The Granaries of Acorn Woodpecker

Woodpeckers are fascinating creatures. They hammer their bills into wood with force so ferocious that it would lead to concussion in any animal. But woodpeckers are equipped with excellent natural shock absorbers that protect their brains against damage caused by rapid and repeated powerful blows, such as a tightly packed brain that prevents it from sloshing around the skull, which itself is composed of compressible sponge-like bone to absorb the energy of the impact, as well as an elongated tongue-bone that wraps around the skull cushioning the brain as it deaccelerates at up to a thousand g. Most woodpecker species peck at tree trunks to build nests, except the acorn woodpecker, that drill holes to store food.

Acorn Woodpecker granaries

An acorn woodpecker hoarding acorns for winter on a utility pole. Photo: Ingrid Taylar/Flickr

As their name implies, the acorn woodpecker’s favorite food is acorn, which is the nut produced by oak trees. When winter approaches, the acorn woodpecker will start hoarding acorns in granaries created out of tree trunks, or anything made out of wood, such as telephone poles and even houses. The acorn woodpecker will drill thousands of holes into the thick bark of living or dead trees and push acorns into them. The holes are made just large enough for a single acorn, and they aren’t pushed too deep into the trunk so that their retrieval is easier, although it isn’t unusual for an woodpecker to put acorns into places where it cannot get them out. In one instance, more than 200 kg of acorns were found in a wooden water tank in Arizona. Acorn woodpeckers will also utilize natural holes and cracks in the bark to store acorns.

Because the acorns are stored in shallow holes, they are visible from the outside, which attracts preys and thus granaries have to be defended. Large granary trees are often defended by several groups of woodpeckers, for they contain enough food (in some cases, up to 50,000 acorns in a single tree) to feed entire families during lean periods. Sometimes the same granary tree is reused over generations to store the winter food supply.

Granaries also require constant maintenance. As acorns dry out and shrink they are moved to smaller holes and the vacated ones are filled by new stock. This is required otherwise the dried out acorns may fall out of their now larger holes. The acorn woodpecker is the only bird to have a centralized food store that is defended communally.

The acorn woodpecker lives mostly in Oregon, California, and the southwestern United States, in the coastal areas and foothills where it is heavily forested with oaks.

Acorn Woodpecker granaries