Modeling collective motion: variations on the Vicsek model H. Chaté, F. Ginelli, G. Grégoire, F. Peruani and F. Raynaud Eur. Phys. J. B, 64 3-4 (2008) 451-456 Published online: 11 July 2008 DOI: 10.1140/epjb/e2008-00275-9