Monday, December 15, 2008

Woodpecker Series - Part I

Wow, a dozen woodpecker species have been sighted in the county where I live here in way-south-central Colorado. I have seen only three in my back yard. I will have to do some research, but according to the Smithsonian, that’s how many might frequent Huerfano County…at least from time to time. A dozen! I’ve actually seen more than the three in my yard (Hairy, Downy and Northern Flicker); near here I’ve also seen Yellow-bellied Sapsuckers and Lewis’s Woodpeckers. I’d love to see an Acorn Woodpecker…they’re way-different looking; something you’d expect to see at a Mardi Gras! Since only four of this dozen have been observed in Denver County, but nine further north and about the same south of here…I figure info on the even dozen is needed here. I wonder just how much info is enough and how much is too much? Who wants to hear this stuff…that I write in order to learn, myself?

I’ll start with a woodpecker I’ve not seen, partly because it’s the first in my list and partly because Bill Schmoker has, again, graciously allowed me to use some of his photographs. This first one is absolutely stunning! (click all photos to see full size)

Acorn Woodpecker - Melanerpes formicivorus

This is a black-backed woodpecker which shows a conspicuous white rump and white wing patches, like a white crescent on each wing, in flight. Both sexes have light eyes on a black and white face with a red crown. It is common and readily seen in open oak woods and mixed woods with oaks. This is a noisy and gregarious bird, with a complex social system, and which keeps communal acorn caches called granaries. The adult has a black head, back, wings and tail, white forehead, throat, belly and rump. The eyes' irises are white on the black face. The adult male has a red cap starting at the forehead, whereas females have a black area between the forehead and the red cap. The white neck, throat and forehead patches are distinctive identifiers. There is often a variable, but usually small, amount of yellow (sometimes with one or more red-tipped feathers) on the throat.

The Acorn Woodpecker is a common inhabitant of foothill and lower woodlands from along the American westcoast and the Southwest, through western Mexico and the highlands of Central America to the northern Andes. This species is closely associated with oaks and acorns and is most commonly found in pine-oak woodlands. It is probably best known for its highly social habits and unique method of storing acorns in specialized trees known as granaries, although group living and acorn storage are not characteristic of all populations. This is one of the sedentary woodpeckers, but at least one population migrates annually and irregular migrations occur elsewhere when local acorn crops fail.

In the U.S., Melanerpine woodpeckers are also represented by the Red-headed Woodpecker, Lewis’ Woodpecker, and possibly also the sapsuckers. Most species in this group are found in Central and South America, too. These woodpeckers use a wide variety of foraging methods, including nut storage, eating fruit, flycatching, and sapsucking, but rarely, if ever, drill for subsurface insects in wood in typical woodpecker fashion. Many in this species exhibit cooperative breeding and other complex social behaviors.

Like at least one-third of the species in the genus Melanerpes, the Acorn Woodpecker is a cooperative breeder and lives in family groups of up to a dozen or more individuals. Birds in these social units store acorns communally and cooperatively raise young. Although acorns constitute a major portion of the diet, particularly during the winter, this species also engages in a wide variety of other foraging techniques including sapsucking, flycatching, bark-gleaning, and seed-eating. All members of the group spend hours and hours storing thousands of acorns in carefully tended holes in trees, telephone poles, and artificial places if natural places are unavailable.


The Acorn Woodpecker is a medium-sized, clown-faced, black and white woodpecker with distinctive red crown, glossy black and white head, white eyes, and white rump and wing patches. As mentioned, there is sometimes a bit of creamy-yellow on the throat. Adult males have solid red crowns (with the exception of the isolated Colombian race); adult females have a wide black band separating the red crown from the white forehead. Males and females are about the same size. Juveniles, prior to the first Prebasic molt, are similar to adult males, with duller colors and lacking the gloss of adults. Younger birds also exhibit dark irises. The plumage of this bird is unmistakable and unlikely to be confused with any other species.


  • Length: 7-9 inches
  • Wingspan: 14-17 inches
  • Weight: 2.29-3.18 ounces
  • Pale or white eye surrounded by black face
  • Front of face white or cream yellow
  • Black around base of bill
  • Top of head red
  • White belly and vent, with fine, dark, lateral, streaks on flanks
  • Black tail
  • Rump white
  • Black streaks extending from chest into belly
  • White patch in wings obvious in flight
  • White bases to outer primaries appear as small white crescent in flight
  • Bill is black


The Acorn Woodpecker adapts well to suburban conditions, so declines in distribution due to human activities are primarily due to habitat degradation and the elimination of snags (standing dead trees), especially those used for acorn storage. A good colonizer, this woodpecker is regularly found outside its normal range sometimes far from breeding habitat.

Most populations are resident, in other words most don’t migrate. In areas where there are large seasonal fluctuations in insects and other foods, year-round residency is dependent on the birds’ ability to store sufficient acorn mast (nut meats) to provide food throughout the winter. Family groups that exhaust their stores often abandon their territories and wander off in search of alternative food. Depending on the magnitude and extent of the crop failure, birds that abandon their territories may find space and acorns close by or be forced to leave the immediate vicinity but remain close enough to return the following spring. Widespread nut-crop failure can lead to permanent disappearance of a large proportion of the population and may lead to a “flight year” in which birds are found in habitats that are normally unsuitable for these birds, including deserts and open grasslands.


All members of an Acorn Woodpecker group spend large amounts of time storing acorns. Acorns typically are stored in holes drilled into a single tree, called a granary tree. The acorns are jammed in so tight that even squirrels can’t pry them out. One granary tree may have up to 50,000 holes in it, each of which is filled with an acorn in autumn. Occasionally they’ll nest in the
same tree.

The Acorn Woodpecker will use human-made structures to store acorns, drilling holes in fence posts, utility poles, buildings, and even using automobile radiators. Occasionally the woodpecker will put acorns into places where it cannot get them out; a few once put 485 pounds of acorns into a wooden water tank in Arizona! In parts of its range, the Acorn Woodpecker does not construct a granary tree, but instead stores acorns in natural holes and cracks in bark. If the stores are eaten, the woodpecker will move to another area, going from Arizona to Mexico to spend the winter, if necessary.

Acorns seem to be emergency provisions; for on mild, winter days these birds catch flying insects. These woodpeckers actually prefer insects, especially flying ants, bees, wasps and beatles. In addition to acorns (both immature and stored), they eat sap, oak catkins, fruit, flower nectar, and occasional grass seeds, lizards, and bird eggs. They often sit at the tops of trees while flycatching, otherwise forages primarily in or near the canopy of trees. Acorns are picked directly off trees; these birds rarely go to the ground except to obtain grit or to pick up acorns inadvertently dropped. Usually, acorns are removed singly from trees, but the bird may also break off a twig holding up to
three acorns.

Sapsucking is a communal affair, with group members congregating at sets of holes that are often used for several years. These sap holes are smaller in diameter and shallower than acorn-storage holes, but are spaced similarly. Their sap holes are readily distinguished from those made by sapsuckers, which are smaller, more closely and linearly spaced, and often girdle the limb.

Insects are preferred food and are eaten at any time of year when weather permits. As mentioned, nuts are supplemental, stored extensively in the fall but then used primarily when conditions render insects unavailable. The increase in the proportion of acorns fed to nestlings with age is probably due to a combination of the greater nutritional demands of larger nestlings and the increasing ability of nestlings to handle larger nut bits as they grow.

Almost any dead or living tree with deep, dry bark may be used as a granary; even parts of trees such as pine cones. Once a group has filled its granary facilities, it may also fill larger cavities such as old nest and roost holes with acorns. These birds store primarily acorns, but almonds, walnuts, hazelnuts, pecans, and piƱon pine nuts are also stored when available. The storage trees are usually mature or dead pines or Douglas firs with thick, soft bark, but dead oak branches and fence posts are also used. The holes made by a colony are used year after year.

The Acorn woodpecker prefers to use pine trees for granaries and for nest cavities, as its wood is quite a bit softer than Oak. Do not be too worried about the survival of your oaks and pines; the Acorn Woodpecker usually only pokes holes through the bark, which wouldn’t harm the tree. In addition, they prefer trees that are already dead.

The acorns are visible in their individual holes, so all members of the group vigorously defend their granaries, which would be valuable food sources for many animals, especially the Lewis’s Woodpecker, which also stores acorns…but also titmice, nuthatches, jays, crows, magpies and squirrels. Groups typically have a primary granary and one or more secondary sites. Acorn Woodpeckers are not storing the acorns in order to eat the insect larvae that ultimately form in the acorn, but for the
acorn themselves.

The woodpeckers collect acorns and find a hole that is just the right size for the acorn. I find it facinating that, as acorns dry out, they are moved to smaller holes; granary maintenance requires a significant amount of the bird's time. Acorns are such an important resource to the California populations, that these birds may nest in the fall to take advantage of the fall acorn crop, a rare behavior in birds.


The species is highly social and usually lives year-round in social units, invariably roosting together in tree cavities. Nighttime temperatures inside roost cavities average significantly higher than outside ambient temperatures and are further augmented depending on the number of birds sharing the cavity. Heat loss is appreciably lessened by roosting communally in cavities, particularly in the presence of wind.

Like other woodpeckers, these hop readily up limbs and around trunks of trees, using their tail as a prop; walking does not occur. As with most woodpeckers, flight is usually undulating, with dips and rises as it flies. However, this trait is not as pronounced as in many other woodpeckers and may even be absent, at least in some areas.

Members of groups communally mob ground squirrels, crows and other relatively large species that come to their granaries and/or attempt to steal stored acorns. Mobbing consists of repeated aerial dives at the offending individual, combined with repeated Karrit-cut displays. It appears to be moderately successful in hastening the intruder’s departure. Acorn Woodpeckers similarly hassle avian predators such as Great Horned Owls, although communal mobbing of such predators has not been reported. Nests are only rarely defended against human intruders. However, groups will attack snakes that approach or attempt to enter nest or roost cavities. The Cooper’s hawk regularly attacks and captures Acorn Woodpeckers and is probably their most significant predator in North America. Other accipiters, falcons, buteos and owls may also capture them. Although direct evidence is lacking, bobcats and gray foxes are attracted to Acorn Woodpecker distress screams, suggesting that these mammals are also at least occasional predators.

Because of this species’ abundance, Native Americans in California, and perhaps elsewhere, utilized Acorn Woodpeckers for food and even developed specialized traps to catch birds as they emerged from their roost holes. Feathers were used for ornamentation on garments: one existing full-size cape is completely covered with red crown feathers from what may have been several thousand individual birds.

More recently, the primary threats to this species are from habitat loss and degradation. In the southwestern U.S. and parts of Mexico, much of the montane, riparian and pine-oak habitat where this woodpecker occurs has been damaged by overgrazing, which has probably lead to substantial population declines in many areas. Additionally, the poor regeneration of oaks in California, which is experiencing results of a disease called "sudden oak death" contributes to habitat loss. Conservation of this species will depend on the maintenance of functional ecosystems that provide the full range of resources upon which the species depends. These include mature forests with oaks capable of producing large mast crops and places for the woodpeckers to nest, roost, and store mast.

Abundant throughout their range, Acorn Woodpecker populations are limited by the availability of acorns and granary sites. Their populations tend to be highly fragmented; as stated, their most significant current threat is habitat degradation. Management practices that preserve the normal age structure of the forest, with an emphasis on the snags and dead limbs used for granaries and nesting, are particularly important. Until recently, many of these trees and snags were removed because they were viewed as hazards or because they might attract lightening strikes. Fortunately, like many woodpeckers, the Acorn Woodpecker, adapt well to the presence of humans, if not persecuted, and readily use human-made structures such as utility poles and buildings for roost and storage locations. These birds will even feed on and store dog food, if it is available.

The Acorn woodpecker does not migrate, however, it will move upslope in the fall. Acorn Woodpeckers generally do not migrate since they have a site-specific food storage system that they enlarge and defend year after year. However populations may wander if local acorn crops fail.


A group of adults may participate in nesting activities; field studies have shown that breeding groups range from monogamous pairs to breeding collectives of seven males and three females, plus up to 10 nonbreeding helpers. Young Acorn Woodpeckers have been found with multiple paternity.

Mating behavior is highly variable, ranging from predominant monogamy in some populations in Arizona and primarily cooperative polygyny in New Mexico to cooperative polygynandry in California. In general, Acorn Woodpecker groups contain one to as many as seven or more male breeders that compete for matings with one to three egg-laying females. In groups with more than one breeding female, the female co-breeders nest jointly and lay their eggs in the same nest cavity. Groups may also contain up to ten male and female nonbreeding helpers, usually offspring of the group produced in prior.

Acorn Woodpeckers breed where there are and pine-oak woodlands; also along riparian corridors and river banks, and in Douglas-fir, redwood, and tropical hardwood forests as long as oaks are present or available nearby. It is found at sea level in southern California, but more generally in mountains up to the distributional limit of oaks (treeline). It is also common in urban parks and suburban areas whenever oaks are present and trees or other natural or human-made structures are available for acorn storage.

All breeding males can mate with any and all of the female breeders of the group. However, reproductive competition within a group is intense and these birds practice the most interesting exercize to enforce synchrony and minimize disadvantages between birds. 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

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 the groups.

Acorn Woodpeckers are unusual in that they produce “runt” eggs in frequencies significantly higher than any other known species. Runt eggs are markedly small eggs, usually do not contain a yolk or are otherwise abnormal internally, and do not hatch. Overall, about four percent of eggs are runts and about twenty percent of clutches contain a runt. 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 somewhat more than ten percent of all eggs laid. Perhaps this is why the egg distruction seems to be a positive rather than a negative behavior.

Nest holes may be used repeatedly for many years; switches occur more frequently after a failed nesting attempt has occured.

As with most woodpeckers, both sexes acquire brood patches and both male and female breeders incubate; nonbreeding helpers incubate only occasionally. Incubation period is eleven days. 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 two females lay, incubation is usually initiated on the second day of laying.

Chicks begin peeping from inside the egg after ten days of incubation. Pipping of the eggshell follows, usually within twelve hours. Because the onset of incubation is gradual, eggs hatch asynchronously, usually in the order in which they were laid. Despite the staggered hatching, the total hatching spread is usually less than 24-hours. Any chick that hatches more than a single day after the first chick is apparently ignored by the adults and usually starves within several days of hatching, regardless of weather or food availability. 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.

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

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.

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. Fledglings continue to be fed by adults for several months, with the termination of most parental care roughly coinciding with the Prebasic I molt; three to four months after fledging. Adults enter the cavity to feed the chicks until the latter are three to four weeks old, after which the chicks usually climb up to the nest entrance to be fed. Nestlings are fed mostly insects, with the estimated proportion of the diet consisting of acorns increasing as the chicks age. 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 a chewed mass.

Like other woodpeckers, adult Acorn Woodpeckers 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. Perhaps this is why newly fledged birds are
so bath-happy.

Nestlings leave the nest approximately
30–32 days after hatching, although runts may take several days longer. Before fledging, nestlings spend increasing amounts of time leaning out of the nest entrance in anticipation of their first flight. 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 will also feed fledglings away from the nest, often delivering food to where one or more 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. Because broodmates remain together for an extensive period of time, often their entire lives, such dominance relationships probably have multiple
fitness consequences.

Some adult birds divide up a fledged brood, each taking a few juviniles, but with this breed brood division is 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.

Interesting Facts:

  • WhatBird says a group of Acorn Woodpeckers are collectively known as a "bushel" of woodpeckers.
  • Formicivorus means 'ant eater'...but these woodies enjoy eating many different insects.
  • Other names for this bird are: California Woodpecker and of course Ant-eating Woodpecker


Listen to songs of this species: click here.



Addendum: If anybody has a photo of one of these birds with the bit of yellow at the throat...and would let me post it here, I sure would appreciate it (and give all the proper credit, too!)


Cheryl said...

Great info Beverly- I added it to my blog: Bay Area Wildlife Blog

Beverly said...

It was a doozie getting all that down, I have to tell ya! LOL

I added an UPDATE to my piece regarding Rossmore with a link to your blog. I hope it helps!

Thanks for stopping by and for your compliments!

Bosque Bill said...

One of my favorite hikes on the SF Peninsula was at Wunderlich Park near Woodside. Up the trail near Salamander Flat was a clearing with a large "bushel" of Acorn woodpeckers. There were several very large Douglas firs used as granary trees around the clearing and you could hear the woodpeckers calling from quite a distance. A wonderful place.

Thanks for your article.

Beverly said...

Hiya Bill,

Thanks for your compliments and for the memories!

Somehow, I’ve never managed to see an Acorn Woodpecker, but I have seen what must have been their (or perhaps Lewis’s) acorns in trees. I was surprised at how tight they were…in the holes. Before I knew they were nuts and placed there by birds, I thought they were some sort of grub! Sheeshhhhhhhhhh

Yeah, I guess firs are better as granaries because the bark is softer and easier to ‘drill’. How cool to see them and the birds. I read they are so social, you can often see 6-8 or even more…in the same tree.

I wish I could find the little video clip I saw once...where some guy filmed a dozen or more flying and out of a tree. It was neat to see...

ehunter said...

Excellent info. I love reading about birds/animals that have interesting behaviors like stashing things. 485 pounds of acorns! I can't even imagine humans collecting that many, let a lone birds. Too bad they put the acorns where they couldn't reap the benefits of their hard work.
I really would like to see one of these birds and a granary.


Beverly said...

LOL Yeah, I thought that was pretty astounding, too! Almost 500#!

Like I said…somewhere I’ve seen an acorn in a tree like the pictures I’ve included show…but I’ve not seen so many as a granary might hold. It’s wild that the birds can get them in there so tight!

Next up, Three-toed Woodpeckers…but they don’t store food like that.

Nice to see you again, thanks!