|Photo courtesy of David Brezinski, US Fish and Wildlife Service|
The scientific name of the acorn woodpecker is Melanerpes formicivorus Swainson. It is a member of the woodpecker family, Picidae [6,7,8,130]. There are 4 recognized subspecies in North America :
Melanerpes formicivorus angustifrons Baird
Melanerpes formicivorus bairdi Ridgeway
Melanerpes formicivorus formicivorus (Swainson)
Melanerpes formicivorus martirensis (Grinnell and Swarth) 
Picus formicivorus Swainson=
Melanerpes formicivorus (Swainson) [6,7]
Melanerpes formicivorus var. angustifrons Baird=
Melanerpes formicivorus angustifrons Baird
Balanosphyra formicivora martirensis Grinnell and Swarth=
Melanerpes formicivorus martirensis (Grinnell and Swarth)
FEDERAL LEGAL STATUS:
No special status
Information on state-level protection status of birds in the United States is available at NatureServe, although recent changes in status may not be included.
Populations of acorn woodpeckers in Washington, Oregon, and California occupy plant communities dominated by Oregon white oak (Quercus garryana) and California black oak (Q. kelloggii); blue oak (Q. douglasii) and California black oak (Q. kelloggii) [149,152,155]; coast live oak (Q. agrifolia), valley oak (Q. lobata), and California black oak [45,90]; and tanoak (Lithocarpus densiflorus) . In south-central Washington, Oregon white oak may be mixed with Douglas-fir (Pseudotsuga menziesii) or ponderosa pine (Pinus ponderosa) . On the Santa Rosa Plateau Reserve in Riverside County, California, acorn woodpeckers occupy habitat dominated by Engelmann oak (Q. engelmannii) and coast live oak . On the east side of the Sierra Nevada, acorn woodpeckers occupy habitat dominated by California black oak and Jeffrey pine (P. jeffreyi) .
At Hastings Natural History Reservation, in Monterey County, California, acorn woodpeckers occupy foothill woodlands, savanna-grasslands, riparian woodlands, chaparral, and mixed-evergreen forest. Dominant tree species in foothill woodlands at Hastings Reservation include blue oak and valley oak. Dominant trees in savanna-grasslands at Hastings Reservation include valley oak, blue oak, and coast live oak. Dominants in riparian woodlands include willow (Salix spp.), California sycamore (Platanus racemosa), coast live oak, and valley oak. Chaparral habitat is dominated by chamise (Adenostoma fasciculatum) and wedgeleaf ceanothus (Ceanothus cuneatus), and mixed evergreen forest is dominated by coast live oak and California black oak [72,91].
Acorn woodpecker populations in the southwestern United States occupy primarily
low-elevation montane plant communities dominated by Gambel oak (Q. gambelii)
and ponderosa pine [20,20,29,49,65,101,123,124,143,145]. They also inhabit the
following plant communities: Arizona canyon riparian forest, dominated by
Arizona sycamore (Platanus wrightii), Arizona walnut (Juglans major),
and Arizona cypress (Crataegus douglasii); Arizona Madrean foothill
forest, dominated by Mexican pinyon (P. cembroides), alligator juniper
(Juniperus deppeana), silverleaf oak (Q. hypoleucoides), and Arizona
white oak (Q. arizonica); and Mexican Madrean foothill forest, dominated by
Mexican pinyon, drooping juniper (Juniperus flaccida), gray oak
(Q. grisea), and Texas madrone (Arbutus xalapensis) .
|One example of acorn woodpecker habitat:
Oregon white oak savanna in Willamette Valley, Oregon.
Photo courtesy of US Fish and Wildlife Service
This review cites data collected primarily from 2 acorn woodpecker subspecies: the Pacific Coast subspecies, Melanerpes formicivorus bairdi, located at the Hastings Natural History Reservation in Monterey County, California (hereafter referred to as "Hastings Reservation"); and the southwestern subspecies, (M.f. formicivorus), located at Water Canyon in the Magdalena Mountains of New Mexico [87,133,134] and the Research Ranch in Elgin, Arizona .
Social organization: Acorn woodpeckers are cooperative breeders with complex social behaviors. The quantity and quality of stored acorns influence group size and composition, reproductive success, and survivorship [67,72,78,122]. Acorn woodpecker groups defend year-round territories, cooperatively store acorns in 1 or more granaries (see Food habits), produce 1 nest at a time, and raise offspring together [72,90,91]. Acorn woodpecker groups consist of 2 to 15 individuals, with an average of 5 or 6 individuals [72,74,78,89,132]. Within groups, there are 1 to 4 breeding males, 1 or 2 (rarely 3) breeding females, and 0 to 10 non-breeding offspring born in previous years. Territories that are continually occupied during the breeding season contain more breeding males and more nonbreeders than territories that are intermittently occupied . Genetic relatedness is high , but close inbreeding rarely occurs [72,74]. At Hastings Reservation, mean adult group size over an 11-year period was 4.4 individuals (SD 2.4) (range not given) during the breeding season. Nearly all groups (97.6%) contained at least 1 breeding male and female. Monogamous pairs of acorn woodpecker were also common . At the Research Ranch and in the Huachuca Mountains of Arizona, mean breeding group size was 2.64 individuals (SD 0.89) (range 2.0 to 5.0). At Water Canyon, mean group size was 2.15 individuals (SD 0.36) (range 2.0 to 3.0) . For comparative breeding data on acorn woodpeckers at Hastings Reservation, the Research Ranch, and Water Canyon, see Koenig and Haydock .
The southwestern subspecies of acorn woodpecker exhibits social plasticity due to marginal habitat. They may either live in highly cooperative resident groups or migrate independently (presumably to oak woodlands in the Sierra Madre of northern Mexico) during winter and return in spring to form temporary male-female bonds. In oak savanna and oak woodland habitats at the Research Ranch, some acorn woodpeckers lived in family groups consisting of 3 birds; they stored acorns in granaries. Most acorn woodpeckers, however, lived independently and stored few acorns. Both types of behavior may be present within the same population at any given time, and acorn woodpeckers may shift between strategies, depending on acorn production. During years of good acorn production, acorn woodpeckers are able to remain resident and form stable social groups. During years of poor acorn production, limited acorn stores are consumed quickly, forcing migration .
Breeding: Acorn woodpeckers are considered opportunistically polygynandrous [72,75], meaning that 2 or more males mate with 2 or more females within a social group. Monogamy is also common [72,132]. Acorn woodpeckers are typically cooperative breeders [72,76,78]. Within a group, up to 4 males may mate with up to 3 females, and females lay eggs in a communal nest [72,78]. Genetic relatedness is high within an acorn woodpecker group; however, close inbreeding is usually avoided. Breeding males are generally unrelated to breeding females. Breeders are typically closely related to each other within each sex. Male breeders are generally siblings or a parent and 1 or more offspring, and the same is true for female breeders . Age of first reproduction at Hastings Reservation is 2.1 years for males and 1.9 years for females .
Breeding and nesting occur from late March to late June and peak in April. Breeding may occur again between late August and mid-October if weather is mild and stored acorns are still available [72,76,77]. Of 1,173 nests found over a 31-year period at Hastings Reservation, 51 nests (4.3%) produced a second brood during fall . In Arizona and New Mexico, mating typically occurs in late June, and a second nesting is rarely attempted during fall .
Reproductive success: Many factors influence reproductive success. They may include availability of stored acorns, abundance of flying insects (see Food habits), reproductive competition, prior breeding experience, territory quality, group size, granary size, and weather [67,72,75]. According to some researchers, availability of stored acorns in spring has the largest influence on reproductive success [72,75,76]. According to Koenig , breeding is impossible if acorn stores have been exhausted by spring. However, according to Roberts , reproductive success is not significantly (P>0.10) related to number of stored acorns, but is related to availability of arthropods, which contain more protein than acorns. Reproductive success may be poor the first spring after a bad acorn crop . At Hastings Reservation, large acorn crops permitted the storage of more acorns and significantly (P≤0.001) increased reproductive success the following spring . Groups of acorn woodpeckers that had not exhausted acorn stores by spring nested earlier, had larger clutches, a higher proportion of eggs that hatched, a higher proportion of young that fledged, and higher survival of young the first winter. The following table compares reproductive success of acorn woodpecker groups with and without stored acorns in May :
|Comparative reproductive success of acorn woodpecker groups at Hastings Reservation *|
|Groups with acorn stores present in May||Groups with acorn stores exhausted by May|
|Mean group size (SD)||4.57 individuals (2.24) (n=139)||3.44 individuals (2.03) (n=43)|
|Mean first laid egg date (SD)||7 May (23.7 days) (n=157)||30 May (36.4 days) (n=20)|
|Mean clutch size (SD)||4.55 individuals (1.03) (n=93)||3.57 individuals (0.65) (n=14)|
|Eggs hatched (%)||69.0 (n=578)||54.2 (n=59)|
|Young fledged (%)||47.6 (n=578)||25.4 (n=59)|
|Hatchlings that fledged (%)||68.9 (n=399)||46.9 (n=32)|
|Mean number of young fledged (SD)||2.89 individuals (1.87) (n=137)||0.51 individuals (1.11) (n=43)|
|Mean number of young alive in February (SD)||1.62 individuals (1.53) (n=104)||0.34 individuals (0.79) (n=32)|
|Groups successful (young surviving through their first winter) (%)||82.7 (n=139)||23.3 (n=43)|
Reproductive competition between joint-nesting females negatively affects reproductive success. Approximately 25% of acorn woodpecker groups consist of 2 or more nesting females sharing a communal nest at any one time. Joint-nesting females compete to lay their eggs in the nest and may destroy eggs laid by co-breeders. Females that are last to lay eggs in the communal nest benefit by destroying eggs laid earlier by co-nesters. Destroyed eggs are compensated for by laying more eggs. Therefore, groups with joint-nesting females lay significantly (P<0.001) more eggs than monogamous females, but reproductive success per female is lower . On a per female basis at Hastings Reservation, monogamous pairs of acorn woodpeckers fledged significantly (P≤0.05) more young on average (2.72 young (SD 1.69), n=29) than groups that consisted of 2 reproductively active females (1.32 young (SD 1.33), n=11) .
At Hastings Reservation, the most important factors influencing reproductive success were prior breeding experience and yearly variation in food, weather, or other ecological conditions. Acorn woodpecker groups in which the breeding membership did not change from the prior breeding season had significantly (P≤0.01) higher reproductive success than groups that experienced turnover in breeding membership . For more information on factors influencing reproductive success at Hastings Reservation, the Research Ranch, and Water Canyon, see Koenig and Stacey .
Incubation period and clutch size: The incubation period averages 11.5 days . For the first week of incubation, 1 male and 1 female tend the eggs. Approximately 1 week after eggs are laid, all birds in the group, regardless of sex, take turns incubating the eggs, and later feeding the young . Overall, adult females incubate eggs more often than adult males, and adult males brood and feed nestlings more often than adult females . Acorn woodpeckers produce at least 1 brood/year across their range, but have been observed producing a second brood at Hastings Reservation . Large acorn woodpecker groups (7 to 8 birds) do not produce more offspring per capita than monogamous pairs , and reproductive success of joint-nesting females is lower than that of monogamous females . Clutch size at Hastings Reservation averaged 4.82 eggs/nest (SD 1.39) for joint-nesting females and 4.36 eggs/nest (SD 1.04) for monogamous females [68,72]. In riparian habitat in the Magdalena Mountains, mean reproductive rate over a 10-year study period was 2.01 young/pair (range 0.05 to 4.00, n = 433) .
Development: Across the acorn woodpecker's range, the complete nesting cycle takes approximately 45 days to complete (4 days for egg-laying, 11 days for incubation, and 30 to 32 days for young to develop before fledging) [68,72,135].
Mortality and survival rates: Major causes of mortality include nestling starvation , predation [73,135], egg destruction by joint-nesting females, and acorn availability . Starvation is a major cause of nestling mortality at Hastings Reservation. Of 693 eggs laid in 180 nests, 16.0% produced nestlings that starved . Predation of eggs is mainly by snakes [55,72,75] and Cooper's hawks (Accipiter cooperii) [91,135] (see Predators). Percentage of juveniles lost to predators at Hastings Reservation was 7.9% (n=31 nests). Most egg mortality occurs from destruction by conspecifics  (see Reproductive success).
At Hastings Reservation, survivorship of acorn woodpeckers was significantly (P<0.001) influenced by acorn availability. For example, the winter following a poor acorn crop, disappearance and probable mortality of females was 46.2% (n=13), compared to 5.8% (n=137) the winter following a good acorn crop . Survivorship was higher on permanently occupied territories compared to those not permanently occupied .
Adult survivorship is higher than juvenile survivorship [75,134]. At Hastings Reservation, mean annual survivorship was 82.4% (n=273) for breeding males and 71.2% (n=302) for breeding females. Survivorship of first-year birds was 57% (n=447) . Over a 9-year study in the Magdalena Mountains, mean annual adult survivorship was 59% (range 38% to 71%, n=397). Mean juvenile survivorship was 35% (range 18% to 64%, n not given) . In Water Canyon, mean annual survivorship was 61.3% for breeding males and 51.5% for breeding females. Survivorship of first-year birds was 37% (n=123) .
The oldest recorded acorn woodpecker at Hastings Reservation was at least 16 years for a male and at least 15 years for a female. The oldest recorded individuals in Water Canyon were at least 9 years for a male and at least 5 years for a female .
Dispersal: Young remain in their natal group for at least the first year . After the first year, offspring either stay in their natal group as non-breeders or disperse to become breeders in vacant spots within other acorn woodpecker groups. Offspring that remain in their natal area are nonbreeding "nest helpers" for several years. They may eventually become breeders within their natal group after the death of breeders [71,72]. Dispersal occurs during spring and may occur during late summer to early fall. In Arizona, acorn woodpeckers may migrate during winter and return in spring to nest in territorial breeding pairs [78,132] (see Social organization). During spring, some juveniles may disperse from their natal group and either join other groups of acorn woodpeckers or colonize abandoned acorn woodpecker territories. During late summer to early fall, dispersal may involve groups of juvenile and adult acorn woodpeckers, and is typically a response to acorn crop failure .
Male acorn woodpeckers dispersed shorter distances and inherited natal territories more often than female acorn woodpeckers at Hastings Reservation. Of 70 breeding males and 87 breeding females that dispersed, mean dispersal distances were 0.34 mile (0.54 km) for males and 0.41 mile (0.66 km) for females, a significant difference (P<0.01) . Of 137 male and 22 female acorn woodpeckers at Hastings Reservation, males inherited 23.7% and females inherited 4.6% of their natal territories .
Territory size and density: Territories are established in habitats that meet acorn woodpecker requirements during summer and winter , and the same territory may be used for generations of acorn woodpeckers . Acorn woodpeckers typically defend permanent, all-purpose territories [66,91]; however, acorn woodpecker groups may disband and leave their territory if an acorn crop fails or a granary tree is destroyed [66,72,106].
Territories typically encompass 10 to 15 acres (4-5 ha) [89,91], and are circular or hexagonal in shape, which may be optimal for energetic efficiency and for storage of fluctuating resources . At Hastings Reservation, territory size ranged from 8.6 acres to 22.2 acres (3.5-9.0 ha), with a mean of 14.8 acres (6.0 ha) and a density of 6.7 territories/100 acres . Most territories at Hastings Reservation are along or adjacent to watercourses or in old fields on hilltops with scattered oaks .
Roberts  found that in California and Arizona, territory size was not strongly related to acorn woodpecker group size. In contrast, MacRoberts and MacRoberts  found a significant positive rank correlation (P<0.01) between group size and territory size in California. As group size increased over time, territory size increased, but the opposite did not occur, so some small acorn woodpecker groups had disproportionately large territories .
In blue oak woodlands in Mendocino County, California, density of acorn
woodpeckers was 25.6 individuals/40 ha . In habitat dominated by blue oak
with varying amounts of coast live oak in Monterey and San Luis Obispo counties,
California, acorn woodpeckers occupied an annual average of 0.2 territories/40 ha
during the spring of 1994 and 1995 .
Acorn woodpeckers are oak woodland specialists . Their demography is limited by acorn productivity and the ability to store mast . Oak-pine woodlands [59,76,82] and habitat containing mature oaks interspersed with grassland are preferred [59,89]. These habitats may occur from sea level to mountainous areas within the distribution of oaks .
Essential habitat elements for the acorn woodpecker include acorn-producing trees, tree cavities, and snags [30,72,122]. Oaks >17.9 inches (45.5 cm) DBH are sought for collecting acorns  and roosting [95,122]. Pines and other conifers are preferred for nesting and for storing acorns due to their soft wood and/or thick bark [95,122]. Storage space (granaries) is a greater limiting factor in acorn woodpecker habitat than acorn availability .
Acorn woodpeckers are generally found in habitat containing more than one oak species . Because different species of oaks do not produce synchronous crops, acorns are usually a reliable food source during fall [70,72,76]. In the southwestern United States, only riparian habitat in canyon bottoms of isolated mountain ranges produces enough acorns to support acorn woodpeckers throughout the winter. Therefore, acorn woodpecker populations are small and discrete in those areas [70,133,134].
Each generation of acorn woodpecker depends on habitat modifications of the previous generation. Habitat previously occupied by other acorn woodpeckers is preferred because it already contains granary trees, sap trees, and trees with cavities for nesting and roosting. Habitat without these features is rarely occupied by acorn woodpeckers [91,122].
Stand composition/structure: Acorn woodpeckers prefer habitat containing oaks [123,124,147], mature forest with large-diameter trees [115,118,155], an open stand structure [34,143,155], and a diversity of oak species [19,70,72,122].
In 3 studies conducted in northern Arizona, oaks were an essential component of acorn woodpecker habitat; however, acorn woodpeckers were found occasionally in pure ponderosa pine forest. Acorn woodpeckers were detected more often in ponderosa pine-Gambel oak woodlands than pure ponderosa pine forest in several sites throughout northern Arizona. Twenty-three stands, defined as contiguous areas of structurally similar forest ≥100 acres (41 ha), were chosen for the study. Twelve stands were located in ponderosa pine-Gambel oak habitat, and 11 stands were located in pure ponderosa pine habitat. In ponderosa pine-Gambel oak stands, acorn woodpeckers were detected most often in old logged areas where trees were 5.0 to 11.9 inches (12.7-30.2 cm) DBH. Within pure ponderosa pine forest, acorn woodpeckers were detected only in unlogged stands with a history of either wildfire or prescribed fire in the previous 20 years . Although not indicated, Gambel oak was more than likely located in the vicinity of pure ponderosa pine stands and was probably used as a food source for acorn woodpeckers.
In a similar study on the Coconino National Forest and the Arizona Army National Guard Camp Navajo, Arizona, acorn woodpeckers occurred in ponderosa pine-Gambel oak stands 3 times more often than in pure ponderosa pine stands. Both cover types had an understory of Arizona fescue (Festuca arizonica) or blue grama (Bouteloua gracilis). Stands were ≥49 acres (20 ha) and contained contiguous areas of structurally similar forest. Stands did not have significantly (P>0.05) different density, canopy cover, diameter, basal area of ponderosa pine, or snag density. The ponderosa pine-Gambel oak habitat contained a mixture of shrub-like and tree growth forms of Gambel oak .
Of 6 study plots dominated by ponderosa pine, Gambel oak, and occasional alligator juniper (Juniperus deppeana) in the Coconino National Forest, Arizona, acorn woodpeckers (3.0 breeding pairs) were found in only 1 plot. The plot with acorn woodpeckers contained the highest density of Gambel oak. Details of the plot composition are shown in the table below :
|Composition of stands in preferred acorn woodpecker habitat in Coconino National Forest, Arizona |
|Relative density||Relative dominance||Relative frequency||Importance value||Absolute density (trees/ha)|
Mature forest containing large-diameter conifers and oaks is an important component of acorn woodpecker habitat. One oak species may sometimes be preferred over other oak species. In a 1988 study in the Six-Rivers, Klamath, and Shasta-Trinity National Forests in California, acorn woodpeckers preferred mature Douglas-fir forest mixed with tanoak and Pacific madrone (Arbutus menziesii) over early seral stages. Mature stands were generally >100 years old and contained trees >24 inches (60 cm) DBH and >131 feet (40 m) tall, with a multi-layered canopy, a well-developed understory, and many standing and fallen trees >35 inches (90 cm) DBH. The brush/sapling seral stage lasts up to 20 years following logging and was characterized as mixed or pure stands of shrubs and Douglas-fir seedlings and saplings <20 feet (6m) tall in association with forbs and perennial grasses. The pole/sawtimber stage persists up to 150 years following logging and consists of trees 20 to 131 feet (6-40 m) tall, with crowns up to 26 feet (8 m) in diameter. The following table shows densities of acorn woodpeckers in different successional stages :
|Density of acorn woodpeckers in 3 successional stages of Douglas-fir forest in northern California |
|Density of acorn woodpeckers (individuals/100 ha)||0||1.0||1.9|
The authors predicted that given logging trends at that time (1988), 85% of mature Douglas-fir stands would be replaced by younger age classes in 50 years, and acorn woodpecker populations could subsequently decline .
Acorn woodpeckers were also detected more frequently in mature forest than in young or old-growth forest in southwestern Oregon and northwestern California. Habitat was dominated by Douglas-fir in association with tanoak and Pacific madrone; stand ages were not given .
In blue oak woodlands in Mendocino County, California, acorn woodpeckers utilized habitat containing large-diameter trees (mean >20 inches (50 cm DBH)). Tree species found on study plots included blue oak, interior live oak, coast live oak, canyon live oak (Q. chrysolepis), valley oak, Oregon white oak, black oak, Oracle oak (Q. × morehus), California buckeye (Aesculus californica), California bay (Umbellularia californica), Pacific madrone, and Oregon ash (Fraxinus latifolia). Acorn woodpeckers excavated cavities most often in blue oak and used Oregon white oak in proportion to its availability. Number of natural tree cavities increased significantly (P<0.001) with tree diameter for all tree species combined. Number of excavated cavities also increased significantly (P<0.001) with tree diameter for all tree species combined and for all species except evergreen oaks and California buckeye .
Acorn woodpeckers generally prefer an open stand structure. In the Siskiyou National Forest, Oregon, acorn woodpeckers selected clearcut areas with residual Douglas-fir trees and either abundant tanoak or California black oak nearby. According to the authors, the clearcut created a habitat structure similar to pine-oak savannas, which may have attracted acorn woodpeckers. Douglas-fir trees within clearcuts had been "topped" by Forest Service personnel to provide wildlife trees. Topped trees were used by acorn woodpeckers for granaries, nests, and perch sites to hawk insects. Other trees used for granaries in the area included sugar pine (Pinus lambertiana) and Jeffrey pine .
In habitat dominated by ponderosa pine in the Coconino National Forest, Arizona, breeding acorn woodpeckers were detected only in a severely thinned plot. Five study plots were chosen in homogeneous stands of ponderosa pine, each containing the same proportion of different size classes of trees and densities of Gambel oak. The following table compares the density of acorn woodpeckers between an unlogged control and various logging treatments :
|Stand characteristics of logged areas and associated breeding densities of acorn woodpeckers in the Coconino National Forest, Arizona |
|Plot||Acorn woodpeckers (pairs/40 ha) detected in 1975||Treatment year||Tree density (trees/ha)||Canopy volume (m³/ha)||Total basal area
|Mean tree height
In blue oak woodlands in Mendocino County, California, acorn woodpeckers required habitat containing low tree density (<100 trees/ha). Tree species found on study plots included blue oak, interior live oak, coast live oak, canyon live oak, valley oak, Oregon white oak, black oak, Oracle oak, California buckeye, California bay, Pacific madrone, and Oregon ash .
The studies cited above indicate acorn woodpecker preference for open stands, but Raphael and others  describe use of mature forest with a multi-layered canopy and "well-developed" understory.
Acorn woodpecker density may be influenced by oak abundance and oak species diversity. Two analyses of Audubon Christmas Bird Count data [19,70] found that abundance of acorn woodpeckers in Pacific Coast populations increased with increasing density of oaks. A field study  did not detect this pattern but found some indication that acorn woodpecker group size was positively related to oak density. In southwestern populations, no relationship was detected between oak abundance and acorn woodpecker abundance [19,70]. Based on Audubon Count data, Bock and Bock  found that Pacific Coast acorn woodpecker populations increased with increasing oak species diversity, up to about 5 species, and then leveled off. They hypothesized that areas with few oak species were subject to random and frequent acorn crop failures, a pattern borne out by Koenig and Haydock's  field research at the Hastings Reservation. Koenig and Haydock  also found that, in Pacific Coast populations, year-to-year variation in acorn woodpecker abundance declined with increasing oak diversity (P=0.01), indicating that populations in areas with high oak diversity are more stable than populations in areas with one or a few oak species. In southwestern populations, Roberts' field data indicated a positive relationship between acorn woodpecker abundance and oak species diversity (P<0.05) , while Bock and Bock's  Christmas Count data did not.
Only 1 study described acorn woodpecker preference for riparian habitat containing Arizona sycamore. In the Huachuca Mountains, acorn woodpeckers were detected in small and large riparian woodland strips dominated by velvet ash (Fraxinus velutina), Fremont cottonwood (Populus fremontii), desert willow (Chilopsis linearis), Arizona sycamore, Arizona walnut (Juglans major), or willow (Salix spp.). They were not detected in riparian habitat dominated by bigtooth maple (Acer grandidentatum). For each tree species, small and large stands were selected in continuous riparian habitat adjacent to open uplands dominated by grass or wooded uplands dominated by oak and/or pine. Control sites for each combination of factors were located in open and wooded areas along drainages with no riparian trees, and several replicate study plots were selected for each combination of factors. Acorn woodpeckers showed a strong affinity for Arizona sycamores regardless of upland vegetation or elevation. Although not indicated, acorn woodpeckers probably obtained food from oaks in nearby wooded uplands. The following table shows characteristics of 13 plots where acorn woodpeckers were detected :
|Riparian habitats used by acorn woodpeckers, Huachuca Mountains, Arizona |
|Dominant tree species||
Acorn woodpeckers are associated with a subset of habitat that is significantly (P<0.05) different than overall habitat. The structure of acorn woodpecker habitat was analyzed for 25 groups of acorn woodpeckers in California and Arizona. Circles were surveyed within a 36-foot (11 m) radius of acorn woodpecker granaries, which were the central activity areas, and in 4 circles expanding 82 to 98 feet (25-30 m) outwards from the center of the circle. Central activity areas contained more tree species (but not more oak species) and more large, high-canopied trees (typically pines) than outside the central activity areas. In addition to granaries, nesting and roosting sites were usually located in the central activity areas. The central activity locus may optimize foraging-flight efficiency when fluctuating amounts of acorns are harvested within a territory. Storage/activity centers were regularly spaced in habitat occupied by multiple acorn woodpecker groups, limiting the potential number of sites where the main granary tree could be located . Burgess and others  found that, in contrast to Roberts'  study, granaries were significantly (P<0.05) clumped across several neighboring acorn woodpecker territories on the grounds of Stanford University. The authors could not contribute clumping of granaries to clumped habitat resources such as acorns, insects, or tree sap. Clumping of granaries across neighboring acorn woodpecker territories may offer the most advantageous group structure for avoidance of predators and efficient foraging .
Nesting: Acorn woodpeckers are strong primary excavators [17,23,24,61,155], creating cavities for nesting and roosting. Cavity excavation is communal and occurs during winter and spring . At Hastings Reservation, nest height averages 27.2 feet (8.3 m) (range 7.5 to 59.1 feet (2.3-18 m)) above ground and nests are most often located in tree trunks. Nests may be constructed in the granary tree but have been recorded up to 0.6 mile (0.9 km) away from the granary tree. Nest holes may be reused. At Hastings Reservation, there is a 50% probability that acorn woodpeckers will reuse nest cavities .
Nest cavities may be constructed in snags, live trees [10,29,55,57,76,129], and large (>3 feet (1 m)) or small (<7 inches (17 cm)) limbs [55,155]. Cavities located in live tree limbs are warmer and have less temperature variance than cavities located in dead limbs . Nest cavities in live tree limbs may also provide more protection from predators such as American black bears (Ursus americanus) than nests located in snags . Snags are typically preferred for nesting, however [29,55,57,129].
Snag use by acorn woodpeckers is common [29,129]; logs may also be used, but details on this habit are not available. In an Arizona pine (Pinus ponderosa var. arizonica) forest in Coronado National Forest, Arizona, acorn woodpeckers preferred nesting in ponderosa pine snags ≥20 inches (50 cm) DBH, with a decay stage in which fine twigs were present, sapwood was sound to rotting, and bark cover was 75% to 100%. Density of snags was 4.0 snags/ha . In the Apache-Sitgreaves National Forest in Arizona, cavity nesting birds, including acorn woodpeckers, preferred snags that retained an average of 90% bark cover (range 60% to 100%) for nesting. Snags that had been dead >5 years and were >19 inches (48 cm) DBH were also preferred. Two acorn woodpecker nests were found in ponderosa pine snags, and 1 nest was found in a quaking aspen snag. Mean tree height of the 3 nesting trees was 40 feet (12 m) (range 30 to 50 feet (9-15 m)) and mean DBH was 19 inches (48 cm) (range 14 to 26 inches (36-66 cm)) .
Acorn woodpeckers do not show a strong preference for snags an all areas. In ponderosa pine-Gambel oak woodlands in western New Mexico, more acorn woodpecker nests were located in live trees than in snags or dead limbs :
|Frequency of acorn woodpecker nests found in snags, dead portions of trees, and live trees (n=15 nests) |
|Snags||Dead portions of trees||Live trees|
Of 238 acorn woodpecker nests found in the Pacific Northwest, 7.8% were located in snags [55,129]. Minimum DBH of snags used for nesting by acorn woodpeckers in the Pacific Northwest is 10 inches (25 cm), and minimum height is 15 feet (5 m) above ground .
Only 7 of 222 nests occurred in snags at Hastings Reservation, which was significantly more than expected based on snag availability (P<0.001). Nest cavities were found in granary trees 42% of the time, which was more than expected (P <0.001) .
Nest tree species: Acorn woodpeckers nest in a variety of tree species, including oak [10,33], ponderosa pine [33,76], Fremont cottonwood , Douglas-fir [34,115,117,118], Pacific madrone (>12 inches (30 cm) DBH) [116,117], and California sycamore .
Small-diameter Gambel oak trees are utilized more often by cavity nesters than small-diameter ponderosa pine . In a study conducted in northern Arizona, acorn woodpeckers nested in Gambel oaks with a mean diameter of 15 inches (38 cm) and ponderosa pines with a mean diameter of 28 inches (71 cm). In areas where ponderosa pine is uncommon, Gambel oak may be particularly important to cavity nesters . In addition, nests may be excavated in live Gambel oak, whereas excavation is uncommon in live ponderosa pine .
In ponderosa pine-Gambel oak habitat in the Black Range, San Mateo, Magdalena, and Zuni mountains in western New Mexico, acorn woodpeckers nested in Gambel oak most often :
|Frequency of acorn woodpecker nests found in 8 tree species |
|Ponderosa pine||Narrowleaf cottonwood
Acorn woodpeckers nested in Douglas-fir and Pacific madrone trees in the Six-Rivers, Shasta-Trinity, and Klamath National Forests in California . Based on the breeding density of 8.6 pairs of acorn woodpecker/247 acres (100 ha), Raphael  estimated that 30 Pacific madrones >12 inches (30 cm) DBH/247 acres are needed to provide nesting substrate each year.
In the Willamette Valley, large-diameter, widely spaced Oregon white oaks provide more cavities for birds than large-diameter Douglas-fir trees .
Population vigor in acorn woodpeckers may be associated with nest site selection. At Hastings Reservation, nests located in California sycamore fledged more acorn woodpeckers than nests in all other tree species (valley oak, blue oak, California black oak, and coast live oak) combined (P=0.05). Mean group size of acorn woodpeckers was significantly larger when nests were located in live limbs (4.5 individuals) compared to dead limbs (3.8 individuals, P=0.01) and when nest cavities were located in California sycamore (4.8 individuals) compared to other tree species (4.0 individuals, P<0.05) .
|Granaries: Granaries are used by acorn woodpeckers for acorn storage [35,45,46,60,72,76,89,106,121,155]. There is usually 1 primary granary in an acorn woodpecker group territory, and 1 or more smaller secondary granaries [46,72,76,91,122,132]. Primary granaries are larger and contain more storage holes than secondary granaries [46,72,91,155]. Of 53 acorn woodpecker groups studied at Hastings Reservation, mean number of granaries within a territory was 2.1 (range 1 to 7) . Acorns are harvested during fall and early winter and stored in holes drilled in the granary. The same granaries are used year after year, and new storage holes are continually created [35,45,46,72]. Dead or live trees are used for granaries, as long as they contain deep, dry bark , and granaries are typically located in tree trunks or lower tree limbs . The cambium layer is rarely penetrated, so little detrimental effect to the granary tree occurs . Eventually, granary trees are lost to fire, rotting, or falling [46,72]. Granaries may also be located in utility poles, fence posts [72,91,106], pine cones [60,91], eaves and wood trim of buildings [72,91,106], and under clay roof tiles . Acorn woodpeckers in San Diego County, California, stored coast live oak acorns in Coulter pine (P. coulteri) cones, which exceeded 12 inches (30 cm) in length . In Engelmann oak and coast live oak habitat in Santa Rosa Plateau Reserve, California, acorn woodpeckers stored surplus acorns in boulders composed of porphyritic basalt .|
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Tree species: Tree species used for granaries vary widely and include oaks, pines, firs, redwoods, sycamores (Platanus spp.), and nonnative species [25,72,91]. Pines are preferred over oaks due to their softer bark [46,89,106], and thick-barked oaks, such as valley oak, are preferred over thin-barked oaks . Granary trees used by acorn woodpeckers on the grounds of Stanford University in California included nonnative Canary Island date palm (Phoenix canariensis) and California palm (Washingtonia filifera) . On the east side of the Sierra Nevada, ponderosa pine  or Jeffrey pine are used for granaries [64,120]. At the University of California's Hopland Field Station, acorn woodpeckers preferred large (>30 inches (75 cm) DBH)) deciduous oaks for granaries. Use of each tree species for acorn storage was significantly different than availability for primary and secondary (P<0.001) granary trees. Within the acorn woodpeckers' territories, Oregon white oak comprised 10% of all trees, yet almost 50% of primary granary trees and >25% of secondary granaries were located in Oregon white oak. Although blue oaks were used extensively for granary trees, use by acorn woodpeckers was less than expected based on availability .
At Hastings Reservation, acorn woodpeckers used valley oak, which had the largest diameter of all trees, most often for granaries. Other granary trees included blue oak, California sycamore, California black oak, and red willow (Salix laevigata) :
|Granary tree characteristics at Hastings Reservation |
|Average DBH (cm)||DBH range (cm)||Average height (m)||Height range (m)||Number of observations||Percentage of total|
|California black oak||62||48-76||13.5||12.0-15.0||2||2|
|Red willow||50||not given||12.0||not given||1||1|
Acorn woodpeckers collect acorns from a few trees within their territories and do not travel far to collect acorns. Transport of acorns at the Sedgwick Reserve in Santa Barbara County, California occurred within a 492 foot (150 m) radius of the oak tree. Thus, granaries within a territory contained a low number of different maternal genotypes of oaks .
Size: Granary size is an important component of territory quality because large granaries result in immediate benefits in food availability. In Water Canyon, acorn woodpeckers that used large granaries containing >3,000 holes had a 58% chance (n=55) of storing enough acorns to last through winter and into the following breeding season. Acorn woodpeckers that used granaries containing <1,000 holes had a smaller chance (9%, n= 62) of their stores lasting through the winter .
Acorn woodpeckers tend to select the largest available trees for granaries. At the University of California's Hopland Field Station, the DBH of granary trees was significantly larger than non-granary trees (P<0.001) . In oak-pine woodlands in Monterey County, California, the largest trees provided ample storage space for acorns, contained rotten limbs, and were likely to die before other trees, thus providing additional storage space. Acorn woodpeckers were observed in an area dominated by ponderosa pine, interior live oak, and canyon live oak (Plaskett Ridge), and an area dominated by sugar pine, tanoak, and canyon live oak (Cone Peak). On both sites, primary granary trees were significantly larger than non-granary trees (P-value not given). Secondary granaries were smaller. Fewer storage holes were created in trees on Plaskett Ridge than Cone Peak, probably because a fire killed many pines 5 years prior to the study, preventing optimal storage use by acorn woodpeckers. Characteristics of storage trees used by acorn woodpeckers are summarized in the table below :
|Physical characteristics of acorn woodpecker granary trees in 2 California oak-pine woodlands |
|Plaskett Ridge||Cone Peak|
|Mean DBH (cm)||Storage holes per tree||Proportion of trees dead (%)||Mean DBH (cm)||Storage holes per tree||Proportion of trees dead (%)|
|All storage trees||81||750||100||148||2,200||47|
|Largest storage tree||96||1,020||100||160||2,900||30|
The immediate area around a granary differs in structure from the surrounding habitat [61,122]. When granary plots were compared to non-granary plots in Oregon white oak woodlands in the Willamette Valley, Oregon, granary plots contained greater oak basal area, shorter shrub height, and larger-diameter granary trees. Three granary trees and 3 non-granary trees were located within each of 20 acorn woodpecker colonies. Habitat was examined within 39 feet (12 m) of granary and non-granary plots. Granaries were selected in the immediate area of high acorn production. Acorn woodpeckers may have selected granary sites with low shrub height to defend against predators and decrease competition for nesting cavities and acorns .
|Habitat characteristics within granary and non-granary plots in Willamette Valley, Oregon |
|Granary plot||Non-granary plot|
|Oak basal area (m²/ha) (SE)||50.1 (4.1)||27.2 (3.0)|
|Shrub height (SE)||18.5 cm (4.3 cm)||45.0 cm (9.5 cm)|
|DBH of granary tree (SE)||64.7 cm (4.7 cm)||53.5 cm (5.5 cm)|
Other: Not all acorn woodpeckers use granaries. At the Research Ranch and the Huachuca Mountains, some acorn woodpeckers did not live in typical family groups but as pairs (see Social organization). Permanent, year-round family groups used granary trees to store large numbers of acorns, and migratory pairs of acorn woodpeckers did not use granary trees. Migratory individuals stored some acorns under loose bark and natural crevices of oak trees and in power poles for immediate use after oaks ceased acorn production .
Average dimensions of nest cavities are: entrance hole= 2 inches (4 cm); cavity depth= 9 to 28 inches (22-70 cm); and cavity width=6 inches (15 cm) . See Nesting for specific nesting requirements.
The acorn woodpecker is omnivorous. Tree sap, insects (Hymenoptera and Coleoptera), and green and mature acorns comprise most of their diet [22,66,72,76,78,90,91,95,122,147]. For a list of insects eaten by the acorn woodpecker, see Koenig and Mumme . Other foods consumed include pinyon pine seeds [91,133], Jeffrey pine seeds , oak buds and catkins , grass seeds , and occasionally lizards, bird eggs , and bats . Olives (Oleo europaea) , cultivated nuts [53,91,96,133], corn (Zea mays), figs (Ficus carica), and wheat (Triticum aestivum) may also be eaten . Grit is collected from the ground (usually road beds) several times a day and ingested .
Foraging techniques include sapsucking, flycatching, and acorn storage, depending on time of year [22,72,89,90,122]. Sapsucking by acorn woodpeckers in California occurs from February to early March, and June to August [72,91]. During winter at Hastings Reservation, sap is extracted from the middle and upper canopies of valley oak and blue oak. During spring, acorn woodpeckers extract sap from California black oak and live oak. During June and July, sap is extracted from live oak [72,89,90,91]. Bast, which is the phloem/cambium removed from the inner bark of trees, may be eaten during winter . Sap holes are reused every year, and new holes are continually added . At Hastings Reservation, flycatching is the primary foraging method during the nesting period in April and May. Insects may also be consumed on warm winter days. Acorn woodpeckers do not usually glean insects from tree bark or drill for wood-boring insects, but flycatch from stumps, uppermost tree branches, or branches just above the grass [72,89,90,91,122]. Insects are the main food item fed to nestlings, and are either fed directly to nestlings or temporarily stored under loose bark or in tree crevices [72,89,90,91,122].
Acorn woodpeckers are "larder hoarders" and store acorns in granaries  (see Granaries). They are also considered "opters", either storing food or migrating depending on food availability . In California, acorns comprise >50% of the acorn woodpecker diet , and in Arizona and New Mexico, acorns comprise 25% to 50% of the acorn woodpecker diet . From late summer to fall, green acorns are eaten directly from trees. At Hastings Reservation, acorn woodpeckers ate green acorns directly from California black oak, valley oak, blue oak, coast live oak, and canyon live oak [72,90,91]. From September to December, acorns are stored in granaries for later use . Stored acorns are usually edible for only 1 year . Acorn woodpeckers are most dependent on stored acorns from September to March, and may use stored acorns through spring and summer if they are available [66,89,90,91]. Use of acorns during the remainder of the year depends on availability of other foods  and whether or not the granary is depleted due to a poor acorn crop . Acorns are the main food stored, but cultivated nuts, pinyon pine seeds, and gray pine seeds may also be stored [69,91,122,133]. Acorn woodpeckers in California may be able to survive winters without stored acorns, depending on availability of other foods .
Acorns are gathered by all group members for 4 to 6 weeks and stored in 1 or 2 granaries. Acorn woodpeckers prefer storing acorns in conifers due to softer bark. Storage holes are usually 0.5 inches (1.3 cm) in diameter and 1.5 inches (3.8 cm) deep. Acorns are inserted with the base facing outward and are pounded into place. As acorns dry and shrink, they must be moved to smaller holes [69,72,89]. Re-storage of acorns usually occurs for at least 6 weeks after harvest. Storage hole construction is an ongoing activity performed by all acorn woodpeckers in a group .
Due to low protein and high tannin content, acorns are a relatively poor food resource for acorn woodpeckers. Available carbohydrates and protein vary little among oak species, but lipid content varies widely and is highest in coast live oak and California black oak. Some species of oak, such as coast live oak and California black oak, contain high amounts of tannins, which reduce nutritional food value by binding protein. Despite high tannin levels, coast live oak and California black oak acorns provide the largest energetic content for acorn woodpeckers. The following table shows composition and energetic content of the acorns most commonly eaten by acorn woodpeckers at Hastings Reservation :
|Composition and energetic content of acorns eaten by acorn woodpeckers in Monterey County, California |
|Oak species||Soluble tannins (%)||Lipids (%)||Protein (%)||Total available carbohydrates (%)||Energetic content (kJ/gm)|
|Canyon live oak||0.398||16.7||3.9||12.6||11.39|
|Coast live oak||0.460||24.3||7.1||13.2||13.23|
|California black oak||0.444||26.5||5.3||14.9||14.07|
At Hastings Reservation, acorn woodpeckers stored a mixture of species of acorns, and selection of different acorn species for storage was unrelated to energetic value. Valley oak acorns were stored by acorn woodpeckers much more frequently, blue oak acorns were stored much less frequently, and coast live oak acorns were stored in roughly the same frequency as the estimated relative abundance of the trees and the estimated relative productivity of the species. The study was conducted during a year when crops of valley oak, blue oak, and coast live oak were excellent, to ensure that acorns of any of the 3 species were abundant enough to be stored in granaries. Acorn woodpeckers also preferred to store acorns that were smaller than those most available and did not discriminate based on insect parasitism .
Acorn woodpeckers store a small amount of acorns considering their energetic needs [69,72]. For example, At Hastings Reservation, 120 to 130 acorn woodpeckers stored an average of 344 acorns/bird each year. This accounted for only 6% to 7% of total yearly metabolic requirements . Acorn storage is apparently unrelated to group size .
Acorn production varies widely from year to year, species to species, and tree to tree. In California oak woodlands, 120 to 130 acorn woodpeckers collected acorns from 5 oak species. The relative proportion of the total of each species varied considerably from year to year but the total number collected was relatively constant, ranging between 42,000 and 45,000 acorns. The following table shows yearly variation in species composition of acorns stored by acorn woodpeckers :
|Yearly variation in species composition of acorns stored by acorn woodpeckers at Hastings Reservation |
Percent of total stored acorns by species
|coast live oak||valley oak||blue oak||California black oak||canyon live oak|
For more information on acorn woodpecker foraging methods and habits in Yolo County, California, see Roberts .
Defense of resources: Interspecific and intraspecific aggression within a territory is common in acorn woodpeckers [59,91,122]. Granaries are the main resource defended; however, roost and nest cavities, sap trees, and hawking perches are also defended [59,89,91,122]. Acorn woodpeckers defend resources from the following mammals and birds: eastern fox squirrel (Sciurus niger) , California ground squirrel (Spermophilus beecheyi) , white-breasted nuthatch (Sitta carolinensis), Steller's jay (Cyanocitta stelleri), oak titmouse (Baeolophus inornatus), northern flicker (Colaptes auratus), western scrub-jay (Aphelocoma californica), spotted towhee (Pipilo maculatus), Nuttall's woodpecker (Picoides nuttallii), European starling (Sturnus vulgaris), ruby-crowned kinglet (Regulus calendula) , yellow-billed magpie (Pica nuttalli), and American crow (Corvus brachyrhynchos) .
Adult acorn woodpeckers sometimes defend primary granary trees from juvenile birds, forcing younger birds to store acorns in secondary granaries until they reach adulthood .
Population trends: Yearly acorn yields [72,75,91] and number of oak species in an area [19,121] may influence population dynamics of acorn woodpeckers. Acorn woodpecker populations are typically larger in areas containing >3 oak species, due to asynchrony in acorn production [19,121] (see Stand composition/structure). Acorn crop failures may lead to "precipitous" declines in acorn woodpecker populations, and may occur every 4 to 5 years .
According to Breeding Bird Surveys conducted in western North America from 1968 to 1991, acorn woodpecker populations increased 0.9% (n=137 survey routes) [54,108,109]. However, in the Sierra Nevada, acorn woodpecker populations decreased 5.3% per year from 1966 to 1996 . Additionally, as of 1997, acorn woodpecker populations were decreasing on Breeding Bird Survey routes in managed ponderosa pine habitat in Arizona and New Mexico .
Predators of the acorn woodpecker include Cooper's hawks [91,135], sharp-shinned hawks (Accipiter striatus) , gopher snakes (Pituophis melanoleucus) , and other snakes . Red-shouldered hawks (Buteo lineatus) and red-tailed hawks (B. jamaicensis) may also prey on acorn woodpeckers .
Acorn woodpeckers influence the composition, structure, and distribution of oak woodlands . The acorn woodpecker is considered by some biologists as a keystone species because it provides tree cavities for birds and other animals that cannot create their own cavities [17,83,152]. In California, the acorn woodpecker is considered a "focal species", which is defined as a species chosen for special attention in a multi-species planning effort. Focal species may be used to guide components of conservation planning such as selection and design of habitat reserves, habitat restoration and management, and population monitoring .
Management considerations for the acorn woodpecker vary by region and vegetation type.
Oak woodlands and forests in the Pacific Northwest: Oak woodland habitats are threatened in western Washington, western Oregon, and California due to urban development, agriculture, logging, firewood cutting, and conifer encroachment from fire exclusion [47,83,93,158]. Additional threats include lack of regeneration of several key tree oak species  and sudden oak death in coast live oak, tanoak, California black oak, and Shreve oak (Q. parvula var. shrevei) in Oregon and California [27,32,38,40,41,102].
In Oregon white oak woodlands in Washington and Oregon, thinning dense stands to increase diameter and production of mast trees  may benefit acorn woodpeckers by creating potential granary trees . For oak restoration and enhancement recommendations in western Washington, see Larsen and Morgan . Wilson and others  recommend maintaining a variety of oak species in acorn woodpecker habitat to ensure acorn production at any one point in time.
Jackman  recommends the following silvicultural management in Pacific Northwest forests for all woodpecker species: 1) place tracts of land >99 acres (40 ha) on 100-year, 200-year, 300-year, or longer rotations, allowing development into mature forest with a minimum of human interference; 2) do not cut standing dead trees; 3) retain dying trees, insect infested trees, dead-topped trees, distorted or wind-broken trees, and trees with heartwood rot on precommercial and commercial thins; 4) retain slash and logs for potential foraging sites; and 5) attempt to duplicate the tree species composition of a stand following timber harvest .
Grassland and chaparral in the Pacific Northwest: Grassland and chaparral (Ceanothus spp.-Manzanita spp.) habitat have been lost and/or degraded in western Washington and western Oregon due to expanding human populations . Altman and others  recommend the following management for wildlife, including acorn woodpeckers, in these habitats: 1) inventory habitat types and their degree of modification, 2) protect high-quality sites and restore degraded habitats, and 3) manage to maintain habitat quality and wildlife populations.
Oak woodlands and oak-pine woodlands in California: Key recommendations for preserving oak woodlands in California include prioritizing the protection of sites with intact oak regeneration, encouraging the replacement of weedy annual grasses with native perennial grasses in the understory, using prescribed fire (see Habitat-related fire effects), and maintaining and enhancing natural vegetation corridors between oak woodlands and adjacent habitats . Stralberg and Williams  recommend the following management for birds in foothill oak woodlands in California: 1) preserve remaining large (>40 acres (16 ha)), undeveloped parcels of oak woodlands; 2) limit subdivisions of rural parcels into small (1.0 to 5.0 acre (0.4-2.0 ha)) ranchettes; 3) manage small parcels of oak woodlands to retain components including large trees, snags, and interior live oaks; and 4) maintain a mosaic of habitat types.
In 1978, Gutierrez and Koenig  recommended the following management in California oak-pine woodlands for acorn woodpeckers: 1) evaluate existing and potential granary trees before removing any and, based on acorn woodpecker territory size, attempt to protect or provide ≥15 primary granary trees/100 acres; 2) if 2 or more granary trees are located close to each other, remove the one with the smallest number of storage holes if removal is necessary at all; 3) maintain old, large, living trees for potential storage trees; and 4) where firewood cutting is permitted, protect granary trees.
Oak-pine woodlands in the southwestern United States: For breeding birds in southwestern ponderosa pine-Gambel oak habitat, Rosenstock  suggests providing a mix of Gambel oak growth forms by retaining and recruiting mature and old-growth oaks wherever possible and enhancing regeneration and recruitment of younger oak growth forms by using prescribed fire and other treatments.
Sudden oak death disease: Sudden oak death disease, caused by a fungus-like water mold, Phytophthora ramorum, may be a threat to acorn woodpeckers [93,102]. Sudden oak death was detected in 1995 and involves tanoaks and red oaks (Erythrobalanus), some of which are endemic to North America. Sudden oak death has caused mortality of coast live oaks in central California [40,41,102] and tanoaks in Oregon and coastal California [27,32,38]. All sizes and ages of tanoak are susceptible to leaf, branch, bole, bark, and/or root infection , and Phytophthora ramorum infection is nearly always fatal to tanoaks, although mature trees may take several years to die . In 2006, Monahan and Koenig  used 3 regression models to predict how sudden oak death disease would impact oak-dependent bird species. The predictions ranged from severe to mild effects. Results were averaged across the 3 models to provide the best current estimated effects of sudden oak death on birds. Using those values, they estimated a 50% decline in acorn woodpecker populations in coastal California .
Snags and coarse woody debris: Acorn woodpeckers use snags and coarse woody debris for storing acorns, nesting, and roosting [10,29,55,57,76,95,129] (see Preferred Habitat). The number of snags available in an area may determine density of cavity-nesting species, including acorn woodpeckers . An abundance of snags of various ages and sizes ensures nesting and foraging opportunities for woodpeckers .
Ohmann and others  suggest retaining large snags and live trees during harvesting and thinning operations for woodpecker species. Based on literature reviews, at least 5 large snags (>12 inches (30 cm DBH))/ha should be retained for cavity-nesting birds .
To ensure availability of dead and dying wood for wildlife habitat in general, Bunnell and others [23,24] suggest using a variety of practices. They recommend retaining preferred species and a range of age classes and sizes of dead wood including some large trees and snags. Trees >12 inches (30 cm) DBH in inland habitat, and conifers >20 inches (50 cm) DBH in coastal habitat would accommodate most bird species [23,24], though acorn woodpeckers tend to prefer larger trees for granaries [46,61,91] (see Granaries). Dead and dying trees can also be retained by limiting salvage logging after fire [23,24].
Grazing: Grazing by livestock may or may not affect acorn woodpeckers. In habitat dominated by gray pine, interior live oak, and blue oak in the western foothills of the Sierra Nevada, acorn woodpeckers were not affected by grazing (P=0.3359). Mean numbers of acorn woodpeckers were 4.8 birds (SD 1.8) on ungrazed plots, and 5.2 birds (SD 1.6) on grazed plots. Possible concerns related to grazing include reduction in shrub, forb, and grass cover; increased abundance of European starlings, which compete with acorn woodpecker for nest cavities; and increased abundance of brown-headed cowbirds (Molothrus ater) which may parasitize acorn woodpecker nests .
According to Zwartjes and others , acorn woodpeckers are a priority species in the southwestern United States, meaning that they should receive greater consideration than non-priority wildlife species in planning related to livestock grazing. In Arizona and New Mexico, intensive grazing by livestock in ponderosa pine woodlands contributes to suppression of low severity fires by reducing fine fuels. Lack of fire encourages shrub growth and conifer encroachment. Dog-hair thickets of young conifers increase the risk of severe fire that could kill mature ponderosa pines. Livestock may also browse deciduous tree seedlings, which affects long-term recruitment of large trees  and may ultimately reduce acorn woodpecker habitat.
Other: In a study conducted in Santa Clara Valley, California, acorn woodpeckers were sensitive to urbanization due to either a shortage of mature trees containing cavities or competition with European starlings for tree cavities . European starlings may compete with acorn woodpeckers for tree cavities; however, acorn woodpeckers may be capable of defending nest holes due to their communal nature . In southern Arizona and New Mexico, acorn woodpeckers provide elf owls (Micrathene whitneyi) , purple martins (Progne subis) [95,129,154], western bluebirds (Sialia mexicana) , and violet-green swallows (Tachycineta thalassina) with nest cavities .
Acorn woodpeckers are sensitive to firewood cutting and rangeland modification in blue oak woodlands in the northern Sacramento Valley, California. Sensitivity was high only when "substantial" differences occurred between cut and uncut woodland conditions .
In coast redwood forests in northern California, acorn woodpeckers use "legacy trees"
. Legacy trees are defined by Mazurek and Zielinski  as old-growth trees
that have been spared during harvest or have survived stand-replacing natural
disturbances. Due to the rarity of legacy trees, it is recommended that their
locations be determined and that they be protected from logging or other physical
HABITAT-RELATED FIRE EFFECTS: In general, birds are usually favored by successional diversity and new growth of food and shelter following fire . The ability of birds to occupy a site after fire depends on pre- and postfire management activities such as logging. Snag-nesting species typically increase following fire and are influenced by postfire snag availability .
Acorn woodpeckers are adapted to habitat with recurring fires of varying severity [39,105,142], which create a mosaic of habitat types [111,112,157]. Acorn woodpecker abundance appears to increase several years following fire [18,62], probably due to an increase in acorn production . Following fire in ponderosa pine habitat, acorn woodpecker use may increase in response to increases in insect populations .
Prior to European settlement in lowlands and foothills of the Pacific Northwest, wildfires were low-to moderate-severity, with short fire-return intervals [1,58,63,85,148]. Frequent, low-severity fires maintained acorn woodpecker habitat by reducing conifers and grasses, initiating sprouting of oaks, reducing fuel loads, and preventing loss of mature oak trees [1,16,52,85,86,101]. Fire exclusion became policy in the Pacific Northwest in the early 1900s , resulting in longer fire-return intervals in lowlands and foothills . Without fire, open-canopy oak savannas become dense oak woodlands, and eventually, conifer forests [1,52]. Consequently, bird species composition has been affected by the encroachment of conifers and nonnative shrubs [43,47,151].
In oak-pine woodlands and ponderosa pine forests in the southwestern United States, low- to moderate-severity wildfires occurred at about 1 fire/decade prior to European settlement [4,36,39,142]. These frequently-occurring fires thinned smaller trees, reduced surface fuel, and invigorated understory vegetation [4,36,136]. According to Pyne  and Moir and others , low-severity fire is one of the most important natural disturbances in southwestern ponderosa pine forests.
Response to fire varies in oaks. Oregon white oak, California black oak, valley oak, coast live oak, and Gambel oak are fire-resistant species, surviving low- to moderate-severity fires [1,3,37,43,44,100,110,137,140,141]. Therefore, acorn woodpeckers in habitat dominated by these species are probably negatively impacted only by severe fire. After high-severity fire, oaks may be completely killed and some may not produce acorns for decades [26,107]. Blue oak is less insulated against fire than other oaks due to thin bark, which tends to flake off as the tree ages . Mature blue oak is resistant to top-kill by low-severity surface fires and most moderate-severity surface fires, but is top-killed or killed by severe fires or the sustained heat of most chaparral fires [99,110]. Interior live oak is sensitive to fire  but readily resprouts . For more information about fire and oaks used by the acorn woodpecker, see the FEIS reviews on oak species (Oregon white oak, California black oak, valley oak, coast live oak, Gambel oak, blue oak, and interior live oak).
Data on acorn woodpecker response to wildfire are sparse. In 3 studies in the southwestern United States, acorn woodpeckers were not detected in unburned habitats, but were detected at least 2 years following wildfire [18,62]. Three and 4 years following the Horseshoe and Hockderffer wildfires in the Coconino National Forest, Arizona, acorn woodpeckers were detected only during the breeding season and only in severely burned areas. Habitat was dominated by ponderosa pine. Details about the wildfire, such as size and severity, were not given :
In the Prescott National Forest, Arizona, acorn woodpeckers were not detected following wildfire in either unlogged or logged burned areas during postfire years 1 and 2. They were detected, however, during spring and fall in an unburned, 5-year-old clearcut. The wildfire burned a total of 29,650 acres (12,000 ha). Within the burn perimeter, there were 11,860 acres (4,800 ha) of ponderosa pine. One-half of the 11,860 acres of the ponderosa pine forest was completely killed. Within the burned area, uncut, partially cut, and clearcut sites were examined during postfire years 1 and 2. Three unburned sites were selected that most closely matched the burned sites with respect to logging. Overall, shrub live oak (Q. turbinella) and Gambel oak were more abundant on burned sites. In the unburned, 5-year-old clearcut site where acorn woodpeckers were found, shrub diversity was greater compared to burned sites. Gambel oak was present in the clearcut, and density of Gambel oak slightly surpassed density of ponderosa pine .
Acorn woodpeckers were detected 4 years after the La Mesa Wildfire in Bandelier
National Monument, New Mexico. Prior to the fire, acorn woodpeckers were not detected
in bird surveys. They were first detected on burned transects in postfire year 6, and
detections increased by postfire year 14. Three transects (Apache, Escobas, and
Burnt) were located in burned areas of mixed-conifer, ponderosa pine-mixed-conifer,
and ponderosa pine/pinyon-juniper habitats. One transect (Frijoles) was located in unburned
pinyon-juniper habitat. On the 3 burned transects, vegetation was killed by
crown fire or scorching on 25% to 80% of the area. No postfire logging occurred. Bird surveys
were conducted 1 breeding season before the fire and in postfire years 1, 2, 4, 6, and 14.
In this habitat, acorn production resumed 4 years after fire, favoring acorn woodpeckers. The following table shows the number of detections of acorn woodpeckers before and after the wildfire :
|Number of detections of acorn woodpeckers/40-ha transects (SE) before and after the La Mesa Wildfire, Bandelier National Monument, New Mexico |
|Transect name||Percentage of area burned by high-severity fire||Prefire year 1||Postfire year 1||Postfire year 2||Postfire year 4||Postfire year 6||Postfire year 14|
|Escobas||33||0||0||0||0||2 (1)||13 (3)|
|Burnt||80||0||0||0||0||4 (2)||4 (1)|
|Frijoles (unburned)||0||0||0||0.3 (0.3)||0||0||0|
In the San Dimas Experimental Forest, California, acorn woodpeckers were occasionally detected in burned grassland (grass species not specified) at 3,000 feet (914 m) but were not detected in unburned grassland or nearby burned or unburned chaparral. Chaparral habitat was dominated by chamise on south-facing slopes. In mesic environments, chaparral habitat was dominated by chamise, ceanothus (Ceanothus spp.), and Nutall's scrub oak (Q. dumosa). Fire severities were not given .
Frequent fire may not be essential in acorn woodpecker habitat. In foothill oak-pine woodlands in the San Joaquin Experimental Range in Madera County, California, acorn woodpeckers preferred nesting in areas containing a high density of logs (1.5% cover of logs ≥ 2 inches (5 cm) diameter). Habitat was dominated by blue oak, interior live oak, and foothill pine. Study areas had been lightly to moderately grazed by livestock for 100 years and lightning fires had been suppressed before they reached 10 acres (4 ha) in size [111,112].
The following table provides fire regime information on vegetation communities in which
acorn woodpeckers may occur, based on the habitat characteristics and
species composition of communities acorn woodpeckers are known to
occupy. There is not conclusive evidence that acorn woodpeckers occur in all of the
habitat types listed, and some community types, especially those used rarely, may have
|Fire regime information on vegetation communities in which acorn woodpecker may occur. For each community, fire regime characteristics are taken from the LANDFIRE Rapid Assessment Vegetation Models . These vegetation models were developed by local experts using available literature, local data, and/or expert opinion as documented in the PDF file linked from the name of each Potential Natural Vegetation Group listed below. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.|
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|Oregon white oak-ponderosa pine||Replacement||16%||125||100||300|
|Surface or low||81%||25||5||30|
|Pine savannah (ultramafic)||Replacement||7%||200||100||300|
|Surface or low||93%||15||10||20|
|Surface or low||78%||13|
|Oregon white oak||Replacement||3%||275|
|Surface or low||78%||12.5|
|Douglas-fir (Willamette Valley foothills)||Replacement||18%||150||100||400|
|Surface or low||53%||50||20||80|
|Oregon coastal tanoak||Replacement||10%||250|
|Mixed conifer (southwestern Oregon)||Replacement||4%||400|
|Surface or low||67%||22|
|California mixed evergreen (northern California)||Replacement||6%||150||100||200|
|Surface or low||64%||15||5||30|
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|California oak woodlands||Replacement||8%||120|
|Surface or low||91%||10|
|Surface or low||78%||13|
|California mixed evergreen||Replacement||10%||140||65||700|
|Surface or low||32%||45||7|
|Surface or low||98%||20|
|Mixed conifer (North Slopes)||Replacement||5%||250|
|Surface or low||88%||15||10||40|
|Mixed conifer (South Slopes)||Replacement||4%||200|
|Surface or low||80%||10|
|Surface or low||74%||30|
|Mixed evergreen-bigcone Douglas-fir (southern coastal)||Replacement||29%||250|
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|Desert grassland with shrubs and trees||Replacement||85%||12|
|Plains mesa grassland with shrubs or trees||Replacement||76%||20|
|Montane and subalpine grasslands with shrubs or trees||Replacement||30%||70||10||100|
|Surface or low||70%||30|
|Southwestern shrub steppe with trees||Replacement||52%||17||10||25|
|Surface or low||25%||35||25||100|
|Interior Arizona chaparral||Replacement||100%||125||60||150|
|Madrean oak-conifer woodland||Replacement||16%||65||25|
|Surface or low||76%||14||1||20|
|Pinyon-juniper (mixed fire regime)||Replacement||29%||430|
|Surface or low||6%||>1,000|
|Pinyon-juniper (rare replacement fire regime)||Replacement||76%||526|
|Surface or low||4%||>1,000|
|Ponderosa pine/grassland (Southwest)||Replacement||3%||300|
|Surface or low||97%||10|
|Riparian forest with conifers||Replacement||100%||435||300||550|
|Riparian deciduous woodland||Replacement||50%||110||15||200|
|Surface or low||30%||180||10|
|Ponderosa pine-Gambel oak (southern Rockies and Southwest)||Replacement||8%||300|
|Surface or low||92%||25||10||30|
|Southwest mixed conifer (warm, dry with aspen)||Replacement||7%||300|
|Surface or low||80%||25||2||70|
|Southwest mixed conifer (cool, moist with aspen)||Replacement||29%||200||80||200|
|Surface or low||36%||160||10|
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|South-central US Grassland|
|Surface or low||93%||3||1||4|
Replacement=Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed=Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects
Surface or low=Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area. [50,80].
Prescribed burning provides habitat diversity for wildlife in some plant communities . In oak woodlands in the Pacific Northwest, prescribed burning has been used for bird conservation by reducing conifer encroachment, stimulating oak seedling recruitment, and creating multi-aged stands [47,58,153]. Limited data is available on acorn woodpecker response to prescribed burning. In the following 2 studies in California and Arizona, acorn woodpeckers declined slightly following low- to moderate-severity prescribed burns [57,84].
Abundance of acorn woodpeckers remained relatively stable 2 to 3 years following
prescribed fires in oak woodlands and oak-chaparral habitat in California
[84,153]. In blue oak-coast live oak woodlands in central-coastal
California, relative abundance of acorn woodpeckers did not change following low- to
moderate-severity prescribed fires. The authors suggest that fall prescribed fires are
likely to have less negative impacts on resident breeding birds in California than
winter prescribed burns because breeding may commence during or after the initiation
of habitat rejuvenation . Similarly, the number of acorn woodpecker
pairs remained relatively stable before and after a prescribed fire in oak-chaparral
woodlands in Kern County, California. Habitat was dominated by blue oak, gray pine,
interior live oak, and wedgeleaf ceanothus (Ceanothus cuneatus). Severity of
the prescribed burn was not described, but the fire generated temperatures
of 156 °F (69 °C) at a depth of 2 inches (0.8 cm) below ground. Acorn woodpecker densities
were compared on a 20 acre (8 ha) plot before the prescribed burn and for 3 years
afterwards. The following table shows a slight decrease in occurrence of acorn woodpecker
pairs in postfire year 3 :
|Occurrence (number of pairs) of acorn woodpeckers before and after prescribed fires in Kern County, California |
|Prefire||Postfire year 1||Postfire year 2||Postfire year 3|
In habitat dominated by Arizona pine on the Coronado National Forest, acorn woodpecker density declined slightly on burned sites. See the Research Project Summary for more details .
Large snags are important habitat features for the acorn woodpecker [10,29,55,57,76,95,129] (see Snags and coarse woody debris). Prescribed fire may either consume or create snags [11,57,157], and generally benefits acorn woodpeckers. In a mixed-conifer forest dominated by ponderosa pine in the Sierra National Forest, California, the greatest loss of preferred snags (ponderosa pines) used by acorn woodpeckers occurred after the first introduction of spring prescribed fire after a long fire-free period (71 years). Snag loss was 34% after the 1st burn and 15% after a 2nd prescribed burn 5 years later, but only 8% on control sites. Initial snag loss was offset by the creation of new snags . Occasional severe fires in oak woodlands may create snags while leaving most large oaks alive, depending on species of oak  (see Habitat-related fire effects).
Large snags should be protected for acorn woodpeckers during prescribed burning when possible [11,57]. In a mixed-conifer forest dominated by ponderosa pine in the Sierra National Forest, California, Bagne  recommend protection of ponderosa pine snags, especially during the first application of prescribed fire in fire-suppressed areas. In Arizona pine forests, Horton and Mannan  recommend protection of large Arizona pine snags (>20 inches (50 cm DBH) by constructing fuel breaks around the trees before conducting prescribed underburns in forests where large snags have been reduced by logging. Salvage logging would likely decrease the density of large snags used for nest cavities [23,24,48]. Large live trees are important to acorn woodpeckers for nesting, roosting, and for granaries [46,60,75,155] (see Preferred Habitat), and may also require protection during prescribed burning.
Specific fire management recommendations for the acorn woodpecker are scarce. Generally, low-severity burning of understory vegetation may be beneficial in most acorn woodpecker habitat (oak woodlands, pine-oak woodlands, and ponderosa pine forests) to decrease conifer encroachment, initiate sprouting of oaks, reduce fuel loads, and prevent loss of mature oak trees [1,4,16,36,39,47,47,52,58,85,86,101,142,153]. According to Huff and others , benefits of prescribed fire to birds are uncertain in oak woodlands and savannas dominated by Oregon white oak or California black oak due to human alteration such as livestock grazing . Fire management considerations for the acorn woodpecker are not available for riparian habitat in the southwestern United States.
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