Melanerpes formicivorusacorn woodpecker

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Geographic Range

The Acorn Woodpecker is found from northwestern Oregon, California, the American Southwest, and western Mexico through the Central American highlands and into the northern Andes of Colombia.

Habitat

The Acorn Woodpecker prefers pine-oak woodlands where oak trees are plentiful. They are also found in riparian corridors, and in Douglas firs, redwood and tropical hardwood forests as long as oaks are available nearby. Urban parks and suburban areas that possess numerous oak trees are often also home to the species.

Physical Description

The Acorn Woodpecker is a medium-sized, black and white clown-faced bird with a red crown, glossy black and white head, white eyes, and white rump and wing patches. There is usually at least one red or yellow tipped feather on the throat. In Colombian populations, the male has a solid red crown while the female has a black band separating the red crown from the white forehead. The wing span of the woodpecker ranges between 13-15 centimeters.

  • Average mass
    73 g
    2.57 oz
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  • Average basal metabolic rate
    0.737 W
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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.

  • Average eggs per season
    4
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  • Average time to hatching
    14 days
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Lifespan/Longevity

Behavior

The species is highly social and usually lives year-round in social units. Group members in temperate habitats do not forage together, but tropical populations often move together. The woodpecker is extremely territorial of granaries and sap trees. The species is generally sedentary, but there are some migrant populations in areas where there are large seasonal fluctuations of insects.

Communication and Perception

Food Habits

The main diet of the Acorn Woodpecker consists of insects, sap, oak catkins, fruit, and flower nectar. Acorns are critical for winter survival. Occassionally, it eats grass seeds, lizards and bird eggs. The bird prefers, however, flying ants and other Hymenoptera and Coleoptera. When foraging, the woodpecker often sits at the tops of trees while flycatching. Most foraging, however, is performed in or near the canopy. The woodpecker rarely goes to the ground except to pick up grit and fallen acorns. Usually, acorns are removed singly from trees, but the bird may also break off a twig holding up to three acorns. Sapsucking is a communal affair and group members congregate at a set of holes that are used repeatedly for several years.

The Acorn Woodpecker stores insects in cracks or crevices and nuts in indiviually-drilled holes in graneries. A granary tree may hold as many 50,000 holes. Holes are usually drilled in dead limbs and in thick bark during the winter. Any dead or living tree with deep dry bark can used as granary. Studies have shown that these granaries are so important that they are one of the main reasons why acorn woodpeckers live in such large families, at least in California. Only a large group can collect so many acorns and also defend them against other groups.

Economic Importance for Humans: Positive

Native Americans in California used Acorn Woodpeckers for food and collected their feathers for ornamentation on garments.

Economic Importance for Humans: Negative

The Acorn Woodpecker is often considered a pest by nut and fruit farmers when the bird feeds on their crops.

Conservation Status

Although the species holds no special status, there are several threats facing the woodpecker. Many of these threats stem from habitat loss and degradation. Overgrazing, poor regeneration of oaks in California, and destruction of oak and pine forests for firewood or development are among the biggest threats facing the species.

Contributors

Marie S. Harris (author), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

visual

uses sight to communicate

References

Koenig, Walter, D. et., al. 1995. The Birds of North America, No. 194.