Acorn Woodpeckers form complex family groups and are cooperative breeders, meaning that offspring may be raised by their parents and helpers who are often older siblings. These social birds also gather and store acorns in large collective granaries which require ongoing maintenance and defense.
This leads to intricate social relationships where helpers and breeders both cooperate and compete. Research has shown that an Acorn Woodpecker helper may benefit from its altruism by inheriting the family territory or enhancing its own inclusive fitness (passing on genes through survival of close relatives). This may explain why siblings form coalitions to compete against others in power struggles for reproductive opportunities.
Check out this great video of Acorn Woodpeckers stocking and guarding their granary, and this California Haikubox recording of a vocalizing Acorn Woodpecker.