Some birds cross whole continents on their way to the breeding grounds, others travel many miles, others again find suitable accommodation in a neighbouring parish--nearly all have a journey to perform, it may be short or it may be long. The flocks of Finches gradually decrease and we observe the males scattering in different directions in search of territories; we watch the summer migrants on their way--small parties halting for a few hours in the hedgerows and then continuing their journey, single individuals alighting on trees and bushes and resting there for a few minutes, and the constant passage of flocks of various dimensions at various altitudes; and we see Fieldfares, Redwings, and Bramblings slowly making their way from the south and the west to their homes in the far north. Occasionally we hear their song, not the emotional outburst customary at this season, but, except in isolated cases, a weak and tentative performance. Gätke speaks of the absence of song on the Island of Heligoland, and refers to the Whitethroat as one of the few migrants that enliven that desolate rock with their melody. On the other hand, many migrants that rest temporarily on the Isle of May sing vigorously.[5] But on the whole there is, I think, no question that the male whilst travelling to its breeding grounds, and, even after its arrival, whilst in search of a territory, sings but little--and that little lacks the persistency characteristic of the period of sexual activity. Yet, when a suitable territory is eventually secured, the nature of the bird seems to change; for, instead of being silent and retiring, as if aware of some end not fully attained, it not only makes itself conspicuous but advertises its presence by a song uttered with such perseverance as to suggest that that end is at length attained. Hence, in a general way, the instinct of song seems to be related to the establishment of a territory.
Now the subsequent course of behaviour tends to confirm this view. We have already had occasion to refer to the fact that the males of some species desert their territories temporarily and join together on ground which is regarded by the birds that associate there as neutral, and that they do so not merely for the purpose of securing food but because they derive some special pleasure from the act of association, and we shall find that the altered behaviour of the male when it leaves its territory to seek food or to join the flock is an important point for us just now.