The movement of animal flocks give us one of the clearest examples of the concept of 'complexity'. Simple interactions between animals lead to patterns that are somehow regular but at the same time difficult to characterise. In this paper we discuss first these models and their predictions about the movement of flocks. We provide novel results about how information is transfered in these systems. We look at recent experiments on locusts and pigeons which show that at least some of the patterns seen in these groups can be explained by the phase transitions and bifurcations that arise from these models. In particular, we look at a phase transition in the marching of locusts and symmetry breaking in the decision-making of pigeons.