Birds on a wire: empirical studies of flocking dynamics in linear aggregates

The dynamics of avian flocking is difficult to study because of the transient, highly mobile and unpredictable nature of bird flocks. However, birds often form clusters when perched on wires and fences, offering a stable platform for studying the formation, internal structure and dynamics of these approximately one-dimensional flocks. Previous studies have examined the distribution of the distances between birds perched on wires, but not the time-dependence of this behavior. We present results from a video study of cliff swallows (\textit{Petrochelidon pyrrhonota}) perched on power lines in Sierra Valley, California. Phenomena studied include how the interbird distance distribution varies as a function of time, what factors influence the addition of further birds to an existing cluster, how the distribution evolves in response to perturbations, and typical timescales for the observed behaviors.