Years ago, I read a science fiction story about a crew that was assigned a peacekeeping mission to a planet whose inhabitants were at war. Apparently, they were killing each other over the rights to a migratory bird. This bird would migrate from one hemisphere to another with the seasons, and the inhabitants of the hemisphere where the birds were not located would all go into psychosis until the birds returned.
However, a segment of the population was not affected by the birds' disappearance, and it was this portion that cared for the afflicted, and pursued the war against the others.
Anyone know the story? I would like to find it again.