In this paper, we explore a behavioral animation of artificial birds that have lived by doing an aggregate motion. We first model individual birds and then propose a behavioral model for an aggregate motion of a flock of birds. In order to represent realistically collision avoidance and flock centering among birds, which are necessary properties in a flock of birds, we consider motive of a flock of birds, and role, velocity, momentum, banking and internal characteristics of each bird. The paper presents the simulation result of the proposed model for a flock of 100 birds.