Flocking for food or flockmates?

Animals in groups behave cohesively, even when those animals are domesticated and are housed in limited environments. But how is such group cohesion maintained? Do animals move in an independent manner, according to their own motivations, or in a social manner, with respect to the movements of others? Here, we use a mathematical model to consider the contributions of social and independent factors in determining group cohesion in clustering hens. The model was based on observed data of singly housed laying hens with additional social attraction factors imposed. We examined the resulting group-cohesiveness when agents were homogenous or heterogeneous in: (i) their attraction towards group members (flockmates) and (ii) their attraction towards resources (food). Provided member agents had similar motivations, groups could maintain high levels of clustering when social attraction was weak. In modelled leader-follower groups, the likelihood of dispersal increased as motivational differences between members increased. Model-fit was tested against clustering observed in twelve replicates of small groups of laying hens housed in the same environment as the singly housed hens used to construct the model. Overall hen clustering most closely matched models of distributed leadership, where all group member agents were independently resource-motivated for the same resource: so hens flocked for food and not for flockmates. In space- or resource-limited environments many of the benefits of group living could be achieved through a combination of conspecific-tolerance and shared motivations, without need for leadership or consensus decision-making.