Territory in Bird Life

by Henry Eliot Howard

The question then arises: Does all this warfare contribute towards the attainment of reproduction? Not far from my house there is a small water meadow, three acres in extent, which for some years has been derelict and is now overgrown with the common rush (Juncus communis) and small alder trees. For three successive seasons I watched the bird life of this meadow, and more especially the Reed-Buntings whose behaviour I was studying at the time. In every respect the meadow was suitable for this bird; there was an abundance of food and numberless situations in which nests could be placed. Each year all the pairs were successful in rearing one, if not two broods, yet the number of pairs never exceeded five—the first year there were three; the second year five; and the third year four. In addition to the four pairs of Reed-Buntings, there were in the spring of 1915, six pairs of Whitethroats, one pair of Lesser Whitethroats, four pairs of Willow-Warblers, one pair of Sedge-Warblers, two pairs of Grasshopper-Warblers, one pair of Chiffchaffs, three pairs of Hedge-Sparrows, two pairs of Tree-Pipits, one pair of Skylarks, one pair of Whinchats, one pair of Flycatchers, two pairs of Song-Thrushes, one pair of Blackbirds, one pair of Redstarts, three pairs of Chaffinches, and one pair of Wrens—in all, thirty-five pairs, whose young were mainly dependent for their living upon the insect life of that meadow and the ground immediately surrounding it. If we allow three young to each pair—and this would take no account of second broods—we arrive at the following result, namely, that one hundred and five young and seventy adults had to be supplied with food from that locality, which would mean, if the search for food were strictly limited to that meadow, that 83 square yards would be allotted to each individual.

Suppose now that the four male Reed-Buntings had each admitted one other male, and that they had secured mates, what would have been the effect upon the whole community? The four additional pairs with their young would have represented twenty individuals, which would have represented a decrease of 8.5 square yards in the space allotted to each individual. The pressure of the bird population upon the means of support would then have been materially increased; and not only the Buntings, but the Warblers, Pipits, and all the rest would have suffered. But the result would have been the same if, instead of the four additional male Reed-Buntings, four males of other kinds had been allowed to enter the marsh, and we can multiply the number four until we arrive at a point when the means of subsistence would no longer have been adequate for the adults, still less for the young. If, then, there were nothing to prevent this happening, many of the birds in that marsh would have no chance of rearing their young successfully. Hence, if the territory is adequately to serve the purpose for which we believe it has been evolved, some provision must have been included in the system to meet the difficulty.

There are three ways by which this may have been accomplished—indirectly, by increasing the size of the area occupied by each individual, and thereby reducing the relative number of each species; or directly, by rendering the fighting instinct of the bird susceptible to stimulation by individuals of other species; or, possibly, by a combination of the two. There were four pairs of Reed-Buntings in the marsh, and their territories covered the whole of it. But inasmuch as other insectivorous birds were established there also, and found sufficient food to maintain both themselves and their families, it is clear that the area these Reed-Buntings occupied was in excess of that which they would have required if they had been the sole inhabitants. And such often appears to be the case. Many a Warbler allocates to itself a space of ground more than sufficient to supply it with all that it needs; so, too, does the Finch, or the Pipit, or the Falcon—if we take no account of kindred species. Thus there is reason to believe that, by limiting the number of individuals in a given locality, this apparently wasteful expanse of territory is serviceable in that it provides against the pressure of the bird population upon the available means of support becoming too great. But though a reduction in the numerical standing of the different species would certainly follow from any increase in the area occupied by the respective individuals, and with even greater certainty would place them in a more secure position as regards their supply of food, yet, when we remember how large a number are dependent upon a supply of insect life for their young, we can understand that it would not alone be a sufficient safeguard against the dangers attendant upon overcrowding. It is here, I believe, that we shall find the true explanation of the hostility; it roughly insures that the number of pairs in any given area does not exceed the available means of support, and indeed it is difficult to imagine how such uniformity of distribution as would free the young from the risk of exposure could be obtained without some such control.

Some birds, however, have no difficulty in finding the necessary food for their young, yet have great difficulty in finding a station where they can rear their young in safety; and the area each one occupies has been reduced to the smallest proportions in order that the maximum number can be accommodated. Here, any increase in the size of the territory would inevitably lead to the extinction of the race, so that nothing stands between failure and success except the ability of the bird to defend its territory. If we study the bird population at one of the breeding stations on the coast, we find, generally speaking, that each kind of bird inhabits a particular portion of the cliff; on the lower ledges are the Guillemots and Kittiwake Gulls; higher up are Razorbills and Fulmars, and at the top, where the cliff is broken and the face of the rock covered with turf and soil, the Puffin finds shelter for its egg. At the same time there is much overlapping; the kind of ledge that suits a Razorbill is equally suitable for a Guillemot or a Fulmar, and so, no matter how successful the Razorbill may be in establishing a territory and preventing intrusion upon it by other Razorbills, it will be all to no purpose if it allows itself to be jostled out of its position by a Fulmar. Hence, inasmuch as breeding stations are limited and competition for territory so severe, only those forms in which the fighting instinct responds freely to a wide range of stimuli will be in a position to maintain a footing upon the cliff.

In trying to estimate the importance of the hostility in its relation to the territory, we must bear in mind that competition varies in different seasons and in different localities. The surface of the land is constantly undergoing modification, partly owing to human and partly to physical agency—forests are cleared; marshes are drained; the face of the sea-cliffs is altered by the erosion of the waves; here the coast may be locally elevated, there locally depressed; and so forth. Many of these changes are slow and imperceptible, many can be observed in our own lifetime. The timber is felled and the undergrowth cleared in some wood, and the following spring we notice a change in the character of the bird population. Migrants which formerly found in it no suitable accommodation now begin to appear, and as the seasons pass by and the undergrowth affords more and more shelter for the nests and an increasing supply of insect life, so their numbers increase until the wood becomes an important breeding station, resonant with the song of many individuals. But slowly the growth increases; the bushes pass into saplings and the saplings into trees, and the undergrowth then disappears just as surely as do the migrants which can no longer find there the conditions which they require.

Or, as an illustration of the effect produced by natural agency, let me describe a change which has taken place in a corner of Co. Donegal. The promontory of Horn Head is bounded on the west by extensive sand-hills, 100 ft. or more in height. On the southern side it is divided from the mainland by a channel, which narrows down to 100 yards or so in width where it fringes the sand-hills, and then widens out again, covering an area of approximately 270 acres. As far as is known in the memory of man, this area has always been tidal. But in recent years a change has taken place, and the blown sand has silted up the channel, with the result that this tidal area has been transformed into a brackish lake. What has brought about the change is not easy to determine. There is evidence, however, of a slow alteration of the level of the shore-line; for in the midst of the sand-hills, situated 150 yards or so from the present sea-margin, and running parallel with it, there is an accumulation of pebbles some 3 feet high by 4 feet deep. This raised beach is now separated from the Atlantic by sand-drifts of considerable height, and consequently there are some grounds for believing that secular elevation is taking place, which, if it be the case, will account for the change in progress. Now the effect on the bird population can be seen even now, and will doubtless become more apparent as the years pass by. Sand-Martins used to find plenty of places to breed amongst the sand-drifts, and moreover do so still. But their nesting sites are constantly changing and disappearing, and the breeding-place of one colony, that was situated in the bank of a stream twelve years ago, is now buried 10 feet or more below the surface of the sand. The area that was once tidal, but is now a brackish lake, is fed by mountain streams, and as the fresh water predominates, so in course of time will it become fringed with vegetation; and instead of the flocks of Curlew, Dunlin, and other waders that, at low water, resorted there to feed, Coots will fight with one another for the possession of territories, and the Wild Duck will teach her young to seek their food.

In whatever direction we turn, we find that many breeding grounds are subject to incessant change. Ancient haunts disappear, new ones come into being, a change which makes life impossible for this bird, as likely as not benefits that one, and so on. There is no stability. Hence in any given district each recurring season there must needs be a large number of individuals which are obliged to seek new stations, and if there were no control over their distribution, if each one were free to establish itself wherever it chanced to alight, this locality might be overcrowded and that one deserted; and, bearing in mind how many species there are that require similar conditions of existence, we can infer that the successful attainment of reproduction would become impossible for many of those individuals so long as each species was indifferent to the presence of the others. On the other hand, if there were no control over the range of the intolerance, the smaller bird would have no chance in competition with the larger, and it is doubtful whether the larger would gain an advantage commensurate with the energy it would expend in ridding its area of the smaller. I have described battles in which the opponents were only distantly related; for instance, the Moor-Hen will attack almost any bird—Partridge, Lapwing, or Starling—that approaches its territory even temporarily. Nevertheless the antagonism between kindred forms is more prevalent, and, as a rule, characterised by more persistent effort; and thus it seems as if the susceptibility of the fighting instinct has its limitations, the degree of the responsiveness being dependent upon the affinity of the opponents.

Suppose now that we take an area inhabited by a number of different species requiring like conditions of existence, divide it into three sections, and imagine that in one they were all sociable, that in another they were all hostile, and that in a third those which were closely related were intolerant of one another. Let us suppose further that each one of them was represented by the full number of individuals that the law of territory would allow. In the first section an individual would establish itself, and, becoming intolerant of its own kind, would exercise dominion over an area roughly sufficient, providing conditions were normal, to insure an adequate supply of food for its young. But it would take no account of other species, and since any number might occupy the same ground, the fact of its having established a territory would not alone suffice to render its supply of food secure. Success in the attainment of reproduction would then become largely a matter of chance, depending upon the number of individuals that happened to settle in this place or in that. In the second section there would be perpetual warfare; for whereas the appropriate organic condition which leads to pairing arises in different species at different times, fresh claimants to occupied ground would constantly be appearing, and the efforts of the inhabitants to preserve their boundaries intact would have to be maintained throughout the whole period of reproduction; and while the stronger or more persistent forms would be more likely to breed, they would do so at the expense of their young, to which they would be unable to devote proper attention, and with an expenditure of energy that would reflect itself upon the future of the race. But the conditions of life in the third section would be such as would be more likely to yield good results. The relations of the different members of the community would be more evenly balanced, for a male would only be called upon to compete with those of its own size and strength. Thus, on the one hand, accommodation would be so divided as to secure the breeding of the maximum number of individuals with the minimum of expenditure of energy, whilst on the other, any undue pressure upon the available means of subsistence would be prevented.

There can be no question that in the latter section a higher percentage of individuals would succeed in rearing offspring. And so, by reason of the fighting instinct being more susceptible or less susceptible according to the affinity of the opponents, a control is established which, while preventing unnecessary extension of warfare, allows for sufficient extension to render the biological end secure.

These, then, are the facts—this the conclusion which can be drawn from them. It may, however, be said of these facts, as it has been said, with even less justification, of the battles between individuals of the same species, that they do not afford evidence of genuine hostility. No doubt there are many naturalists who could supplement these facts with others in which the conflicts resulted in bodily injury, or terminated fatally, or at least were of a more determined kind. But I have already drawn attention to the fact that, so long as a definite result is attained, the severity of the struggle and the amount of injury inflicted are matters of small moment. Let us, however, run over the substance of the argument, and then briefly refer again to this point of view.