More info for the terms: association, avoidance, basal area, competition, cover, density, forbs, frequency, importance value, locus, mast, natural, nonnative species, prescribed fire, relative density, relative dominance, relative frequency, selection, shrub, shrubs, snag, tree, wildfire
Acorn woodpeckers are oak woodland specialists [93]. Their demography is limited by acorn productivity and the ability to store mast [76]. 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 [76].
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 [101] 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 [72].
Acorn woodpeckers are generally found in habitat containing more than one oak species [76]. 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 [123]. 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 [124].
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 [147]:
Composition of stands in preferred acorn woodpecker habitat in Coconino National Forest, Arizona [147] Relative density Relative dominance Relative frequency Importance value Absolute density (trees/ha) Ponderosa pine 86.8 91.9 74.3 253.0 59.7 Gambel oak 13.2 8.1 25.7 47.0 9.0
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 [118]:
Density of acorn woodpeckers in 3 successional stages of Douglas-fir forest in northern California [118]
Successional stage
Brush/sapling Pole/sawtimber Mature 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 [118].
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 [115].
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. Ã 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 [155].
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 [34].
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 [143]:
Stand characteristics of logged areas and associated breeding densities of acorn woodpeckers in the Coconino National Forest, Arizona [143] Plot Acorn woodpeckers (pairs/40 ha) detected in 1975 Treatment year Tree density (trees/ha) Canopy volume (m³/ha) Total basal area
(m²/ha) Mean tree height
(m) Control 0 n/a 646 19,370 26.7 15.5 Silviculturally cut 0 1974 236 17,039 23.3 13.2 Strip cut 0 1970 181 6,526 12.4 11.5 Severely thinned 3 1969 69 3,990 7.9 11.0 Clearcut 0 1967 0 0 0 0
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 [155].
The studies cited above indicate acorn woodpecker preference for open stands, but Raphael and others [118] 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 [121] 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 [19] 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 [70] field research at the Hastings Reservation. Koenig and Haydock [70] 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) [121], while Bock and Bock's [19] 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 [139]:
Riparian habitats used by acorn woodpeckers, Huachuca Mountains, Arizona [139] Dominant tree species
Stand size
(small ≤200 m) (large ≥1,000 m)
Upland vegetation Velvet ash large wooded Fremont cottonwood large wooded small wooded open Desert willow large wooded small wooded Arizona sycamore large wooded small wooded open Arizona walnut large wooded open small open Willow small open
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 [121]. Burgess and others [25] found that, in contrast to Roberts' [121] 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 [25].
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 [72]. 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 [76].
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 [55]. 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 [10]. 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 [57]. 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)) [129].
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 [10]:
Frequency of acorn woodpecker nests found in snags, dead portions of trees, and live trees (n=15 nests) [10] Snags Dead portions of trees Live trees 5 2 9
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 [13].
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) [55].
Nest tree species: Acorn woodpeckers nest in a variety of tree species, including oak [10,33], ponderosa pine [33,76], Fremont cottonwood [57], Douglas-fir [34,115,117,118], Pacific madrone (>12 inches (30 cm) DBH) [116,117], and California sycamore [55].
Small-diameter Gambel oak trees are utilized more often by cavity nesters than small-diameter ponderosa pine [33]. 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 [33]. In addition, nests may be excavated in live Gambel oak, whereas excavation is uncommon in live ponderosa pine [123].
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 [10]:
Frequency of acorn woodpecker nests found in 8 tree species [10] Ponderosa pine Narrowleaf cottonwood
(Populus angustifolia) Quaking aspen
(Populus tremuloides) Gambel oak Other* 4 1 1 9 0 *pinyon pine (Pinus edulis), southwestern white pine (Pinus strobiformis), boxelder (Acer negundo), and Douglas-fir
Acorn woodpeckers nested in Douglas-fir and Pacific madrone trees in the Six-Rivers, Shasta-Trinity, and Klamath National Forests in California [117]. Based on the breeding density of 8.6 pairs of acorn woodpecker/247 acres (100 ha), Raphael [117] 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 [47].
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) [55].
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) [91]. 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 [60], and granaries are typically located in tree trunks or lower tree limbs [76]. The cambium layer is rarely penetrated, so little detrimental effect to the granary tree occurs [106]. 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 [91]. 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 [60]. 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 [91]. Granary tree
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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 [106]. 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) [25]. On the east side of the Sierra Nevada, ponderosa pine [53] 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 [155].
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) [91]:
Granary tree characteristics at Hastings Reservation [91]
Tree species
Average DBH (cm) DBH range (cm) Average height (m) Height range (m) Number of observations Percentage of total Valley oak 99 28-153 14.5 2.4-24.0 88 78 Blue oak 65 28-90 11.5 3.05-18.3 17 15 California sycamore 74 36-128 16.4 9.6-24.0 5 4 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 [45].
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 [75].
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) [155]. 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 [46]:
Physical characteristics of acorn woodpecker granary trees in 2 California oak-pine woodlands [46] 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 [61].
Habitat characteristics within granary and non-granary plots in Willamette Valley, Oregon [61] 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 [132].