BRITISH WARBLERS
Why should males be so strongly impelled to reach their breeding grounds? An answer to this question is, I believe, to be found in the necessity for occupying a breeding territory. Each male, that is to say, must secure a position wherein the rearing of its offspring can be safely accomplished, and this is the first and most important step towards reproduction. I must explain what is meant by a "breeding territory." Let us suppose that we watch a male of one of our common species—a Reed Bunting for instance—and accurately record its movements each day during the first few hours of daylight from about the second week in February, what do we find? Not that its behaviour is of a casual description, not that it is here to-day and gone to-morrow, but that all its movements are subject to a routine which becomes increasingly definite as the season advances. There it is in the same plot of ground, in the same bush or clump of bushes, taking short flights first in one direction and then in another, attacking other males that come within a certain radius and exhibiting in all its actions a strong disposition to make its own just that one particular corner of the universe. Those few acres wherein it performs this routine of activities and awaits a female I have termed a breeding territory, the dimensions of which vary, according to the species and according to the environment, from the few square miles of the larger birds of prey, to the few square inches on a ledge of rock which is all that the Guillemot requires. I have, perhaps, in the earlier parts of this work, expressed myself unguardedly when speaking of boundaries, and the critic is perfectly justified in taking me to task for seeming to imply a definiteness which does not really exist. In the history of the Marsh Warbler I therefore tried to make it clear that the word should be interpreted somewhat liberally. The conception of a boundary must not be that of a line definitely delimiting an area the exact extent of which the bird is cognisant of, but of the normal extent of range which has become habitual and has, I suppose, been
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