Flocks of birds and how they seem to move together have always fascinated any observer of them. New research from the Universities of Exeter and Cambridge reveals for the first time that, contrary to current models used to explain the movement of flocks, the differences between bird species and social relationships between individuals play a critical role in determining the dynamics of mixed-species flocks. The unified behavior of bird flocks has puzzled scientists for hundreds of years. One naturalist from the turn of the century even suggested telepathy may be involved. There have since been less esoteric explanations, including mathematical models that show that repeated interactions among individuals following simple rules can generate coordinated group movements. However, these models usually rely on the assumption that individuals within groups are identical and interact independently, which may not reflect reality.