  In real flocks, it was revealed that the angular density of nearest neighbors
shows a strong {\it anisotropic structure} of individuals by very recent
extensive field studies by Ballerini et al [{\it Proceedings of the National
Academy of Sciences USA} {\bf 105}, pp.1232-1237 (2008)]. In this paper, we
show that this empirical evidence in real flocks, namely, the structure of
anisotropy also emerges in an artificial flock simulation based on the {\it
BOIDS} by Reynolds [{\it Computer Graphics} {\bf 21}, pp.25-34 (1987)]. We
numerically find that appropriate combinations of the weights for just only
three essential factors of the BOIDS, namely, `Cohesion', `Alignment' and
`Separation' lead to a strong anisotropy in the flock. This result seems to be
highly counter-intuitive and also provides a justification of the hypothesis
that the anisotropy emerges as a result of self-organization of interacting
intelligent agents (birds for instance). To quantify the anisotropy, we
evaluate a useful statistics (a kind of {\it order parameters} in statistical
physics), that is to say, the so-called $\gamma$-value defined as an inner
product between the vector in the direction of the lowest angular density of
flocks and the vector in the direction of the moving of the flock. Our results
concerning the emergence of the anisotropy through the $\gamma$-value might
enable us to judge whether an arbitrary flock simulation seems to be {\it
realistic} or not.
