Fig. 2

A hypothetical example of abundances of several bird species across a gradient, when (a) their distributions are independent of each other and (b) their distributions coincide. In (b) if the species that coincide are all flocking species, it suggests an effect of the flock system or a flock leader. However, mechanistic studies are required to demonstrate that the species interact and that this is not simply the result of habitat filtering. Figure modified from Terborgh (1971)