How do large shoals of fish and flocks of birds change direction very quickly without colliding with each other? We humans have the greatest difficulty achieving even two-dimensional travel without walking into each other, yet mass formations of fish and birds seem able to manage it in three dimensions. I have seen that very young fish are capable of this feat, so it would not appear to be a trait that is learned. How do these animals communicate the order to change direction and also the new direction that they are going to take?Ray Wijewardena, Colombo, Sri Lanka