After 10+ tribes, I've decided it's just dumb luck. I've completed the collections in 4 or 5 villages without even trying. One especially lucky village had all of the feathers, all the shells and all but one rare coral before the first babe-in-arms was 5 years old. As soon as I started clearing the rubble, I had pieces appear every few minutes and was done all collections before I started the statue or sewing hut.

Other tribes I have run the kids ragged collecting, used the chief to help point out rares, and I still can't get that one last feather or shell.