We provide a biologically motivated technique for modeling and animating bird flocks in flight, which produces plausible and realistic-looking flock animations. While most previous approaches have focused on animating cluster formations, this article introduces a technique for animating flocks that fly in certain patterns, so-called line formations. We distinguish between the behavior of such flocks during initiation and their behavior during steady flight. Our simulation of the initiation stage is rule-based and incorporates an artificial bird model. Our simulation of the steady-flight stage combines a data-driven approach and an energy-savings model.