Acorn Woodpecker

Melanerpes formicivorus

Order:
Piciformes
Family:
Picidae
Sections

Behavior

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Acorn Woodpecker in flight.

Flight is usually undulating with dips, during which the wings are partly closed and pressed against the sides of the body, interspersed with short periods of flapping and an upward sweep before alighting.

© Nancy Christensen , California , United States , 29 October 2017
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Acorn Woodpecker in flight.
© Scott Logan , California , United States , 5 April 2019
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Acorn Woodpecker sunbathing.

Sunbathes on warm, sunny days; birds sit with their wings partially spread out and apparently fall asleep for up to several minutes at a time.

© Alison Davies , California , United States , 9 December 2018
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Acorn Woodpecker bathing.
© Brooke Miller , California , United States , 24 July 2019
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Acorn Woodpeckers defending nest cavity.
© Dick Dionne , Arizona , United States , 20 February 2009
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Acorn Woodpeckers interacting.
© David Sutton , California , United States , 24 October 2018
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Acorn Woodpeckers interacting.
© Lee Friedman , California , United States , 1 May 2018
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Group of Acorn Woodpeckers.

Highly social, usually lives year-round in cohesive social units.

© Bruce Mast , California , United States , 9 June 2018

Locomotion

Walking, Hopping, Climbing, etc.

Birds hop readily up limbs and around trunks of trees, using tail as a prop. Walking does not occur.

Flight

As in most woodpeckers, flight is usually undulating with dips, during which the wings are partly closed and pressed against the sides of the body, interspersed with short periods of flapping and an upward sweep before alighting (9). However, this trait is not as pronounced as in many other woodpeckers and may even be absent, at least in some areas (67).

Self-Maintenance

Preens commonly; one adult female radio-tracked at Hastings Reservation while feeding nestlings preened 7.6% of 14 daylight hours (19). Does not allopreen. Sunbathes on warm, sunny days; birds sit with their wings partially spread out and apparently fall asleep for up to several minutes at a time. Birds regularly bathe when suitable water sources are available. Anting not recorded.

Agonistic Behavior

General

Within-group agonistic behavior is relatively rare, except at the nest when birds will squabble, particularly prior to and during egg-laying, and indirectly among joint-nesting females that engage in egg tossing (see Sexual Behavior: Mating System). Dominance relationships are established early among siblings, possibly within the nest (97) and limited evidence exists that these relationships are maintained as adults (98). The frequency of fights decreases over the first several months after fledging and is subsequently low. Males are dominant over females.

Power Struggles

The most dramatic agonistic interactions occur during power struggles, intense fights occurring in association with the filling of reproductive vacancies within groups (94, 99). Reproductive vacancies occur when all the breeders of 1 sex die or disappear and are generally filled by unrelated birds from outside the group to the exclusion of a related helper of the missing sex still present in the group. The right to fill such vacancies may be contested by up to 20 or more nonbreeding helpers from other social groups who fight, often in unisexual sibling units, in contests lasting days or weeks and involve nonstop fighting, chasing, and vocalizations associated with intruders such as Urrks, Garricks, and drumming. Power struggles occur over vacancies of either sex but are generally more intense over female vacancies. The length of and number of intruders involved in power struggles are positively correlated with territory quality as measured by size of storage facilities. Power struggles are generally won by the largest participating coalition of siblings. Winners remain and share breeding status in the new territory, whereas losers return home to their natal groups and resume their status as nonbreeding helpers.

During power struggles and other kinds of territorial skirmishes, birds vocalize and chase extensively, occasionally making contact and grappling with adversaries. While grappling, birds peck at each other's heads and drop, sometimes hitting the ground and remaining for several seconds before disengaging and flying away.

Nest-Hole Competitors

Acorn Woodpeckers attack potential nest-hole competitors, particularly European Starling (Sturnus vulgaris), which regularly use Acorn Woodpecker nest holes in California (100). Attacks are most frequent once the woodpeckers have initiated nesting in a particular cavity; prior to initiating a nest, they do not successfully defend preferred nest holes against persistent intruders. Although European Starlings can adversely affect the breeding of individual groups, there is no evidence that they are affecting Acorn Woodpecker populations (101).

Mobbing

Members of groups communally mob California ground squirrel (Otospermophilus beecheyi), American Crow (Corvus brachyrhynchos), and other species that either live in granaries or attempt to steal stored acorns. Mobbing consists of repeated aerial dives at the offending individual combined with repeated Karrit-cut displays, and often appears to be moderately successful in hastening the intruder's departure. Acorn Woodpeckers similarly hassle avian predators such as Great Horned Owl (Bubo virginianus). Nests are only rarely defended against human intruders. However, groups will attack snakes that approach or attempt to enter nest or roost cavities (see Predation).

Spacing

Territoriality

Highly territorial, especially of granaries and sometimes sap trees, often defending them not only against conspecifics but a wide range of potential heterospecific competitors (69, 76, 102). At granaries, interspecific defense rates are up to several times more frequent than intraspecific territorial interactions. The former, which are primarily related to competition for acorns and granary defense, peak in autumn, whereas the latter, associated more with searching for breeding vacancies, peak in the spring (76).

Mean territory size at Hastings Reservation is 6 ha (47). Territory size is highly variable, however, and often open-ended when abutting unsuitable habitat, e.g., at right angles to streams in mountainous canyons (103). Birds, both nonbreeding helpers and breeders, systematically leave their home territories regularly and go on "forays" during which helpers search for reproductive vacancies up to at least 15 km away (104; Barve et al. submitted); the drivers of foray behavior by breeders is as yet unclear. All group members occasionally wander outside their normal home range in pursuit of acorns and water (91).

Territories without stores are often defended less vigorously than those with stores, since conspecific intruders commonly attempt to rob acorns when the latter are present (105). All group members help defend the territory, but territorial defense is highly sex specific, with birds primarily defending against conspecifics of the same sex while sometimes ignoring intruders of the opposite sex. Nonbreeding helpers defend territories at least as vigorously as do breeders (76).

Individual Distance

Unlike some cooperative breeders, Acorn Woodpecker is not a contact species, nor do birds allopreen. In general, it is unusual to observe > 2–3 individuals in close proximity to each other. Exceptions include: during pre-roost and post-roost mounting (see Sexual Behavior), during power struggles, while sapsucking, while mobbing ground squirrels (Otospermophilus spp.), and when mate-guarding during the early spring.

Sexual Behavior

Mating System

Highly variable, ranging from predominantly monogamy in some populations in Arizona and primarily cooperative polygyny in New Mexico to cooperative polygyny-polyandry (polygynandry) in California. In general, Acorn Woodpecker groups contain 1–8 male breeders that compete for matings with 1–4 egg-laying females (106, 49). In groups with > 1 breeding female, the female cobreeders nest jointly and lay their eggs in the same nest cavity (107). Groups may also contain up to 10 male and female nonbreeding helpers, usually offspring of the group produced in prior years (see Breeding: Cooperative Breeding).

Reproductive competition within groups is intense (107). The most extreme form of interference occurs among joint-nesting females, who regularly destroy eggs laid by their cobreeders. Although females appear to be unable to discriminate between their own eggs and those laid by others, a female will usually destroy any eggs laid in her joint nest until she has laid her first normal egg. More than one-third of eggs laid in joint nests are destroyed in this fashion. Once all females have begun a normal laying sequence, egg destruction usually ends. Eggs are removed and carried intact to a nearby tree where they are temporarily stored and then consumed piecemeal, usually by several different individuals and frequently by both the female that removed it and the cobreeder (usually a sister or other close relative) that laid it. Because eggs laid late in the laying sequence are more likely to be lost to brood reduction, females that begin laying later than their cobreeders are at a significant disadvantage. However, by destroying early eggs, last-to-lay females enforce synchrony and minimize the disadvantages of laying late eggs (108, 109).

Reproductive competition among mate-sharing males is less dramatic but still significant. Males often attempt to disrupt copulation attempts by their cobreeders and regularly participate in joint mate-guarding, whereby males attend and closely follow the breeder female beginning 7–25 d before laying of the first egg and terminating 1–2 d before clutch completion (110). Mate-guarding is only observed in groups with ≥ 2 breeder males; comparable behavior, at least in the form of close attendance of the female, is absent in groups that contain only a single male. This suggests that close attendance of breeding females by male Acorn Woodpeckers is an expression of within-group reproductive competition, not a means by which males prevent females from copulating with males from outside their social unit (111).

Infanticide, another form of reproductive competition in Acorn Woodpeckers, has been described by Stacey and Edwards (112). In groups where the lone breeding female or male dies or otherwise disappears during nesting, a replacement breeder that immigrates into the group may destroy any eggs or nestlings belonging to its predecessor. Such behavior appears to occur regularly at Water Canyon, New Mexico, but only early in the breeding season when the potential for renesting is high (112). Birds joining groups late in the breeding season neither kill young nor help feed them (106). At Hastings Reservation in central coastal California, infanticide by immigrants has not been observed under natural conditions. However, similar behavior can be experimentally induced in groups with cobreeding males by temporarily removing 1 cobreeding male prior to egg-laying, then returning him after completion of the clutch. Returning males often destroy eggs, forcing the group to renest (113). Egg destruction by males under natural conditions has also been observed (67, RLM, WDK).

Sex Ratio

At Hastings Reservation, the juvenile sex ratio at the time of the Postjuvenile molt is 54.0% males (n = 1,273). This small but significant difference is due to a combination of differential brood reduction and higher postfledging mortality of females, both in turn a consequence of female eggs hatching on average slightly later than male eggs (114). This significantly male-biased sex-ratio is not readily explained by the idea that males help more than females (the “repayment” model; 115), local resource competition, or sexual size dimorphism (116). No evidence has also been found for facultative sex-ratio manipulation related to annual difference in resources or territory quality (116). Subsequently, breeder females benefit less from the high survivorship correlating with cobreeding than do males, further contributing to an adult population that is about 60% males; breeding females experience relatively high rates of mortality during spring and summer (72).

Pair Bond

Courtship displays and pair-bonding as traditionally defined are absent. In nonmigratory populations, however, breeding males and females usually remain together on the same territory throughout their lives, and close attendance of breeder females during their fertile period by breeder males occurs regularly in groups containing ≥ 2 of the latter (110; see Mating System).

Copulations are infrequent and rarely observed. Precopulatory and postcopulatory displays are absent or rudimentary. In groups where only 1 male and 1 female are breeding, copulations are brief and uneventful. In groups containing > 1 potentially breeding male, multiple males may attempt to mount a female in succession, and males often attempt to interfere with copulation attempts by their male cobreeders (117, 47, 106, 111). Females appear to attempt to hide copulations from other males in the group, but they ultimately may mate with > 1 male during egg-laying, leading to multiple paternity within clutches (118, 119, 107; see Breeding: Cooperative Breeding). This behavior leads to parental uncertainty among males and may result in all males helping to raise the young, whether or not those males are also related to each other (106).

A similar behavior involving pseudocopulations, termed preroost mounting, mimics normal copulatory behavior in many respects but serves a more social function because sperm is not transferred between individuals. The entire group, or a large part of it, assembles and often sits quietly for a few minutes, following which birds hop over to and mount each other. Mounting may involve any group member mounting any other, including both heterosexual and homosexual combinations.

Extra-Group Copulations

Rare or absent. Electrophoretic work in California found an incidence of 4% extra-group paternity (119). DNA fingerprinting of 51 nestlings from monogamous pairs at Hastings Reservation found no evidence for extra-group paternity and only 1 case of apparent extra-group egg-dumping (120). More recently, no extra-group fertilizations or egg parasitism was detected among 386 nestlings from 127 nests analyzed using multilocus DNA fingerprinting (121).

Social and Interspecific Behavior

Degree of Sociality

Highly social, usually lives year-round in cohesive social units (see Sexual Behavior, and Breeding: Cooperative Breeding). In temperate habitats, group members usually operate independently and do not forage together in their territories. However, in Belize and possibly other tropical habitats, group members often move together (83). Group living has also been observed in all populations studied outside the United States, including Mexico, Belize, Honduras, Costa Rica, Panama, and Colombia (66).

Play

Not studied. Most early descriptions of "play" behavior (e.g., 122) appear to be attributable to interactions during foraging behavior or to power struggles (see Agonistic Behavior).

Nonpredatory Interspecific Interactions

Regularly and vigorously defends granaries and stored acorns against a variety of acorn-eating competitors, including titmice, nuthatches, jays, crows, magpies, and squirrels (102). Less frequently, may also attempt to displace heterospecifics from acorn-bearing oaks, sap trees, and favored flycatching perches or loafing sites (69, 47, 119). During the nesting season, occasionally competes with other cavity-nesting species over access to suitable nest cavities (see Agonistic Behavior).

Predation

Cooper's Hawk (Accipiter cooperii) regularly attacks and captures Acorn Woodpeckers in both California and New Mexico (47, 72, PBS) and is probably the most important predator of adults and juveniles in North America. Other accipiters, falcons, buteos, and owls may also capture this species (123, 124). Although direct evidence is lacking, bobcat (Lynx rufus) and gray fox (Urocyon cinereoargenteus) are attracted to Acorn Woodpecker distress screams, suggesting that these mammals are also occasional predators (95).

Nest predation is relatively rare at Hastings Reservation, California, accounting for < 10% of nest losses, but is considerably higher in Water Canyon, New Mexico. The most important nest predators are snakes, particular bull/gopher snakes of the genus Pituophis, which occasionally kill not only eggs and nestlings but also brooding adults (49, 125, PBS). Nest holes are occasionally taken over by woodrats (Neotoma spp.), chipmunks (Tamias spp.), small owls, and feral honey bees (Apis melifera), but no predation has been specifically attributable to such events.

When a low-flying accipiter or large falcon is detected, it usually elicits an Alarm Call that alerts other group members of the potential danger. Response is typically to seek refuge in a nearby roost hole or nest cavity or to move to the side of a branch opposite the approaching predator and remain motionless.

Birds captured either by humans or Cooper's Hawk often produce shrill, loud distress calls which are very different from Alarm Calls. Such calls do not generally attract other Acorn Woodpeckers, either within or outside the social unit (95; see Sounds: Vocalizations).

Nesting birds sometimes mob potential nest predators, remaining and giving repeated Karrit-cut and Karrit displays when their nests are approached by a climbing snake or, less commonly, by humans (see Agonistic Behavior). Birds occasionally swoop at and deliver pecks to the snake's head or body; they have even been observed to pull a bull snake out of a cavity, forcing it to fall to the ground (PBS).

Recommended Citation

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