  The nature of the phase transition in a system of self-propelling particles
has been extensively studied during the last few decades. A theoretical model
was proposed by T. Vicsek, {\it et. al.} [Phys. Rev. Lett. {\bf 75}, 1226
(1995)] with a simple rule for updating the direction of motion of each
particle. Based on the Vicsek's model (VM) [1], in this work, we consider a
group of animals as particles moving freely on a two-dimensional space. Due to
the fact that the viewable area of animals depends on the species, we consider
the motion of each individual within an angle $\varphi=\Phi/2$ ($\Phi$ is
called angle of view) of a circle centered at its position, of radius $R$. We
obtained a phase diagram in the space ($\varphi$, $\eta_c$) with $\eta_c$ being
the critical noise. We show that, the phase transition exists only in the case
of a wide view's angle $\varphi \geq 0.5\pi$. The flocking of animals is an
universal behavior of the species of prey, but not the one of the predator. Our
simulation results are in good agreement with experimental observation [2].
