Acorn Woodpecker

Melanerpes formicivorus

Order:
Piciformes
Family:
Picidae
Sections

Diet and Foraging

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Figure 3. Acorn Woodpeckers store acorns in drilled holes.

Acorn Woodpeckers typically store acorns in drilled holes, to be used as food at later dates. Some large trees contain 1000s of such holes. Drawing by J. Schmitt.

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Acorn Woodpecker flycatching.

Generally captures insects by flycatching high above the canopy or by gleaning off tree limbs.

© Kathleen Kent , California , United States , 6 June 2017
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Acorn Woodpecker with acorn.

Acorns usually removed singly from trees, but Acorn Woodpeckers may also break off twigs containing up to 3 acorns at once or eat particularly large acorns in situ.

© Blake Matheson , California , United States , 22 October 2018
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Acorn Woodpeckers foraging on bananas.
© Georges Duriaux , Jinotega , Nicaragua , 4 January 2018
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Acorn Woodpecker foraging for flower nectar.
© Anne (Webster) Leight , Arizona , United States , 17 June 2017
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Acorn Woodpecker storing acorn in granary tree.

The habit of storing nuts in individually-drilled holes in granaries is unique to this species.

© Zach Millen , California , United States , 28 September 2017
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Acorn Woodpecker storing acorn in granary tree.

An individual granary tree may contain only a few or as many as 50,000 holes, each of which is typically filled with an acorn in autumn.

© Dorna Mojab , California , United States , 17 December 2017
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Acorn Woodpecker granary tree.

An individual granary tree may contain only a few or as many as 50,000 holes.

© Cris Whetstone , California , United States , 16 January 2017
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Acorn Woodpecker granary tree.

Almost any dead or living tree with deep, dry bark may be used as a granary. This individual is using a palm tree.

© Jon Tveten , California , United States , 5 January 2019
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Acorn Woodpecker granary tree.

Holes are reused annually, accumulate with time, and may eventually be drilled in almost every available limb of a granary tree, although preferred sites are those on the underside of limbs and other partially protected sites.

© Thomas Johnson , California , United States , 25 January 2019
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Felled granary tree exposing a large acorn store.
© anthony guizar , California , United States , 13 February 2016
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Acorn Woodpecker at Red-naped Sapsucker sap holes.
© Alane Gray , California , United States , 4 April 2019
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Acorn Woodpecker at hummingbird feeder.
© Angela Kenny , California , United States , 1 March 2019
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Acorn Woodpeckers drinking.

Frequently drinks from water collected by holes in trees. Also drinks occasionally from springs and other groundwater sources.

© David Sutton , California , United States , 22 October 2018
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Acorn Woodpecker drinking.
© Christine Sparks , California , United States , 9 July 2018

Feeding

Main Foods Taken

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Video: Male Acorn Woodpecker foraging.
© Larry Arbanas, Arizona, United States
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Video: Male Acorn Woodpecker foraging.
© Timothy Barksdale, California, United States
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Video: Female Acorn Woodpecker at hummingbird feeder.
© Timothy Barksdale, Arizona, United States
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Video: Female Acorn Woodpecker drinking.
© Larry Arbanas, Arizona, United States

Insects, especially flying ants and other Hymenoptera and Coleoptera, acorns (both immature and stored), sap, oak catkins, fruit, flower nectar, and occasional grass seeds, lizards, bats (62), mice (ML136885531), birds (63, 64), and bird eggs (65, including their own; see Behavior: Sexual Behavior). For a review of the food habits of birds from studies conducted outside of the United States, see Flores and Ardón (66).

Microhabitat for Foraging

Often sits at the tops of trees while flycatching, otherwise forages primarily in or near the canopy. Acorns are picked directly off trees; birds rarely go to the ground except to obtain grit and to pick up acorns inadvertently dropped.

Food Capture and Consumption

Generally captures insects by flycatching high above the canopy or by gleaning off tree limbs. Flycatches primarily in specific flights in which an individual targets and catches a single insect during a sortie (47). Flights are often nearly vertical, lasting up to 43 s but commonly closer to 5 s. Usually 1 sortie every 1–2 min, but up to 6/min recorded (47). Only rarely digs into wood for insects or their larvae.

Acorns usually removed singly from trees, but may also break off twigs containing up to 3 acorns at once (67) or eat particularly large acorns in situ (68). Acorns are eaten piecemeal after removing or pecking through the shell and are generally consumed at “anvils,” locations on the upper surfaces of horizontal limbs where cracks and crevices provide a suitable holdfast (69, 47). See also Food Selection and Storage.

Sapsucking is a communal affair, with group members congregating at sets of holes often used for several years. Sap holes are smaller in diameter and shallower than acorn storage holes but are spaced similarly. Sap holes are readily distinguished from those made by sapsuckers (Sphyrapicus), which are smaller, more closely and linearly spaced, and often girdle the limb.

Diet

Based on stomach analyses of 84 birds from California collected in all months except February, April, and July (70), acorns constitute 53.3% of the diet, followed in importance by fruit (22.9%), ants (8.1%), other Hymenoptera (7.3%), beetles (2.9%), other insects (4.5%), and other vegetable matter (1.0%). A comparable study of 72 birds collected in all months from Oregon (71) yielded 59.8% acorns, 15.8% other vegetable matter, 10.4% ants, 7.3% beetles, 0.5% other Hymenoptera, 4.7% other insects, and 1.5% fruit.

Nestlings are fed insects and acorns pieces, with the proportion of the latter increasing with nestling age (19; see Breeding: Parental Care). Based on food samples taken from nestlings at Hastings Reservation collared with pipecleaners, 39% of samples contained acorn fragments, whereas all contained arthropod fragments, including Hymenoptera (68%), Coleoptera (50%), Homoptera (25%), Hemiptera (25%), Lepidoptera (21%), Dermaptera (18%), Neuroptera (14%), Diptera (14%), Plecoptera (11%), Orthoptera (11%), and Arachnida (4%) representing at least 40 families (72).

Flycatches at any time of the year when insects are available. Consumes acorns directly off the tree starting when they approach maturity. In California, sapsucking is done primarily in early spring (February and early March) and again in midsummer (July and August), especially when birds have run out of stored acorns (47, WDK). In New Mexico, sapsucking occurs almost exclusively in the spring (PBS). Apparently only the sap itself, and not the “bast,” or wood tissue, is consumed (47).

Dietary Preferences

Insects are preferred food and are eaten at any time of year when weather permits. Acorns are supplemental, stored extensively in autumn but then used primarily when conditions render insects unavailable. The increase in the proportion of acorns fed to nestlings with age (see Breeding: Parental Care) is probably due to a combination of the greater nutritional demands of larger nestlings and the increasing ability of nestlings to handle larger acorn bits as they grow. Stable-isotope analyses indicate that access to stored acorns allows breeders to provide nestlings with a greater proportion of protein-rich insects while subsisting themselves on a higher proportion of relatively low-quality acorns (73). At Hastings Reservation, sap is also supplemental adult food, eaten in conjunction with stored acorns in late winter and following the exhaustion of stored acorns in the summer.

Preferences among different species of acorns have not been determined, although circumstantial evidence suggests little or no preferences despite significant differences in the composition of acorns of different species (74).

Food Selection and Storage

Insects captured by flycatching may be stored for extended periods in cracks or crevices (59, 69). More evident is the habit of storing nuts in individually drilled holes in granaries, which is unique to this species.

An individual granary tree may contain only a few or as many as 50,000 holes (75), each of which is typically filled with an acorn in autumn (Figure 2). Generally groups have 1 primary and 1 or more secondary storage trees; of 53 groups sampled by MacRoberts and MacRoberts (47), the mean number of granary trees per group was 2.1 (range 1–7). Holes are drilled primarily in the winter and are made in dead limbs and in thick bark without penetrating the cambium and phloem layers associated with sap. Consequently, storage holes do not compromise living trees. Holes are reused annually, accumulate with time, and may eventually be drilled in almost every available limb, although preferred sites are those on the underside of limbs and other partially protected sites.

Large granaries are the aggregate result of dozens of generations each drilling relatively few holes per year. Birds do not drill holes all at once, and thus the length of time needed to drill a hole is not known. Using a reasonable estimate of 1 h/hole, 5 birds (3 breeders and 2 nonbreeding helpers) spending an average of 2.7% (breeders) and 1.4% (nonbreeding helpers) of their time drilling new holes (76) could be expected to drill about 477 holes/yr. Assuming no loss of old holes, such a group would require > 8 yr to produce a modest granary of 4,000 holes and > 100 yr to drill the large granary with 50,000 holes recorded by Dawson (75). Birds generally make new holes only when they already have stored mast, and thus granaries are the product of a positive feedback system whereby groups with more holes can store more nuts which allows them to make more holes. This pattern continues until the granary wood rots and the tree or limb falls to the ground.

Almost any dead or living tree with deep, dry bark may be used as a granary, even parts of trees such as pine cones (77). Human-made structures such as utility poles, fence posts, wood-sided buildings, undersides of roof tiles, and even automobile radiators may be used. Once a group has filled its granary facilities, it may also fill larger cavities such as old nest and roost holes with acorns.

Birds store primarily acorns (both Quercus [true oaks] and Notholithocarpus [the tanoak]), but almonds, walnuts, hazelnuts, pecans, and piñon pine nuts are also stored when available (78, 47, 79). The ability of this woodpecker to store vast numbers of acorns is confirmed by examples in which birds store acorns in structures such as walls or hollow trees where they subsequently fall through and become unretrievable. A total of 62,264 such acorns were counted in the door and window casings of one such unused house in the Sierra Nevada (78), and 220 kg of acorns apparently stored in a single season were retrieved from a wooden water tank near Flagstaff, Arizona (80). There are records of birds storing unsuitable items, including small stones, scraps of bark, fruit pits, scales of old pine cones, and even complete Douglas-fir (Pseudotsuga menziesii) cones (78, WDK). Although acorn storage occurs throughout the species' range, its extent is variable, being almost universal in California, irregular in southwestern United States, Mexico, and Central America, and apparently only occasional in Colombia (81, 82, 66). When storage occurs in tropical areas, it is sometimes done in bromeliads, under loose bark, and in other natural locations rather than in granaries (67, 83). Mast stores play a critical role in allowing groups in temperate areas to remain on territories during the winter when few alternative food sources are available (see Distribution, Migration, and Habitat: Nature of Migration) and to breed successfully the subsequent spring.

Both observational (84) and genetic studies focusing on stored acorns (85, 86) have found that acorns are usually harvested from a small subset (< 10) trees located within about 150 m of the granary, although birds will foray considerably farther to find acorns if necessary. Birds appear to store acorns that are smaller than those available but do not have strong preferences for particular species of acorns, despite differences in energetic content (74).

Stored acorns are often infested with insect larvae, especially those of wasps, moths, and weevils. Such larvae are almost certainly eaten when found. However, there is no truth to the idea that stored acorns are nurseries for insect larvae (the “grub theory”; 87), and birds do not appear to select acorns depending on whether they are parasitized by insects or not (74). Most insect larvae pupate and leave acorns as the acorns dry out, and thus few larvae remain by the time stored acorns are consumed.

Nutrition and Energetics

Acorns contain water-soluble phenolic compounds known as tannins, which in many species have the deleterious effect of precipitating and thus reducing the availability of proteins. In contrast to California Scrub-Jays (Aphelocoma californica), a sympatric species also highly dependent on acorns, Acorn Woodpeckers are able to survive > 2 wk when fed nothing but acorns with little or only modest loss of body mass (88). However, body mass loss is greater when fed high-tannin coast live oak (Quercus agrifolia) acorns than lower-tannin valley oak (Q. lobata) or canyon live oak (Q. chrysolepis) acorns (88), suggesting a detectable detrimental effect of tannins. This detrimental effect is apparently due at least in part to lower digestive efficiency (89).

Metabolism and Temperature Regulation

Average field metabolic rate of 4 adults estimated using the doubly labeled water technique during spring breeding attempts was 195 kJ/day, considerably lower than that which would be predicted based on values for other nonpasserines (19). The physiological ecology of the Acorn Woodpecker is unusual in at least two ways: first, the relationship between resting metabolic rate of adults and ambient temperature (Ta) exhibits no thermoneutral zone; and second, basal metabolic rate is high compared to other nonpasserines (19).

This species almost invariably roosts in tree cavities. Nighttime temperatures inside roost cavities average significantly higher than outside ambient temperatures and are further increased 1.2–6.0°C depending on the number of birds sharing the cavity (90). Heat loss is significantly lessened by roosting communally in cavities, particularly in the presence of wind.

Nestlings have been documented (ML30413551) leaving the nest cavity in times of extreme heat (Richard Fischer Sr., personal communication).

Drinking, Pellet-Casting, and Defecation

Frequently drinks from water collected by holes in trees. Also drinks occasionally from springs and other groundwater sources, with individuals recorded visiting a single source of water up to 3 times daily during the dry season (91).

Recommended Citation

Koenig, W. D., E. L. Walters, P. B. Stacey, M. T. Stanback, and R. L. Mumme (2019). Acorn Woodpecker (Melanerpes formicivorus), version 2.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.acowoo.02