Tsuru no Sugomori

An earnest tale of animal cooperation, as two out-of-place chickens try to make their home on an island inhabited by cranes. When they are attacked by an eagle, their adoptive parent cranes are killed, but the chickens organize a counterstrike. Eventually, peace is restored to the island. Based on a story by Teru Takakura, this anime was made over a three-year period by around 80 members of the Japan Film Broadcasting Industry Association under the pseudonymous lead of Shibayama.

(Source: The Anime Encyclopedia)