Reproduction
Mating systems of Acorn Woodpeckers range from monogamy in some populations to cooperative polygyny. Generally, Acorn Woodpecker groups contain 1-7 male breeders that compete for matings with 1-3 egg-laying females. In groups that contain more than one female breeder, the female cobreeders lay their eggs in the same nest cavity. There is often extreme reproductive competition between joint-nesting females, who regularly destroy eggs laid by their cobreeders. After females have established a normal laying sequence, egg destruction stops. Reproduction competition between males is displayed by attempts by a male to disrupt copulation between another pair. Courtship and pair-bonding displays are absent.
Nest cavities are drilled into large dead or living limbs in trees or snags, which may contain granaries. The inside of the nest cavity is lined with fresh wood chips, and nest holes may be used repeatedly for several seasons. Average clutch size for a group with more than one female is five white, elliptical eggs. The average clutch size for a singleton female is four eggs. Eggs are laid at approximately 24 hour intervals. The incubation period is 11 days and both male and female breeders incubate. Once the chicks have hatched, all group members participate in providing food. Nestlings leave the nest after 30-32 days.
