  Behavioral contagion and the presence of behavioral cascades are natural
features in groups of animals showing collective motion, such as schooling fish
or grazing herbivores. Here we study empirical behavioral cascades observed in
fish schools defined as avalanches of consecutive large changes in the heading
direction of the trajectory of fish. In terms of a minimum turning angle
introduced to define a large change, avalanches are characterized by
distributions of size and duration showing scale-free signatures, reminiscent
of self-organized critical behavior. We observe that avalanches are generally
triggered by a small number of fish, which act as effective leaders that induce
large rearrangements of the group's trajectory. This observation motivates the
proposal of a simple model, based in the classical Vicsek model of collective
motion, in which a given individual acts as a leader subject to random heading
reorientations. The model reproduces qualitatively the empirical avalanche
behavior observed in real schools, and hints towards a connection between
effective leadership and avalanche behavior in collective movement.
