Birds
Acorn Woodpecker
Melanerpes formicivorus · also called acorn woodpecker
The acorn woodpecker is a clown-faced bird of western oak woodland with two remarkable habits. It builds granaries — drilling thousands, sometimes tens of thousands, of small holes into a single tree and jamming an acorn into every one, a store the family guards and maintains for generations. And it lives in cooperative groups where several adults breed together in one nest.
Overview
Acorn woodpeckers live in oak and pine-oak woodland from Oregon and California through Mexico to the northern Andes. They are about 21 cm long, black and white with a red cap and a pale, staring eye. They eat stored acorns, plus insects caught in flight, tree sap, and fruit.
The granary
A group drills a chosen granary tree — or a snag, pole, or wooden building — full of small holes, sometimes tens of thousands, and wedges one acorn tightly into each. As acorns dry and shrink, the birds move each one to a snugger hole to stop it being pushed out or stolen. A granary is used and added to across many bird generations.
Cooperative and joint breeding
Groups of up to about fifteen birds hold a territory, and young often stay for years to help raise siblings. Several co-breeding males and females may lay in one nest cavity; early on, the females destroy one another’s eggs until they synchronise laying, after which they cooperate on the mixed brood.
Status
The acorn woodpecker is Least Concern, common and stable, but tightly tied to oak woodland, so long-term loss of oaks is the main concern. Its fortunes rise and fall with the size of each autumn’s acorn crop.
Acorn Woodpecker: common questions
Why does the acorn woodpecker store acorns in holes instead of burying them?
Storing food in tree holes suits a bird that lives in one place year-round and defends a territory. A buried cache can be dug up by squirrels, jays, and other animals, and it is hard to guard, whereas a granary tree studded with acorns is right in the middle of the group's territory where the birds can watch it and drive off thieves. The tight fit of each acorn in its own hole also makes pilfering slow and awkward. The trade-off is the enormous amount of work involved in drilling the holes and maintaining the store, which is only worthwhile because the group is permanent and the granary lasts for decades.
How many acorns can a granary hold?
The largest granaries hold staggering numbers. Individual granary trees have been found with more than fifty thousand holes, and while not all are filled at once, a big, well-stocked granary can contain tens of thousands of acorns. Because the holes are reused and added to year after year by successive generations of the same family group, a granary represents the accumulated labour of many birds over a very long time, and losing one, for example when a granary tree falls or a pole is replaced, is a serious blow to the group that depends on it.
What is joint nesting, and why do the females destroy each other's eggs?
In many acorn woodpecker groups there is more than one breeding female and more than one breeding male, and the females lay their eggs in the same nest cavity and then share incubation and chick-rearing. The problem is that a female gains by having her own eggs make up more of the clutch, so early in the laying period each female removes and often eats eggs laid by the others. This destructive phase ends once the females synchronise and begin laying on the same day, at which point it is no longer possible to tell whose egg is whose, and the birds switch to full cooperation on the resulting mixed brood.
Do acorn woodpeckers damage buildings?
They can. A bird looking for granary sites does not distinguish between a dead tree and the wooden siding, eaves, fascia, or utility poles of a house, and a group can drill hundreds or thousands of storage holes into a building over time, sometimes packing them with acorns. The damage is cosmetic to structural rather than a health hazard, but it can be costly. Deterrents such as visual scares, netting, and filling and covering existing holes have mixed success, and because the birds are protected, dealing with a determined group usually means making the wood unattractive rather than removing the birds.