The constant variation of the shape and internal structure of huge flocks of starlings (Sturnus vulgaris) when they circle above their sleeping site at dawn has amazed observers for ages. Variation of shape occurs also in the flocks of other species of birds, more so than in fish schools (fish schools are usually oblong). The causes of this variability, however, are mostly unknown up till the present time. We investigate these causes with the help of a computer model that generates a travelling group by self-organisation. In the present paper, we use a model, called StarDisplay, the flocking patterns of which resemble both qualitatively and quantitatively those of real birds (in particular starlings). In this model, individuals coordinate with neighbours through attraction, alignment and avoidance of collision (just as in models of fish schools). This model is supplemented with a few specifics of starling behaviour, namely 1) simplified aerodynamics of their flying movement, 2) a low and constant number of interaction-partners and 3) preferential movement above a ‘sleeping area’.
As to shape, we measure the relative proportions of the flock and the longest dimension in respect of its movement. We show that variation of flock shape is due to two main causes, namely a) the aerodynamics of flying, b) the subgroup formation in a flock. Subgroup formation arises from large size of the flock, low number of interacting partners, and heterogeneity of the environment. We explain how this comes about and thus construct hypotheses that can be tested empirically.