Each year the different pairs have been more or less successful in rearing their young; each year the young can be seen accompanying their parents up to the time when the sexual instinct arises; and yet the actual number of pairs is on the whole remarkably constant, and there is no perceptible increase. It seems as if the numbers of three and two respectively were the maximum the headland could maintain. But this is no exceptional case; it represents fairly the conditions which obtain as a rule amongst those species, granting, of course, a certain amount of variation in the size of each territory determined by the exigencies of diverse circumstances.
If we take a given district, and devote our attention to the smaller migrants that visit Western Europe each returning spring for the purpose of procreation, we shall find that the movements of the males are subject to a very definite routine. This, however, is not true of every male; some may be wending their way to breeding grounds at a distance; others may be seeking the particular environment to which they may be adapted; others again, having found their old haunts destroyed, may consequently be seeking new.
Of all this there is evidence. Small parties of Chiffchaffs pass through a district on their way to other breeding grounds, flitting from hedge to hedge as they move in a definite direction with apparently a definite purpose; Reed- Warblers settle in a garden or plantation,
