Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/46

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then there would be nothing to prevent any number of the same species congregating within a very restricted area. Consequently the food supply, which might have been ample for the progeny of one pair of each of those species, would become exhausted in the immediate vicinity of the nest. The parent birds, therefore, instead of seeking and finding food close at hand, would be compelled to travel further and further afield, and thus the quick supply of food, so essential to the welfare of the offspring, would not be forthcoming, and correspondingly the interval for brooding, which is of such importance during the early stages of growth, would be diminished, with results that could only be detrimental, and in cold wet seasons disastrous to the offspring. Apart from these general considerations, my belief that the effect of this law of territory has resulted in securing an efficient food supply for the offspring in their early stages of growth is based, in some measure, on the behaviour of certain birds, both at the commencement of the period of sexual reproduction and after the young are able to take care of themselves. If a pool, surrounded by agricultural land and occupied by a number of Moorhens, is kept under observation early in the season, it will be noticed that each morning at daybreak the different individuals will wander over the land in search of food. Now select some one particular pair, owning a territory, and notice their behaviour. Any intrusion upon their domain is resented; it is only necessary for an unpaired individual, wandering amongst the rushes, to overstep the boundary, to call forth a peculiar cry from the owner, followed by a determined attack. Presently one or two individuals or pairs fly or run out on to the fields to feed, causing a stir amongst the whole community; one bird follows another, the pair we are watching swim towards the shore, join their neighbours, and in company with them peaceably search for food. The fields are clearly regarded as neutral ground, the territory itself being probably not capable, so early in the season, of supplying the owners with food. But when the young are hatched, the conditions