Each acorn is hammered into its own hole, snugly enough that squirrels and jays struggle to prise it out. As acorns dry and shrink the birds move them to tighter holes, a maintenance task that occupies much of the autumn and winter. Granary trees with more than 50,000 holes have been recorded.
The granary anchors an unusual social system. Groups typically hold several breeding males and one or more breeding females that lay into a shared nest, alongside non-breeding helpers from previous broods. Females competing for the joint nest have been observed removing each other's eggs until laying synchronises.
A granary represents years of accumulated labour, so the tree itself becomes the contested resource; power struggles after a breeder dies can draw in birds from surrounding territories. What is not settled is how much the stored acorns actually contribute to overwinter survival relative to other foods, which include sap, insects and flycatching.