Territory in Bird Life

by Henry Eliot Howard

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 astonishing 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, Grasshopper-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 assuredly there were, inhabiting the locality; that they had also resorted, which assuredly 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 promptitude? 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, circumstances. 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 astonishingly 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 cumulative 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 illustration 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 ownership 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 quantity of food which she hurriedly distributes and immediately settles down to brood; and if forcibly prevented from returning, her attitude 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 principal 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 sex to discover the other with reasonable promptitude.