California Partners in Flight Corinna Lu October 1998

 SPECIES: Acorn woodpecker (melanerpes formicivorus)

 STATUS: No special status

 MANAGEMENT STATUS: No special status

 range map:

 I. Historical distribution and abundance:

 II. Current breeding distribution: The acorn woodpecker breeds from western North America to northern South America. This includes Baja California, with an isolated population in southern Baja California in the Laguna Mountain-Cape region, Mexico, Belize, Guatamala, El Salvador, Honduras, Nicaragua and Costa Rica. In the United States, they breed in Arizona, New Mexico and western Texas as well as most of California westof the Sierra Nevada mountain range. In California, Acorn woodpeckers range from sea level to over 2000 meters in elevation, the limits of oak distribution. (MacRoberts and MacRoberts,1976)

 They prefer savanna, oak woodland and montane forests with oaks FOR CALIFORNIA, BE MORE SPECIFIC In California, acorn woodpeckers range from sea level to over 2000 meters in elevation, the limits of oak distribution (MacRoberts and MacRoberts 1970). It is present in the western Sierra Nevada foothills, Coast Ranges, Klamatch Range, and in the eastern Sierra Nevada from Modoc County (Ziener, Laudenslayer, Jr., Mayer and White, 1990).

 ECOLOGY: Please be as specific as possible in regards to bioregion (when possible)

 I. Average territory size: MacRoberts and MacRoberts (1976) found average territory SIZE in central coastal California from 3.5 to 9 ha, with an average of 6 ha, while Swearingen (1977) found the average territory size in the same area to be 2.4 ha, with a variation of 0.9 to 3.0 ha. In the Central Valley, he reports average territory size to be 4.7 ha with variation of 1.5 to 8.1 ha.

 Trail (1980) found that resource availability between years significantly affects group and territory size.

Roberts (1979) found that territory size is not strongly correlated to group size. A positive rank correlation was found between group size and territory size by MacRoberts and MacRoberts (1976), but the correlation was not always consistant and did not hold true for small groups.

[The average distance to the nearest neighboring territory in California was found to be 127 miles (Roberts, 1979).]

II. Time of occurrence and seasonal movements The acorn woodpecker does not normally migrate. Occasionally, they will search out new territory nearby when the acorn crops fail. Exceptions include populations in Arizona, where the acorn resources are not adequate to support a winter population. The populations generally do not build granaries but store acorns in natural crevices. As acorn production ceases and resources dwindle, the birds migrate, returning in the spring.

A. Arrival date on breeding grounds: Not applicable

B. Departure date from breeding ground: Not applicable

C. Spring migration period: Not applicable

D. Fall migration period: Not applicable

E. Extent of wintering in California: Acorn woodpeckers do not migrate in California.


III. Migration stop-over needs/characteristics: Not applicable.

IV. Nest type: The acorn woodpecker is a cavity nester, with nests generally in the same tree that serves as the main storage tree, or granary.

V. Foraging strategy: Sap, buds, catkins, insect, and acorns are important sources of food for adult birds. They tend to catch insects on the wing, and not bore into trees. Acorn woodpeckers almost always catch only one insect per flight and never forage for insects on the ground or in low brush.

VI. Displays: Short (1982) has observed a Bowing Display, or up and down movement of the head, together with Yacup Calls, followed by a Hunched Posture, or an "extreme lowering of the head and bill and hunching of the back (showing off the crown patch)." Wind Spreading Displays were observed when other birds were near each other and were "used aggressively toward intruding Acorn woodpeckers."

VII. Social Organization: The Acorn woodpecker is a cooperatively breeding, polygynandrous bird that lives in social groups of two to 15 individuals. Traditional pairing is rare but has been known to occur, particularly if a lack of resources make cooperative breeding inefficient (Stacey and Bock, 1978). The groups generally contain one to four breeding males, one to two breeding females and zero to fifteen nonbreeding offspring (MacRoberts and MacRoberts, 1976). There is nearly always a familial relationship between the birds in one group. The social organization of this bird is inextricably linked with acorns because they allow the population to remain non-migratory. Within migratory acorn woodpecker populations, there are only traditional monogramous pairings with no fidelity from year to year (Stacey and Bock, 1978).

A. Typical breeding densities: No data.

Note: Roberts (1979) states that the average distance to the nearest neighboring territory was 127 miles. That seems a little strange to me, so I didn't put it in. I wasn't able to find any other figures for breeding densities, though.

B. Mating System: The Acorn woodpecker is a communally breeding bird and most commonly breed in groups of 2 to 15 on a communal territory. Koenig and Mumme (1987) state that at the Hastings Natural Reservation in central coastal California, a variety of breeding groups are present including groups with one male-female breeding pair, one breeding male and 2 breeding females (polygynous trio), groups with 3 breeding males and one breeding female (polyandrous quartet), and groups with 3 breeding males and 2 breeding females (polygynandrous quintet).

 C. Delayed breeding: Nonbreeding birds in groups are often physiologically capable of breeding but do not begin to breed for several years. Koenig and Stacey (1990) state that reproductive success can be maximized by remaining in a high quality territory even as a nonbreeder than to leave the group early and breed in a lower quality territory.

 D. Post fledging biology of offspring: Offspring fledge after 30-32 days and are idependent at two months. They are fed insects for the first 2 weeks, then begin feeding on sap and flycatching for themselves. It takes young juvenile birds approximately six weeks to become adept at handling and cracking acorns.

 E. Post breeding social behavior: No data.

 VIII. Clutch Size: Mean clutch size of single females ranges from 3.7 to 4.9 eggs in the Hastings Natural Reservation in central coastal California. Mean set size (number of eggs laid in all nests in a group, including groups of more than one female) in the Hastings Natural Reservation ranges from 3.7 to 5.3 eggs (Koenig and Mumme, 1987). Ritter (1938) reported clutch sizes ranging from three to seven eggs, and has observed up to seventeen. This number does not reflect the number laid by one female, but by all the breeding females in the group.

 IX. Incubating sex (female/male): Mumme, R.L, W.D. Koenig, and F.A. Piltelka (1990) found no significant difference between males and females in incubation attendance, but did find significant difference between males and females in other aspects of nest care such as brooding, hourly feeding rate per nest and per nestling, and percentage of feedings in which fecal sacs were removed. On the whole, females spend more time caring for the young than males, and breeders spend more time caring for the young than non-breeders.

 X. Incubation period: 11-14 days.

 XI. Nestling period: 2 months.

 XII. Development at hatching: No data.

 XIII. Number of broods: Bent (1939) states that acorn woodpeckers have from one to three broods per year.

 XIV. Who tends the young: Within a group, both male and female breeders and nonbreeders tend the young, although breeders spend more time tending the young than nonbreeders and femals spend more time than males. The nonbreeding helpers are usually offspring of the breeders.

 XV. Diet:

 A. Major food items: Acorn woodpeckers feed on stored, immature and mature green acorns, sap, and flying insects, as well as occasional fruits and eggs of other birds. Proportions of these food items change depending on the season. MacRoberts (1970) states that at Hastings Natural History Reservation in central coastal California, flycatching is "undoubtedly the major foraging method of these birds throughout April and May, only to be replaced in importance by sapsucking in June and July."

 Acorn woodpeckers store acorns in granaries, utilizing this resource in the winter when other food items are scarce. They prefer standing snags to living trees because the wood is softer. Similiarly, they are usually in soft trees such as pines, eucalyptus and sycamores (Ritter, 1938) but it is not uncommon for them to be in oak trees. Telephones, fence posts and even trim on houses have also been used (MacRoberts and MacRoberts, 1976; Ritter, 1938). There is at least one granary per territory and as many as seven have been found on the Hastings Preserve (MacRoberts and MacRoberts, 1976).

 Acorns play an important role in the ecology of the acorn woodpecker due to the fact that they provide a steady energy supply for the birds in the winter, allowing them to remain nonmigratory. Despite this, only 4.6% to 16% of the bird's energy in the winter comes from stored acorns (Koenig and Mumme, 1987; Stacey and Koenig, 1990). One exception is the population in Colombia, where acorn woodpeckers have been observed living year-round in communally breeding groups although they do not store acorns or rely heavily on them (Kattan, 1988). Instead, their main diet year-round seems to be insects, saps and fruit.

 A. Drinking: The acorn woodpecker needs drinking water daily and will cross territory boundries to obtain it.

 XVI. Site Fidelity: Territories boundaries remain the same even when birds new to the area occupied it (MacRoberts and MacRoberts, 1974, Stacy and Ligon, 1987). This implies that the resources within the area are very specific and closely match the needs of a certain group size.

XVII. Timing of Breeding: The breeding season at the Hastings Natural History Preserve is from April to June, with the majority of nests begun between April 15th and May 15th (Koenig and Mumme, 1987). Fall breeding also occurs from August to September, although rare. Breeding in southern Baja California begins in June to October, in Arizona from April until August, in Mexico, from May to October, and in Costa Rica, from March to August (Short, 1982).



Acorn woodpeckers are most commonly found in savanna, oak woodland and montane forests with oaks. Hurley, Robertson, Brougher, and Palmer (1981) state that the prime habitat type needed in order to increase population levels and densities, extend existing ranges, or move into heretofore unoccupied areas consists of blue oak savanna at the pole/medium tree stage or large tree stage with less than 40% canopy cover, digger pine/oak at the pole/medium or large tree stage with less than 40% canopy cover, or black oak woodland at the pole/medium tree stage or large tree stage with less than 40% canopy cover and with two or more species of oak present. Suitable stand structures are 1 to 2 acres of large oak/pine surrounded by open oak or oak/conifer stands. Trail (1980) found a statistically unsignificant trend towards territories in pine/oak woodlands to be larger and contain larger granaries than oak-juniper or oak woodland territories.

Roberts (1979) found that there were significant mean differences between the area with the most activity (central circle) and surrounding areas in the territory (peripheral circles). The central circle had more tre species although not more oak species, a higher percent canopy cover, and "more trees of moderate to (especially) large size." However, "the fundemental difference between central and peripheral circles is that each central circle includes the storage tree of a woodpecker group, which generally also contains the roosting and nesting cavities for the group. This means that acorn woodpeckers are associated with a subset of their habitat that is significantly different from the overall habitat."

 I. Nest Site

A. Substrate (species): Often, a large tree that serves as the granary in a group is also the nest site. These trees are normally large with broad canopy cover. At Hastings Reserve, the granary/nesting tree was found to be the largest tree in the area (Gutierrez and Koenig, 1978). Softwoods such as pines are preferred, but nests and granaries are also found in oak. In New Mexico, cottonwoods, oaks and dead pine are used (Stacey, 1979).

B. Height of nest: Short (1982) reports that "nest site varies greatly in height and in choice of trees but is virtually always in a dead stub or dead tree. …usually the nest is more than 4 meters above the ground. Ziener, Laudenslayer, Jr., Mayer and White (1990) states that nest height varies from 1.8 to 30 meters.

C. Height of plant: No data

D. Objects/Plants concealing nest: No data

E. Percent nest cover: No data

F. Average nest tree DBH: No data


II. Vegetation surrounding the nest: measurements from 0.04 ha (0.1 acre) plots

A. Canopy cover: Canopy cover between 30% and 70% was reported as ideal for blue oak savanna habitat on the west slope of the Sieera Nevada mountain range in Fresno County, California (PG&E habitat suitability index model, 1986).

B. Average top canopy height: No data

C. Dominant plant species in canopy: No data

D. Average shrub cover: No data

E. Co-dominant plant species in canopy: No data

F. Dominant shrub species: No data

G. Co-dominant shrub species: No data

H. Average forb cover: No data

I. Dominant forb species: No data

J. Co-dominant forb species: No data

K. Ground cover: No data.

1. logs: No data

2. grass/sedge: No data

3. water: No data

4. leaf litter: No data

5. rock: No data

6. bare ground: No data

7. other:

L. Slope: No data

M. Aspect: No data

N. Tree DBH: No data.

O. Snags: Snags are often used as granaries. The holes are normally drilled into the bark only and not the wood.

P. Distance to water: Less than 0.25 miles was reported as ideal by Hurley, Robertson, Brougher and Palmer (1981).

 III. Landscape factors

A. Elevation: Less than 3000 feet is ideal (Hurley, Robertson, Brougher, and Palmer, 1981).

B. Fragmentation: No data.

C. Patch size: Hurley, Robertson, Brougher, and Palmer(1981) report that a territory size of greater than 15 acres is ideal.

D. Disturbance (natural or managed): No data.

E. Adjacent land use: No data.

F. Climate: No data.

G. Other: No data.


IV. Notes: An oak species diversity of greater that four would increase the population numbers of acorn woodpeckers in an area (Hurley, Robertson, Brougher, and Palmer1981). Bock and Bock (1974) found that the northern limits of the acorn woodpecker in southwestern Arizona were determined by the diversity of oaks present and not the limits of the Quercus genus in general. This is not the case in for the entire distribution of acorn woodpeckers. In California, Roberts (1979) found no correlation between woodpecker density and oak species diversity, although he also did in Arizona. Similarly, Koenig and Mumme (1978) found no correlation in the Hastings Reserve in Monterey County.


SPECIAL FACTORS: Factors influencing occurrence and viability.

 I. Brood parasitism:

No data.

 II. Dietary:

No data.

 III. Sensitivity to human-induced disturbance: Acorn woodpeckers seem to be unaffected by human-induced disturbance and need only an adequate concentration of oak trees to support a group of birds (Lu, 1996,unpublished thesis)

 IV. Pesticide use: No data

 V. Predators: In the Hastings Natural History Reservation in the Santa Lucia Mountains, Monterey County, California, the main predators are Cooper's and Sharp-shinned Hawks (Accipiter cooperi and Accipiter striatus). Nocturnal predators most often contribute to nesting failure and include the northern pygmy owl, gopher snakes and long-tailed weasels (Mustela frenata) (MacRoberts and MacRoberts, 1978; Koenig and Mumme, 1989). Short (1982) also reports that Lewis's woodpecker occasionally competes for acorn stores, although usually acorn woodpeckers are able to overpower the bird.

 VI. Exotic species invasion/encroachment: No data

 VII. Other/Notes:



 MANAGEMENT ISSUES AND OPTIONS: Verner and Boss (1980) report that the decline and ultimate elimination of oaks could threaten the existence of this species in California.


 ASSOCIATED BIRD SPECIES: A list of other species that would benefit from management of the target species. This section to be completed after the above sections have been compiled.