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Bird Species Spotlight: Acorn Woodpecker

by Jake Jacoby | October 11, 2016

Copyright Jake JacobyLooking like wide-eyed clowns, acorn woodpeckers live in large groups in oak woodlands. I saw my first one during a recent trip to Costa Rica in February this year. Their social lives are endlessly fascinating as they store literally thousands of acorns each year by jamming them into specially made holes in trees. One bird of the group is always on alert to guard the hoard from thieves, while others race through the trees giving parrot-like calls. Their breeding behavior is equally complicated with multiple males and females combining efforts to raise young in a single nest.

Acorn woodpecker closeup © Jake Jacoby

Acorn woodpeckers also have a very complicated social system. Family groups hold and defend 15-acre territories, and young woodpeckers stay with their parents for several years and help the parents raise more young. Several different individuals of each sex may breed within one family, with up to seven breeding males and three breeding females in one group.

Acorn woodpecker in tree © Jake Jacoby

All members of an acorn woodpecker group spend large amounts of time storing acorns. Acorns typically are stored in holes drilled into a single tree, called a “granary tree.” One granary tree may have up to 50,000 holes in it, each of which is filled with an acorn in autumn. Also, the acorn woodpecker is not above using structures made by humans to store acorns, drilling holes in fenceposts, utility poles, wood-sided buildings, and any other available wooden structure.

Acorn woodpecker on tree stump © Jake Jacoby

Acorn woodpeckers drill holes primarily in the winter, in the thick bark of dead limbs where the drilling does no harm to a living tree. Each year they reuse old holes and add some new ones. The acorns are wedged so tightly in their holes that they’re very difficult for other animals to remove. After they’ve been stored for a while, the fit becomes looser as the acorn dries out; group members will periodically check their stored acorns and move the loose ones to smaller holes. In addition to the acorns, the woodpeckers will feed on insects (particularly ants) and fruits and seeds. They will also feed on sap, digging pits in bark or visiting those previously made by sapsuckers.

In groups with more than one breeding female, the females put their eggs into a single nest cavity. A female will usually destroy any eggs in the nest before she starts to lay, and more than one-third of all eggs laid in joint nests are destroyed. Once all the females start to lay, they stop removing eggs. The woodpeckers will usually excavate multiple cavities, any one of which may be used for nesting (the rest are used for nocturnal roosting). The nesting cavity is usually about 6 inches in diameter and up to 2 feet deep which may be reused for many years. Acorn woodpeckers do not build a nest within the cavity, but during the digging process a layer of fresh wood chips accumulate on the bottom. They replenish the chips throughout the nesting period by pecking away at the cavity walls.

Acorn woodpecker on tree branch © Jake Jacoby

In the beginning, only breeding females incubate the eggs, but later other group members also incubate. Incubations lasts about 11 days and all group members help brood and feed the young. The young leave the nest after about 30 days, although fledglings return to the nest to roost and feed. Acorn woodpeckers will normally raise one or two broods each year.

Why do birds have different colored eyes?

Eye color isn’t tied to one group of birds or another, but a common pattern is a change in eye color as immature birds grow to adulthood. Bald eagles, ring-billed gulls, and ducks such as goldeneyes and scaups have brown eyes as youngsters, and yellow eyes as adults. Red-tailed hawks reverse this pattern, with their eyes changing from yellow to brown. And, the yellow eyes of a juvenile Cooper’s hawk turn deep red as they reach maturity. The brown eyes of a juvenile roseate spoonbill turn deep red as well when they reach adulthood.

Not all birds’ eyes change color as the bird’s age. But for those whose eye color appears to signal adulthood, this is likely an adaptation that helps them gauge the maturity—and suitability—of potential mates.

About the Author

To see more of Jake's work as well as his favorite photographs check out his Flickr page.

One thought on “Bird Species Spotlight: Acorn Woodpecker

  1. I’ve lived with acorn woodpeckers for the past twenty years…must have a hundred saved images and 10X that discarded. They adapt well to the urbanization of oak woodland as long as some trees are left. They adapt well to human structures, storing their acorns under the shingles of my roof until it raised up enough to leak; filling the roof-top vent pipes with acorns (I now have them covered with wire mesh); and filling my 20 bluebird boxes with acorns every winter. But, as I told my neighbor, they were here long before our houses were so they’ve got first rights.

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