceases to be so readily stimulated. A point is reached where territory is no longer strictly held, and tolerant in dividualism supervenes, several pairs foraging over the same ground without serious friction and even breeding side by side where suitable sites exist with convenient access to an abundant food-supply. The final stage is reached where y =not k but, say, z ook. In this case there is enough food for i oo pairs of falcons within the effective range of one. It will therefore be unnecessary for the young of the original pair to be expelled from the territory as they grow up. On the contrary they will remain, thriving on the abundant resources, and some sort of colony of falcons will arise, subject to a numerical limit of roughly a hundred pairs. There will, so far as is known, be no direct sanction for enforcing this limit, such as the territory system imposes upon individual pairs, but if the colony seriously exceeds it the increasing drain will begin to tell upon the victims, or actually drive them away, and stringency of food resources will drastically redress the bal ance—not by actual starvation, but by forcing a sufficient number of falcons to seek quarters elsewhere. In practice, clearly k would represent a considerable number of birds, and the immense con centrated population required to yield anything like look would only rarely be found. But if the food of the falcon were, say, grasshoppers instead of birds the demand would be much more easily satisfied, and it is actually among birds of prey feeding chiefly on insects, very small mammals and fishes that large com munities usually occur, those depending upon the higher forms rarely passing beyond tolerant individualism.
Mutatis mutandis the same conditions apply to all birds, for scientifically considered all are equally birds of prey. Thus the British robin (Erithacus rubecula melophilus), which inhabits woods, gardens and hedgerows, has a food-supply consisting of such minute insects, and so limited in proportion to its effective foraging range and the susceptibility of its nestlings to exposure, that it is forced by economic pressure to adopt strict territory, from which all other robins are excluded. In autumn and winter males and females hold separate domains, which often adjoin, and may be fused as the breeding season approaches to form a terri tory for a pair. The cosmopolitan starling (Sturnus vulgaris) is an example of a species of wider foraging range and more catholic diet, nesting in cavities not abundantly available, which commonly adopts the solution here called tolerant individualism, though for all other purposes gregarious. There may be several pairs breed ing on a single house and bringing food from the same meadow ; they work independently and almost ignore one another's pres ence. The rook (Corvus frugilegus) differs in maintaining the community intact through the breeding season, many nests being built on a single tree or in adjoining ones, their occupants working in company and combining to repulse an enemy. Individual pairs attempting to build independently within the communal domain are at an obvious disadvantage, and their nests are liable to be destroyed. Of ten the formation of communities may be brought about by restriction of breeding quarters, as in the case of the cosmopolitan sand-martin or bank-swallow (Riparia r. riparia), or of the gannets and petrels, which breed on rocky islets.
The bird population and the manner in which it is distributed at various seasons are therefore controlled within fairly narrow limits by the amount of suitable food available and the periodic demand for nesting sites. But it is not to be supposed that such fundamental considerations are in any sense perceived or acted upon by the birds themselves. They do not, for example, settle the extent of their territories by elaborate calculations regarding the amount of food these may be expected to yield of ter the young are hatched.
Male.—The process, which has been most clearly analysed by Eliot Howard in the case of the English buntings, is much more primitive. In spring, in response to some internal organic change, the males of these species begin to isolate themselves from the flocks in which they have spent the winter. This isolation is at first only partial; each male continues to roost and feed with the flock but retires with increasing frequency to a particular domain, consisting say of a strip of hedge and its surroundings, or a patch of scrub, where he sings persistently and drives off other males of the same species. The true song, uttered at the top of the voice, is closely associated with territory-holding, and is rarely or never delivered in a finished form unless the singer is on or in search of a private territory. It is not to be confused with the low inward sub-song or undersong, sometimes called recording, which is uttered indis criminately by females and birds of the year as well as by males assembled in winter flocks when singing in chorus. While there are many examples intermediate between these two, whose signifi cance is a controversial question (they may simply express joie de vivre), it is no longer open to doubt that song in its most highly developed forms has the effect of proclaiming ownership of territory, thereby warning rivals that there is a prior claim on the ground, and informing females that an eligible male is in possession. It has therefore a double biological value in obviating the waste of energy involved in needless collisions—for no male will approach the singer except deliberately to offer battle—and in attracting mates, for it is undoubtedly the females which wan der most in search of mates, the males being tied to their terri tories. The acceptance of this view involves abandoning the old vague definition of song from a musical standpoint, since all per formances having the same purpose must fall in the same class. Thus larks and pipits, for example, sing as they fly, using a par ticular style of flight reserved for that purpose; the appeal to the eye and the appeal to the ear can only arbitrarily be separated. And just as the hidden nightingale appeals to the ear alone, so apparently other territorial species appeal to the eye alone, such birds which always occupy prominent observation posts. The soaring of birds of prey may sometimes serve a similar purpose.
By degrees contact with the flock is relinquished, and the male spends all his time in the chosen territory, where by wandering restlessly about he rapidly develops a customary "beat," and a favourite singing stand. Till rival males settle in the vicinity this force of custom is the only limit to his boundaries. He is constantly patrolling them without knowing why, and on meeting a rival he is impelled to drive him away. But the strength of this impulse fluctuates; if the clash occurs on the very outskirts of the territory and happens to be nearer the chosen nest-site of the invader, the invader will probably carry the day, for the full powers of resistance are only stimulated when the nest-site and the immediately adjoining area vital to the food supply of the young are at stake. On such prov ocation a bird appears to become almost invincible against mem bers of its own species. This accounts for the fact that while out lying parts of territories are often lost, and even whole territories change hands before breeding begins, it seems exceedingly rare for a pair with a nest and eggs to be deprived of their domain by birds of the same species, however severe the pressure. Thus the size of the territory may be reduced by competition to the minimum essential to the safety of the brood, but it cannot be re duced indefinitely. That such psychological reactions are in some way linked with the question of food supply seems clear from the fact that a sudden extraordinary local concentration of prey like a swarm of locusts, a plague of mice, or a rise of may-fly, will at once cause a rush to the spot on the part of species interested and a complete local breakdown of any territorial system that has previously been in force. Immediately it becomes more profitable to fetch supplies for the brood at a distance than to collect them on the home domain the most exclusive species will temporarily change their mode of life. It becomes both impossible and unnec essary for those in possession of the scene of plenty to expel the host of trespassers attracted by it, and on the other hand the foragers have no more concern in maintaining the integrity of their own territories when there is better hunting outside them.
The Adjustment of Territory Under Economic Pres sure.—Territory has therefore to be understood not as a rigid system of partition into small areas each occupied by a jealous pair, but as a delicately adjusted balance created and maintained by economic pressure and varying greatly in its details from time to time and from place to place according as the pressure varies. For the sake of clarity the foregoing sketch has had to be given a rather simpler and more clearly defined shape than is often found in practice; it is merely intended to illustrate some of the principles upon which territory is based, and must not be taken as immutable law or exhaustive treatment. There is, for ex ample, a considerable proportion of non-breeding birds, fluctuat ing from season to season, and generally largest in the case of the larger species, which are slow in coming to maturity. In the case of such migrants as many storks, terns and gulls, immature birds often remain throughout the breeding season in the winter quar ters of the species, while others accompany the adults. But apart from immaturity it is undoubtedly common for fully grown speci mens quite capable of breeding to fail to do so. Some of these remain in flocks or parties, wandering about over ground where no territory is held by pairs of their own species. A large number of males take up territories, but are so late in doing it, or so unfa vourably placed, that they fail to attract a mate, and often desert the territory in search of one. It is from this class that solitary birds noticed in towns, or away from their natural environment, in the height of the breeding season are frequently drawn; it is also, among songsters, responsible for an utterly disproportionate share of the bird-music of that period. The hens, with no obliga tion to tie themselves down to a territory, wander much more freely, and both sexes seem always ready to fill at a moment's notice any vacancies occurring through misadventure among the mated birds. While satisfactory data respecting the ratio of non breeding to breeding stock have yet to be secured this is undoubt edly an important factor, amounting in ascertained cases to as much as 4o per cent.
That different species compete with one another for food is clear, but except with regard to nesting sites the frequency and manner in which this rivalry finds vent are still uncertain. The raven (Corvus corer) and the peregrine falcon or duck-hawk (Falco peregrinus) are almost world-wide forms which often hold overlapping territories and come into collision with one another freely. But there are probably few species which coincide in range and have a diet and breeding habits so nearly identical that rivalry between the two is as severe as between members of a single race. No doubt rivalry is more often potential than effec tive ; it is not in practice a question of "taking the bread out of one another's mouths," but simply of a situation from which such a crisis would rapidly develop if the existing checks were removed. In averting such a state of famine, enemies are of the greatest biological value to a species. The dominant forms on a given area will therefore comprise a community whose activities are partly complementary, partly competitive. The study of such interrelations is the subject of Ecology (q.v.). There is however one aspect to be dealt with here—that of bird population.
