  We present a general framework for modeling a wide selection of flocking
scenarios under free boundary conditions. Several variants have been considered
- including examples for the widely observed behavior of hierarchically
interacting units. The models we have simulated correspond to classes of
various realistic situations. Our primary goal was to investigate the stability
of a flock in the presence of noise. Some of our findings are counter-intuitive
in the first approximation, e.g., if the hierarchy is based purely on dominance
(an uneven contribution of the neighbors to the decision about the direction of
flight of a given individual) the flock is more prone to loose coherence due to
perturbations even when a comparison with the standard egalitarian flock is
made. Thus, we concentrated on building models based on leaderfollower
relationships. And, indeed, our findings support the concept that hierarchical
organization can be very efficient in important practical cases, especially if
the leaderfollower interactions (corresponding to an underlying directed
network of interactions) have several levels. Efficiency here is associated
with remaining stable (coherent and cohesive) even in cases when collective
motion is destroyed by random perturbations. The framework we present allows a
the study of several further complex interactions among the members of flocking
agents.
