When the Curlew flies to that part of the mud-flat which is first exposed by the receding tide, and there associates with others, it does not then do so because it has any interest in its fellows, nor because they serve as an attraction, but because it is constrained by hunger--in other words, the association is determined by accident. But when, during periods of repose, it sees a flock, flies to it, and takes up a position in the midst of it, it does so not because suitable accommodation is lacking--not therefore because of external constraint--but because it derives some pleasure from satisfying something in its organic complex. We speak of this behaviour and of the emotion which characterises it as the _gregarious instinct_: by which we mean that the inherited nature of the Curlew, as a tribe, is so constituted that, given the appropriate internal conditions and adequate external stimulation, every individual will respond in a similar manner--that is, the behaviour is primarily determined by racial preparation. This is what we mean by the _gregarious instinct_ biologically considered. We may resolve our own experience in relation to the crowd into its simplest constituents, project our own primitive feelings into the Curlew, and say that the bird feels uneasiness in isolation and satisfaction in being one of the flock. But in truth we know nothing, save by analogy, of the correlated psychical state. All the knowledge we possess is derived from a study of the objective aspect of the behaviour, which in simple terms may be expressed thus: the individual is drawn towards its companions; there is a relation between the size of the flock and the strength of the attraction; and all Curlew behave similarly under similar circumstances.
This instinct controls the movements of many birds from early autumn to the commencement of the breeding season. And so powerful is the control that the individual is suppressed and its activities subordinated to the welfare of the community as a whole. Flocks of Waders roam about the tidal estuaries in search of food, and different kinds of Gulls assemble there and preen their feathers or sleep; Warblers alter their mode of life, and in the osier bed, or amongst the elders, seek their food together in peace; Finches, Buntings, Pipits, and Wagtails, though food is everywhere abundant, gather themselves together respectively into bands which, as winter approaches, grow into flocks and even into composite flocks; and as the Warblers leave for the south, so their places are filled by flocks of Thrushes and Finches from the north. In whatever direction we turn, when the days begin to shorten, it is the community, not the individual, that thrusts itself upon our attention; and throughout the winter continues to be the outstanding feature of bird life.