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Acorn Woodpecker

Melanerpes formicivorus ORDER: PICIFORMES FAMILY: PICIDAE

IUCN Conservation Status: Least Concern

Reminiscent of a troupe of wide-eyed clowns, Acorn Woodpeckers live in large groups in western oak woodlands. Their social lives are endlessly fascinating: they store thousands of acorns each year by jamming them into specially made holes in trees. A group member is always on alert to guard the hoard from thieves, while others race through the trees giving parrotlike waka-waka calls. Their breeding behavior is equally complicated, with multiple males and females combining efforts to raise young in a single nest.

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At a GlanceHelp

Measurements
Both Sexes
Length
7.5–9.1 in
19–23 cm
Wingspan
13.8–16.9 in
35–43 cm
Weight
2.3–3.2 oz
65–90 g
Relative Size
Smaller than a Northern Flicker; larger than a Downy Woodpecker.
Other Names
  • Pic glandivore (French)
  • Carpintero de bellota (Spanish)

Cool Facts

  • In 1923, American ornithologist William Leon Dawson called the dapper Acorn Woodpecker “our native aristocrat.” Dawson wrote: “He is unruffled by the operations of the human plebs in whatever disguise…Wigwams, haciendas, or university halls, what matter such frivolities, if only one may go calmly on with the main business of life, which is indubitably the hoarding of acorns.”
  • The Acorn Woodpecker has a very complicated social system. Family groups hold 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.
  • 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.
  • The Acorn Woodpecker will use human-made structures to store acorns, drilling holes in fenceposts, utility poles, buildings, and even automobile radiators. Occasionally the woodpecker will put acorns into places where it cannot get them out. Woodpeckers put 220 kg (485 lb) of acorns into a wooden water tank in Arizona. In parts of its range the Acorn Woodpecker does not construct a granary tree, but instead stores acorns in natural holes and cracks in bark. If the stores are eaten, the woodpecker will move to another area, even going from Arizona to Mexico to spend the winter.
  • In groups with more than one breeding female, the females put their eggs into a single nest cavity. A female usually destroys 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 oldest Acorn Woodpecker on record was at least 17 years, 3 months old. This live bird was identified in 2009 by its colored leg band, which it had been wearing since 1992.

Habitat


Open Woodland

Acorn Woodpeckers live year-round in oak and pine-oak woodlands of western Oregon, California, and the Southwest through Mexico and Central America. They also live in other habitats with oaks present or nearby, including streamside forests, Douglas-fir forests, redwood forests, tropical hardwood forests, suburban areas, and urban parks. Though found as low as sea level, they are more common in mountains, ranging up to the elevation limit of oak trees.

Food


Insects

Acorn Woodpeckers eat acorns and insects (and other arthropods). The woodpeckers harvest acorns directly from oak trees and are famous for their habit of storing nuts—primarily acorns, but also almonds, walnuts, hazelnuts, pecans, and pinyon pine nuts—in individually drilled holes in one or more storage trees. These are known as granaries and can have upwards of 50,000 nuts stored in them. The birds drill the 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 periodically check their stored acorns and move the loose ones to smaller holes. Besides converting many kinds of live and dead trees into granaries, Acorn Woodpeckers often store acorns in structures like utility poles, fenceposts, and wood-sided buildings—a practice that has brought them into conflict with more than a few protective homeowners. Despite their association with acorns, Acorn Woodpeckers prefer to catch flying insects when those are available. They hunt for ants, beetles, and other insects by flying out from high perches. They may hunt insects at any time of year, often storing them in cracks or crevices. Besides nuts and insects, Acorn Woodpeckers also eat fruit, sap, oak catkins, and flower nectar, along with occasional grass seeds, lizards, and even eggs of their own species. In the spring they gather in groups to suck sap from small, shallow holes in tree bark, often using the same sets of sap holes for several years.

Nesting

Nesting Facts
Clutch Size
3–6 eggs
Number of Broods
1-2 broods
Egg Length
0.9–1.1 in
2.3–2.7 cm
Egg Width
0.7–0.8 in
1.8–2 cm
Incubation Period
11 days
Nestling Period
30–32 days
Egg Description
White.
Condition at Hatching
Blind, featherless, and helpless.
Nest Description

The cavity is usually about 6 inches in diameter, and it may be 8 inches to more than 2 feet deep. Acorn Woodpeckers do not build a nest within the cavity, but during the digging process a layer of fresh wood chips usually accumulates on the bottom. They replenish the chips throughout the nesting period by pecking away at the cavity walls.

Nest Placement

Cavity

Acorn Woodpeckers excavate multiple cavities, any one of which may be used for nesting (the rest are used for nocturnal roosting). They dig cavities in dead or living limbs, large or small, either in the granary (storage) tree or any other large tree. The woodpeckers reuse nest holes for many years.

Behavior


Bark Forager

Acorn Woodpeckers are such unusual birds with such complicated social behavior that they have given rise to one of the longest-running behavioral studies of birds. They live in family groups of up to a dozen or more individuals, and they cooperate in raising young and in gathering, storing, and guarding food. Even their approach to cooperative breeding is unusually complex. Some groups have multiple breeding males and females, and all of a group’s breeding females lay their eggs in a single nest. Each female destroys any eggs that are present before she begins laying, resulting in the demise of more than one-third of the total eggs laid in joint nests. Once all the females have started laying their own eggs, their destructive behavior stops and they remove the debris to a nearby tree. There each egg is gradually eaten by several individuals—often including the female who laid it. Throughout the year, Acorn Woodpeckers collect acorns and wedge them tightly into holes they’ve made in tree bark. Acorn Woodpeckers fiercely defend these acorn granaries against other groups and any other species that might rob the stores. They also defend 15-acre territories around the granary. They occasionally wander outside the territory in pursuit of acorns and water. Birds that help at nests but don’t get to breed often go out looking for breeding vacancies in other groups, up to about 10 miles away.

Conservation

status via IUCN

Least Concern

Acorn Woodpeckers are numerous and their populations have been stable since 1966, according to the North American Breeding Bird Survey. Partners in Flight estimates the global breeding population at 5 million, with 30 percent living in the U.S. and 57 percent in Mexico. They rate a 9 out of 20 on the Continental Concern Score and they are not on the 2012 Watch List. While Acorn Woodpeckers are common in oak woodlands, their numbers have probably declined since historic times because of development and habitat degradation, including overgrazing and loss of oaks due to disease and habitat conversion. Oak, pine-oak, and streamside forest has been converted to other uses throughout the Acorn Woodpecker’s range. In the Southwest and parts of Mexico, overgrazing has damaged mountain pine-oak habitats and streamside forests, probably reducing the Acorn Woodpecker population substantially. California populations, though not currently declining, have an uncertain future because of slow oak forest regeneration. Other threats include having nest holes taken over by European Starlings, an aggressive introduced species. Occasionally, people shoot Acorn Woodpeckers to keep them from eating nut and fruit crops. However, Acorn Woodpeckers have also shown the ability to colonize new habitats such as suburban neighborhoods, using human-made structures for roosting and acorn storage.

Credits

Range Map Help

Acorn Woodpecker Range Map
View dynamic map of eBird sightings

Migration

Resident (nonmigratory), except for one population near the Huachuca Mountains of southeastern Arizona. These birds usually leave their breeding grounds in winter (possibly heading to Mexico’s Sierra Madre) as they are unable to store enough acorns to make it through winter in most years. Other populations may wander irregularly when local acorns are scarce.

Backyard Tips

Acorn Woodpeckers may visit seed and suet feeders near oak woodlands within their range. If Acorn Woodpeckers have discovered your wood siding and begun making holes in it, they can be very difficult to get rid of. People have had some success with hanging strips of shiny ribbon from the eaves or putting balloons in front of the siding to scare the birds away; the surest fix is to switch to an impenetrable siding material. Here's more about keeping away woodpeckers.

Find This Bird

Acorn Woodpeckers are usually pretty easy to find if you take a short walk through open oak or pine-oak forests in their range. Listen for their loud, parrotlike squawks and look for Acorn Woodpeckers perched atop bare treetops. In flight, pay attention to the pattern of three black-and-white flashes—one on each wing, plus the white rump. Keep an eye on the trees as you walk, and you might find one riddled with acorn-filled holes all the way up the trunk and main branches. This is the granary tree, the main food storage “pantry” created and used by communal groups of these fascinating woodpeckers.

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Going Nutty for Acorn Woodpeckers: Story and photographs in Living Bird magazine

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