Some time before 1984, in my childhood, 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 who cared for the afflicted, and pursued the war against the others. I think that they had three 'genders' for lack of a better term: men, women, and eunuchs. One gender was called "merldives" I think.