Stored acorns are an important food resource, both during the winter and for successful reproduction the following spring.
Groups can even breed in the fall when the acorn crop is particularly good.
They have one of the most bizarre mating systems of any bird in the world. All individuals within the group are close relatives except that cobreeder males are not related to joint-nesting females. Here's a nest. After checking the hole, we nail the cut piece of wood back in place. There is also a great deal of competition for reproduction within groups.