Acorn Woodpecker

Melanerpes formicivorus

Order:
Piciformes
Family:
Picidae
Sections

Breeding

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Figure 2. Annual cycle of breeding, molt, and migration of the Acorn Woodpecker in central coastal California.

Thick lines show peak activity, thin lines off-peak. Although most individuals breeding in this and other populations do so in spring, some breed in the autumn as well. See text for details.

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Acorn Woodpecker excavating hole.

Excavates its own nest holes in whatever large trees are available.

© Alexander Viduetsky , California , United States , 6 November 2016
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Acorn Woodpecker at nest hole.
© Ketury Stein , California , United States , 26 May 2019
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Acorn Woodpecker at nest hole.
© Aidan Coohill , California , United States , 5 June 2018
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Adult Acorn Woodpecker feeding young.

All group members continue to feed remaining nestlings as well as free-flying juveniles after the first chicks have fledged.

© Rhys Marsh , California , United States , 6 September 2018

Phenology

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Video: Female Acorn Woodpecker excavating nest.
© Larry Arbanas, Arizona, United States, 9 May 2006
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Video: Female Acorn Woodpecker feeding juvenile.
© Brian Sullivan, California, United States, 14 September 2010

Figure 2. Depends on local conditions. In central coastal California, breeding is extended and bimodal. The primary season begins in April or rarely March (earliest first-egg date at Hastings Reservation is 17 March), peaks around 1 May, and extends though late June (rarely July). Within this time period, groups may renest or, if successful relatively early in the season, have second nests. Second nests tend to occur following years of good acorn crops and mild winters when stored acorns are still available late in the season, and are often started several days before young from the earlier nest have fledged. There is a strong overall seasonal decline in reproductive parameters, with clutch size, fledging success, and overwinter survival of fledglings being largest when nests are started early in the spring (126). The secondary breeding season, which only occurs in about one-third of years and is usually much less pronounced than the first (see Table 1), is during the autumn, with first egg dates in August through late September (latest first-egg date at Hastings Reservation is 23 September), with the latest young fledging in early November. Autumn nests tend to occur in years when the acorn crop is large and summers relatively warm (127).

In New Mexico, breeding rarely begins before May and there is no secondary autumn season, although second nests may occasionally fledge as late as early September. In Arizona, breeding is delayed until the monsoon season in late June, and renesting is rare (51).

Nest Site

Excavates its own nest holes in whatever large trees are available. Nests may or may not be in the granary (see Diet and Foraging: Food Selection and Storage); at Hastings Reservation, nests have been recorded up to 0.9 km away from the granary.

Each territory contains multiple cavities, several of which may be used for nocturnal roosting and any one of which may be used for nesting. Both large (> 1 m diameter) and small (17 cm diameter) limbs are used, as are both dead and living limbs in trees and snags. At Hastings Reservation, birds preferred to nest in cavities in dead limbs in large valley oak (Quercus lobata) and sycamore (Platanus racemosa); cavities tended to face east where temperature extremes are relatively buffered (128). Preferred nest cavities were not necessarily the most successful, however, suggesting that there are ecological constraints on nest-site selection (129). In New Mexico, most nests are in Fremont cottonwood (Populus fremontii) and ponderosa pine (Pinus ponderosa).

Nest

Nest holes may be used repeatedly for many years; at Hastings Reservation, about 50% of nesting attempts are in a previously-used cavity.

Inside diameter of nests is usually about 15 cm. Height of nests at Hastings Reservation ranges from 2.3 to 18 m (mean 8.3 m), and nest depth ranges from 22 to 70 cm (mean 40.0 cm). Deep nests are usually the result of annually deepening a traditional cavity in a limb with some degree of heart rot. Hole angles vary from –87° to +30° relative to the vertical, although holes facing upward are rare.

Although no nest is built, there is usually about 1 cm of fresh wood chips in the bottom of the nest cavity by the time eggs are laid. These wood chips, pecked from the cavity wall by the adults, are replenished throughout the nesting period.

Eggs

Shape and Color

Eggs are flat white, elliptical, smooth, and have no markings. When fresh they are translucent, but they turn an opaque flat white after several days of incubation.

Size

Mean (± SD) size based on 756 eggs from North America measured by Koenig (130) is 25.2 ± 2.00 mm (length) and 19.2 ± 1.16 mm (breadth). Egg size at Hastings Reservation is positively related to temperatures prior to egg-laying and negatively related to the food supply as indexed by the prior autumn’s acorn crop, but is not affected by the presence of helpers (131), contrary to the prediction of the “concealed helper” hypothesis that laying females reduce their investment in egg size or quality when assisted by helpers (132).

Mass

Weight of 35 freshly laid eggs from 10 different females at Hastings Reservation is 5.3 g, approximately 6.8% of mean female mass of 77.5 g and about 90% of the value predicted by the Piciformes equation of Rahn et al. (133, 19).

Acorn Woodpeckers are unique in that they produce “runt” eggs in frequencies significantly higher than any other species (134). Runt eggs are markedly small eggs between 17–22 mm in length and 14–17 mm in breadth (130), usually do not contain a yolk or are otherwise abnormal internally, and do not hatch (134). Overall, about 4.3% of eggs are runts and about 21% of clutches contain a runt (134). Runts are particularly closely associated with nests produced by joint-nesting females, where they are laid as the first egg of a clutch about half the time and overall constitute > 10% of all eggs laid (109).

Egg Laying

Eggs are laid at approximately 24-h intervals, usually in early to mid-morning. All group members frequently inspect the nest cavity during and after oviposition. Although up to 3 females can lay eggs in the same nest, only a single case (out of 58 offspring produced by 16 groups of monogamous pairs) of possible intraspecific egg-dumping by a female from another social unit (120). Egg destruction is common when females share a nest (108, 109; see Behavior: Sexual Behavior).

Clutch Size

Mean (± SD) set size (the number of eggs incubated, including nests in which > 1 female contributed eggs) for 224 egg sets collected is 5.06 ± 1.06 eggs and increases slightly with latitude (Spearman rank correlation of set size with latitude r = 0.14; 135). Mean set size at Hastings Reservation is 4.37 ± 1.08 eggs for clutches of singleton females and 4.98 ± 1.76 for all nests including joint-nesting females. Clutch size is slightly but not significantly larger in previously-used nest cavities than in newly excavated cavities (136).

Incubation

Incubation period 11 d. Both male and female breeders incubate and both sexes acquire brood patches; nonbreeding helpers incubate only occasionally. Onset of incubation is gradual, with the proportion of time spent in the nest increasing over the period of egg-laying. Full incubation is usually reached on the day the penultimate egg is laid. In nests where 2 females lay, incubation is usually initiated on the second day of laying.

Incubation bouts by individual breeders are up to 1 h in length and usually end with the arrival of another bird. Change-over activities are minimal. While inside the nest hole, birds frequently perch on the inside of the nest cavity near the nest entrance for extended periods rather than incubate the eggs. Relative contributions to incubation summarized in relation to sex, breeding status, and age are presented by Mumme et al. (137). In general, nonbreeding helpers incubate little (mean percent attendance ranging from 1.1 to 4.7%), whereas breeder females incubate slightly more than breeder males (mean attendance by females ≥ 2 yr = 26.0%, for males = 21.6%). Overall incubation attendance (percent of daylight hours ≥ 1 bird was inside the nest) at 51 nests studied at Hastings Reservation averaged 83.4% (138). These values vary considerably from group to group depending on composition and other factors. Nocturnal incubation and brooding are performed by 1–2 dominant breeder males; breeder females and helpers occasionally spend the night in the nest as well.

The hardiness of eggs against temperature stress is unknown. Observations at Hastings Reservation indicate that clutches can survive at least 1 night of neglect during full incubation and still hatch successfully.

Hatching

Chicks begin peeping from inside the egg after 10 d of incubation. Pipping of the eggshell follows, usually within 12 h. Whether adults assist chicks in hatching is unknown. Because the onset of incubation is gradual, eggs hatch asynchronously, usually in the order in which they were laid (114). Despite the staggered hatching, the total hatching spread is often < 24 h. Any chick that hatches > 1 d after the first chick is apparently ignored by the adults and usually starves within several days of hatching, regardless of weather or food availability (114). Even interchick intervals of a few hours have significant fitness consequences. Relative to their later-hatching broodmates, early-hatching chicks fledge in better nutritional condition, rank higher in the fledgling dominance hierarchy, enjoy higher postfledging survivorship, and attain larger adult size (114).

Eggshells usually remain within the nest until most or all of the young have hatched. They are then removed by the adults. Eggshells are dropped from the nest entrance, carried off by departing adults, or occasionally eaten. Unhatched eggs are usually removed by adults, but not until several days after the other eggs have hatched.

Young Birds

Mean mass at hatching is 4.7 ± 0.8 g, or 5.9% of adult mass (19). Hatchlings are blind, naked, and highly altricial. Although they lie prone for several hours after hatching, hatchlings soon balance upright, making extensive use of the large heel pads with which they hatch. Hatchlings beg with Tse and Rasp calls while waving their heads back and forth with open mouths. They close their mouths on and attempt to swallow anything they touch, including each other's heads. Such interactions appear to be accidental; competition is best described as scramble rather than interference, at least during the first week when most brood reduction takes place. Hatchlings are phototonic, responding to shadows blocking off the light entering the nest cavity by begging.

Brood reduction is common, with 16% of nestlings perishing due to starvation between hatching and fledging (72). Of these, 76% perish within 3 days of hatching, and young from eggs hatching more than 24 h after the first egg hatches almost invariably die, regardless of group size or ecological conditions (114, 139). The adaptive significance of such extensive brood reduction remains unknown.

Hatching mass doubles in 24–36 h, depending on hatch order, after which mass increase is almost linear for 3 wk. Overall nestling growth is represented by the logistic equation m(t) = 82.19/(1 + 9.79x e – 0.226 t), where m(t) is mass on day t (19). Weight recession prior to fledging is typical, though the timing and severity are variable. Eyes open at about 10 d, egg tooth disappears by about 12 d, Juvenile feather tracts appear by about 14 d, and feathers erupt from their sheaths starting at about 21 d. Up until fledging, chicks tend to locomote backward, presumably an adaptation preventing them from falling out of what are often nearly horizontal nests. Tse and Rasp vocalizations give rise to Squee and Trtrtr calls 3–4 wk after hatching. Fledging occurs in 30–32 d at a mass of about 80 g. At Hastings Reservation, male nestlings and fledglings are slightly heavier and have slightly longer bills than do females, as is true in adults. Otherwise, feather development is identical between the sexes. Hatch order and seasonal date, but not group size, affect the condition of nestlings (114).

Parental Care

Brooding

Chicks are brooded by all adult group members, although as during incubation, breeders brood more than nonbreeding helpers and females brood slightly more than males (137). Thermoregulatory ability is initially poor and increases linearly with mass, reaching 50% of adult capabilities at a mass of about 54 g, generally attained when nestings are 13–14 d old. Brooding also declines almost linearly with time after the first 4–5 d of the nestling period, covering 60–65% of the time early in the nestling period and decreasing to 10–15% by the time nestlings are 12 d old (140). Consequently, chicks are subject to periods of hypothermia resulting from brooding adults leaving the nest to forage well before the chicks are able to thermoregulate competently on their own (19).

Feeding

All group members normally help feed chicks beginning with the hatching of the first chick in the nest. Feeding rates are highest for breeder females, followed by breeder males and nonbreeding helpers (137, 114, 140). Fledglings continue to be fed by adults for several months, with the termination of most parental care roughly coinciding with the Prebasic I molt, 3–4 mo after fledging. Adults enter the cavity to feed the chicks until the latter are 3–4 wk old, after which the chicks usually climb up to the nest entrance to be fed. Nestlings are fed mostly insects (see Food Habits: diet, above), with the estimated proportion of the diet at Hastings Reservation consisting of acorns increasing from 17% for nestlings 1–5 d old to 19% for 9–12 d old nestings to 31–42% for nestlings over 23 d old (19, 73). Acorns are broken up into small pieces when fed to chicks. Insects, which are captured primarily by flycatching and to a lesser extent by bark-gleaning, are fed singly or in boluses. Stable-isotope analyses indicate that the availability of stored acorns allows adults to provide nestlings with more protein-rich insects while maintaining themselves on relatively protein-poor, low-quality acorns (73).

The rate at which individual chicks are fed depends on, among other things, group size, with a mean feeding rate of nests at Hastings Reservation of 0.15 feed/min for pairs increasing to 0.33 feed/min for groups of 8 or more birds. Mean (± SD) feeding rate over all nests was 0.24 ± 0.15 feed/min; the maximum observed rate was 0.98 feed/min for 2 groups, size 9 and 13 birds. Adults appear to be unselective in apportioning food to nestlings, delivering food to whatever chick begs most vociferously, but this has not been tested. Patterns of load-lightening or compensatory care—the degree to which birds reduced their effort with increasing group size—are detailed in Koenig and Walters (140), while patterns of chick provisioning vis-à-vis brood size have been examined experimentally by Koenig and Walters (141). Provisioning also appears to be related to the behavior of other birds in the group, resulting in either clumping of visits or alternating visits (142).

Nest Sanitation

Hatchlings produce fecal sacs which are ingested by adult birds for the first week or so after hatching. Subsequently, fecal sacs are removed by the adult birds, 61% by breeder males (137, Koenig and Walters 2012 [Behavioral Ecology 23: 181-190]). Adults brooding chicks often produce small quantities of fresh wood chips from the nest-cavity walls. These chips adhere to feces and absorb much of the associated free liquid. Adults then remove both the fecal sacs and the chips sticking to them, yielding a relatively clean nest environment. However, in the week prior to fledging, the cavity often becomes so crowded with nestlings that adults cease to remove fecal material. From this point until fledging, nestlings may become very soiled. As this occurs, the rich organic substrate of accumulating fecal matter on the nest floor provides an excellent environment for a variety of invertebrates.

Cooperative Breeding

General Patterns

Common and complex throughout much or all of the species' range. Average breeding-group size ranges from a mean of 4.4 individuals (maximum 15) at Hastings Reservation, where the species is a permanent resident, to marginally greater than 2 individuals at the Research Ranch in southeastern Arizona, where the species is migratory (51). However, small groups of 2–3 individuals are the most common breeding units in all populations studied to date.

The mating system is opportunistic polygynandry (polygyny-polyandry). Within a group, 1–8 male cobreeders compete for matings with 1–4 joint-nesting females who lay their eggs in the same nest cavity (see Behavior: Sexual Behavior). Superimposed on this breeding core of individuals may be 1–10 nonbreeding helpers, offspring fledged by the group in prior years that assist the breeders in feeding young. Nonbreeding helpers may be up to 5 yr old. Thus, 2 major forms of cooperative breeding are exhibited: helping at the nest by nonbreeding offspring and plural breeding by mate-sharing males and joint-nesting females.

Within a population, all combinations of group composition may occur simultaneously. For example, between 1975 and 2018 at Hastings Reservation, 40.7% of breeding groups contained 1 breeder male and 1 breeder female, 49.6% contained ≥ 2 breeder males, 25.2% contained ≥ 2 breeder females, and 15.5% contained ≥2 breeder males and females. Of these same groups, 56.2% contained ≥ 1 nonbreeding helpers, and 26.9% contained ≥ 1 nonbreeding helpers of both sexes. Evidence for joint nesting has not been directly observed in either Arizona or New Mexico, where no group watched intensively thus far has contained more than 1 breeder female (51, 143). However, joint nesting in the southwestern United States is suggested, at least historically, by the occurrence of unusually large clutches present in egg collections (WDK) and the incidence of runt eggs, which are associated with joint nesting and occur with equal frequency in collected egg sets from the southwestern U.S. and California (134). A summary of group composition at 3 intensively studied U.S. populations is provided in Table 2.

At Hastings Reservation, cobreeding males and joint-nesting females are almost always close relatives. Of 9 sets of definitively joint-nesting females where relatedness was known, 6 (67%) were sisters, 1 (11%) was a mother and her daughter (2 daughters in 1 year), and 2 (22%) were apparently unrelated (109). Of 200 group-years involving cobreeding males, 102 (51%) were brothers, 55 (28%) were a father and son, 41 (20%) were a combination of the prior 2 categories, and 2 (1%) were apparently unrelated (WDK, MTS). This contrasts sharply with the situation at Water Canyon, New Mexico, where 50% of cobreeding units consist of apparently unrelated individuals (106).

The mechanism determining reproductive roles within groups appear to be a combination of reproductive competition and incest avoidance (144, 106, 145, 146). Cobreeders share parentage within groups (106, 108, 118, 119, 49, 121). Joint-nesting females share maternity equally; thus each female is the mother of, on average, half the offspring fledged from joint nests. In contrast, there is strong reproductive skew, defined as the degree to which parentage is biased toward a subset of cobreeders, among cobreeder males (147, 148, 107). The male garning the majority of paternity, however, often switches from one nest to the next, suggesting that paternity is largely an “all or none” event within a nest but that the overall probability of success may be nearly the same among cobreeder males over many nesting attempts (149).

Group offspring at Hastings Reservation act as nonbreeding helpers within their natal group as long as a potential parent (defined as any of the cobreeders contributing to the nesting effort at which they were raised) is still present. However, elevated hormone levels and slightly enlarged testes indicate that helpers are reproductively competent (MTS). Offspring can inherit and cobreed in their natal group along with their same-sexed parent(s) following the disappearance and replacement by unrelated individuals of their parent(s) of the opposite sex (146). This results in various combinations of parents and their offspring cobreeding within the same group. Reproductive vacancies are not filled by offspring from within the group. Indeed, the presence of offspring of the same sex as a vacancy can delay replacement, sometimes for > 1 yr (99, 150). Thus, genetic relatedness within groups is high, but breeder males are generally unrelated to breeder females and inbreeding is rare (144, 145, 121). Close relatedness among cobreeders of the same sex is not the case in all populations, however, as demonstrated by the situation in Water Canyon (106).

Results of Cooperative Breeding

Fledging success increases with group size, more so in Water Canyon than at Hastings Reservation (106, 72, 49). At Hastings Reservation, individual feeding rates of both sexes and both breeders and helpers decrease similarly with group size, but overall feeding rates increase both overall and per nestlings with increasing group size (137, Koenig and Walters 2012 [Behavioral Ecology 23: 181-190]). Survivorship is significantly higher for all birds living in larger groups in Water Canyon (143) and among breeder males at Hastings Reservation (72).

In Water Canyon, increased survivorship and a higher expected lifetime reproductive success combine to make philopatry of young birds on high-quality territories the preferred option, whereas juveniles born on low-quality territories almost always leave to seek breeding opportunities elsewhere (143, 151). At Hastings Reservation, dispersal and independent breeding is a superior alternative to nondispersal and helping under almost all circumstances, and delayed dispersal is forced on offspring by ecological constraints, specifically the difficulty of acquiring access to suitable storage facilities (72, 149) as well as a nest cavity (ELW, WDK). Helpers are estimated to gain 0.13 offspring-equivalents of fitness via delayed dispersal and helping behavior, two-thirds of which come from increasing the reproductive success of relatives and one-third of which comes from enhancing the survivorship of relatives (152). The reproductive benefits of helpers varies significantly depending on the food supply as indexed by the prior autumn’s acorn crop; helper males confer significantly greater reproductive benefits to groups following good acorn years, while the effects of helper females is small and unrelated to the acorn crop (Koenig et al. 2011 [American Naturalist 178: 145-158]).

Among males, enhanced survivorship and reproductive success compared to males breeding solitarily is sufficient to explain the incidence of mate-sharing both in Water Canyon and at Hastings Reservation (106, 72). For females, the situation is more complex, but joint-nesting females have slightly higher rates of annual survival and an increased probability of acquiring a high-quality territory, partly compensating for reduced annual reproductive success (72, 153). The main driver of joint-nesting in this species, however, appears to be habitat saturation, as joint-nesting is more common when the population density is high and independent breeding opportunities are low (107).

Comparative Social Behavior

Of the 21 species of Melanerpes recognized by Short (4), at least 8 are known or probable cooperative breeders. Unfortunately, no congener has yet been studied in detail. Of particular interest would be comparative studies of those that are not cooperative breeders. For example, although typically nomadic—a behavior that precludes the development of granaries that can be passed between generations—some populations of the noncooperatively breeding Lewis' Woodpecker in California are both permanently resident and locally sympatric with Acorn Woodpeckers (31, WDK, MTS). Both species store acorns, but Lewis' Woodpeckers do not excavate granaries and do not live in family groups. Similarly, Red-headed Woodpeckers in the eastern U.S. store acorns and other kinds of mast, but they do not excavate their own holes (154). In comparison, the dependence of Acorn Woodpeckers on granaries would appear to be critical to their complex social behavior. However, this does not explain the apparently highly social behavior of at least 6 species of Melanerpes found in Central and South America that, despite marked dietary similarities with Acorn Woodpeckers, are not known to depend on structures comparable to granaries. This paradox is completed by the observation that Acorn Woodpeckers live in groups even in Central and South America where they are less dependent on granaries and in some cases apparently do not store food at all (82, 66).

Thus, although we have considerable information on the apparent costs and benefits of cooperative breeding in Acorn Woodpeckers, we know virtually nothing about why this species, along with several other congeners, has evolved complex sociality while other species of Melanerpes have not. Determining the reasons for such differences in breeding systems remains a major challenge in the field (155).

Brood Parasitism by Other Species

Interspecific brood parasitism not recorded.

Fledgling Stage

Nestlings leave the nest approximately 30–32 d after hatching, although runts may take several days longer. Prior to fledging, nestlings spend increasing amounts of time leaning out of the nest entrance in anticipation of their first flight, and may climb out of the cavity entirely and hang on the limb close to the nest to thermoregulate when conditions are particularly hot (PHOTO). Although some chicks climb out of the nest and move about on the nest limb, most leave the nest via flight. Not only do fledglings return to the nest to roost, they also return to be fed. The slight hatching asynchrony in this species results in asynchronous fledging, so fledglings often return to the nest to intercept adults arriving with food for the unfledged chicks. Adults also feed fledglings away from the nest, often delivering food to where ≥ 1 fledglings sit quietly in vegetation. Upon an adult's arrival, fledglings compete noisily for the food. Fights among broodmates in the weeks prior to and after fledging are not uncommon and may occur independent of food deliveries. Such fights appear to establish dominance relationships within the brood, with earlier-hatching and thus larger brood members consistently dominating later-hatching broodmates (97). Because broodmates remain together for an extensive period of time, often their entire lives, such dominance relationships probably have multiple fitness consequences.

Immature Stage

Brood division not apparent. All group members continue to feed remaining nestlings as well as free-flying juveniles after the first chicks have fledged. When second nests occur, juveniles from the first nest sometimes help provision nestlings, but also enter the nest and intercept food intended for the nestlings.

An average of 65% of fledglings survive to the termination of parental care, which roughly corresponds with the first Prebasic molt. Postfledging survivorship is highest when groups are able to maintain acorn stores into summer. At Hastings Reservation, fledglings from spring and autumn nests enjoy higher postfledging survivorship than those from summer nests (114).

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