Group(s) of birds assembled together. Two basic factors cause birds to aggregate: 1) congregation into groups due to social behaviour (flocks) 2) groupings as a result of ecological factors alone (food, shelter, etc.). There are different types of flocks: bad-weather flocks (as a reaction to climatic conditions); they often migrate away from deteriorating conditions. Seasonal flocks: especially in the temperate zone passerine species form flocks in winter following the breeding season in which pairs occur dispersed. Permanent flocks: some species occur in flocks throughout the year (e.g., many seabirds, finches, weavers); they are so-called gregarious species. Flocks may consist of a single species (monospecific) but also mixed flocks occur. In the latter case these are usually species with different feeding habits (mixed foraging party). "Gaggle" refers to a flock of geese.

Alternative forms for flock : flocks, gaggle.