Continuum modeling of the equilibrium and stability of animal flocks

► We use a continuum model for an animal flock to explore equilibria and stability. ► We find finite flocks for pressures P(ρ)=cργ, when γ>1. ► Flocks with γ>2 support a constant density for large flock number. ► Flocks with 1<γ≤2 have a inner density that grows with flock number. ► Planar flocks are unstable to long-wavelength perturbations along their surface.