Acorn Woodpecker

Melanerpes formicivorus

Order:
Piciformes
Family:
Picidae
Sections

Demography and Populations

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Figure 4. Relative abundance of Acorn Woodpecker during the breeding season.

Based on data from the North American Breeding Bird Survey, 2011–2015. See Sauer et al. (2017) for details.

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Figure 5. Regional trends in Acorn Woodpecker breeding populations.

Based on data from the North American Breeding Bird Survey, 1966–2015 (Sauer et al. 2017). Data show estimates of annual population change over the range of the survey; areas of increase are shown in blue and declines are shown in red. See Sauer et al. (2017) for details.

Measures of Breeding Activity

Estimated age at first reproduction at Hastings Reservation is 2.1 yr for males and 1.9 yr for females (72). Of offspring surviving to the Postjuvenile molt, approximately 32% of males and 40% of females breed when they are 1 yr old, and 72% of males and 77% of females breed by the time they are 2 yr old. A small proportion of birds do not attain breeding status until they are 5 yr old or, for males, when they are 6 yr old. A summary of reproductive parameters for the three well-studied North American populations is given in Table 1.

A variety of factors influence reproductive success of Acorn Woodpeckers. The most important of these are the prior autumn's acorn crop and the availability of acorns from the prior autumn stored in a group's granary. Following good acorn crops at Hastings Reservation, groups are more likely to have stores remaining in the spring and initiate breeding earlier, have larger clutch sizes, and fledge more than 5 times as many young as groups in years following a poor acorn crop or with no stores remaining. A similar, although not as dramatic, enhancement of reproduction is shown at Water Canyon, where groups with acorns remaining fledge more than twice as many young as those without stores (143). Stored acorns are sometimes fed directly to older nestlings, but the primary means by which stored acorns enhance reproduction is apparently by allowing adults to feed a greater proportion of the insects they catch to nestlings (73). The availability of acorns during the winter also results in adults in better physical condition for breeding. This is demonstrated by the larger and earlier clutches of birds with access to stores and an increased probability that birds without stores will fail to initiate any breeding attempt whatsoever during the season. Acorns also enhance reproductive success at Hastings Reservation, but not in Arizona or New Mexico, by increasing the probability that groups will attempt a second nest in the spring and by facilitating autumn nesting (127).

Life Span and Survivorship

At Hastings Reservation, California, overall annual survivorship is 86.5% (n = 2,396 bird-years) for breeding males and 83.8% (n = 1,602) for breeding females; values for Water Canyon, New Mexico, are 61.3 (n = 155) and 51.5% (n = 103). Estimated survivorship of first-year birds parallels that for adults, being about 48.6% (n = 4,101) at Hastings Reservation and 37% (n = 123) at Water Canyon (49, WDK, ELW); survivorship based on re-sightings of color-banded individuals.

Apparent survivorship in both study areas is significantly higher in groups with additional adults and, at least at Water Canyon, in groups living on high-quality territories (143). At Hastings Reservation, apparent survivorship of breeding males is greater on high-quality territories (those with large acorn storage facilities) and when assisted by helpers, especially when the acorn crop is large the prior autumn. Among breeder females, survivorship is lower among birds that nest jointly than among females nesting singly but otherwise unaffected by ecological conditions (156). Females are particularly at risk during the spring and summer, suggesting that reproduction entails a greater cost or increased risk to females than males (see also 107).

The oldest recorded birds in the wild are an 18 year-old male and a 17 year-old female at Hastings Reservation, and a male at least 9 yr old and a female at least 5 yr old at Water Canyon. These differences in longevity between sites reflect in part differences in the length of the two studies. In captivity, a male held for most of its life at the Lindsay Museum in Walnut Creek, California, lived to be 24 yr old (and is still alive as of this writing; E. Nardi, personal communication); a captive female kept by WDK lived to be 19 yr old.

Disease and Body Parasites

Diseases

Diseases have not been studied.

Parasites

Soon after hatching, nestlings may be parasitized by fast-running hematophagous mites and lice that hide under the wings and in skin folds. One of these includes the cimicid swallow bug (Oeciacus vicarius), but otherwise these have not been identified or studied. Botfly larvae (Philornis spp.) are conspicuously absent, presumably due to the lack of nesting material in which larvae can hide.

Causes of Mortality

Predation is most likely the primary cause of mortality among adults, although the potential importance of diseases and epidemics is unexplored. In years of poor acorn supply, a large fraction of the population may emigrate from an area, returning if the crop failure is relatively localized (48). If the crop failure is over a wide geographic scale, a large proportion of the population may perish directly or indirectly to starvation during the winter. Because sympatric species of oaks do not necessarily produce acorns synchronously (157), the probability of such disastrous crop failures probably varies depending in part on the species diversity of oaks in an area (52, 53). Data from Hastings Reservation, where oak diversity is relatively high, and from Water Canyon, where it is relatively low, support this hypothesis. At Water Canyon, a crop failure leading to total abandonment of all territories has occurred in 2 of 20 (10%) yr (158). At Hastings Reservation, where 3 to 5 species of oaks are common, total abandonment of the population has not been observed, although poor acorn crops leading to a decline of > 20% of the adult population have occurred in 4 of 38 (11%) yr (48, 159).

Losses of eggs are due to a variety of causes including, in order of importance, hatching failure, egg destruction by conspecifics, desertion, and nest-site competition (72). Hatching failure is higher in groups containing > 1 breeding male and/or female (160). Sources of nestling mortality, also in order of importance, include starvation (brood reduction), predation, desertion, weather, and adult death. Infanticide by immigrants also occurs (112), although it probably accounts for only a small proportion of overall nestling mortality.

Range

Extra-territorial movements show 2 peaks: an initial, smaller peak in the autumn as young birds explore their surroundings, and a larger peak in the spring associated with dispersal (105). Early studies using radio-tracking at Hastings Reservation (104) indicated that there are few differences in movement patterns between the sexes during the first year, but by the second year of life nonbreeding females engage in forays (flights by individuals to neighboring territories primarily searching for reproductive vacancies) at rates significantly higher than those of males. Overall foray rates by nonbreeders increased from below 0.5/h in their first year to 1–2/h in their second year. Foray length also increased, averaging about 0.3 km in the first year to > 1 km for nonbreeder females in their second year. For the first 2 months after fledging, juveniles spent much of their time following older individuals throughout their territory, gradually expanding their home range such that by 4–8 months after fledging their home range was larger than those of the breeders in the group.

More recent work at Hastings Reservation using an automated radio-telemetry array and solar-powered tags has confirmed the widespread extent of foray behavior, but indicates that both helpers and breeders commonly engage in this behavior (Barve et al. submitted). This study also found that birds residing in low-quality territories (those with small acorn storage facilities) tended to foray greater distances than birds residing on high-quality territories and that birds living in large groups tended to spend more time foraying.

Dispersal itself is discussed in detail by Stacey and Ligon (143), Koenig and Mumme (72), Hooge (104), and most recently by Koenig et al. (161). Dispersal is rare prior to the first spring after fledging. The overall proportion of young that eventually inherit and breed in their natal territory is 23.7% of males and 4.6% of females; these proportions are 60.7% of males (n = 229) and 20.2% of females (n = 109) that survive to their first spring (161). Dispersal is frequently conducted in unisexual sibling coalitions: an estimated 68% of males and 43% of females disperse with at least 1 same-sexed sibling.

Based on observations of banded birds, dispersal distances are low, averaging 0.22 km for males and 0.53 km for females (161). However, radio-tracked nonbreeders regularly foray up to 15 km away from their natal territory, and the mean dispersal distance among 8 females that dispersed while being radio-tracked was 6.1 km, an order of magnitude farther than estimates based on direct observation of banded birds (104). These findings, along with the species' good colonizing ability (50; see Distribution, Migration, and Habitat: Historical Changes), indicate that the Acorn Woodpecker is relatively vagile.

Home ranges at Hastings Reservation based on minimum convex polygons average 5.5 ± 2.0 SD ha for male breeders, 4.9 ± 2.32 ha for female breeders, 13.2 ± 7.6 ha for male nonbreeders, and 89.3 ± 56.9 ha for female nonbreeders, who foray more extensively than birds in other age/sex classes (104). Comparable home ranges have not been determined at Water Canyon, but individuals frequently wander considerable distances and have been observed > 5 km from their core area (PBS).

Population Status

Numbers

Within the United States, often the most abundant woodpecker species within its range, with a maximum density on Christmas Bird Counts of 7.96 individuals/h in the northern Central Valley of California, considerably greater than the maximum density of 4.72 individuals/h for Northern Flicker (Colaptes auratus), the woodpecker with the next highest recorded winter density in North America (162). Using data from the North American Breeding Bird Survey (BBS), the Acorn Woodpecker population was estimated at 2,000,000 individuals for the United States from 2005 to 2014 (163). No data on population abundance from elsewhere in the species' range.

Trends

BBS data (1968–2017) for the United States showed the population increasing by 0.3% per year (n = 209 survey routes; 164), including annual increases in the Coastal California (0.15%), Sierra Nevada (0.3%), and Northern Pacific Rainforest (0.41%) bird conservation regions (164). Over a 45-year period (1970–2014), BBS data indicated that the Acorn Woodpecker population for the United States increased by an estimated 34% (163). No data on population trends from elsewhere in the species' range.

At Hastings Reservation in central coastal California, the population has increased significantly over the past several decades, increasing from 52 individuals in 1980 to 219 birds in 2013 (Hagemeyer et al. submitted). This increase was due to increased number of groups rather than larger group size and appears to be driven by increased productivity due to broad-scale canopy regrowth during this time period (165).

Population Regulation

Populations are limited by the availability of acorns, nesting cavities, and suitable granaries (166, 72). The latter is ultimately determined by acorns as well, but is independent insofar as areas of acorn abundance are underutilized unless adequate storage facilities exist (158). Territorial behavior sets an upper limit to the number of breeders (48). The propensity to joint nest is more likely when population density is high (107). Group size is limited by granary size and social factors, primarily inbreeding avoidance and reproductive competition, which inhibit breeding by subordinates (167, 99, 146, 150). Seasonal decline in reproduction limits highly fragmented populations, like many in the southwestern United States, may depend upon immigration and metapopulation exchange for their continued persistence (168).

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