Pileated Woodpecker

Woodpecker Pileated Gerry Sutton June 2013
The Pileated Woodpecker is the largest of the 25 extant North American picine species

Common Name: Pileated Woodpecker – The prosaic common name is a statement of fact; they peck wood for both food and shelter. Pileated describes the bird’s striking red capped pileum, which is the name for the top of a bird’s head from the bill to the nape of its neck.

Scientific Name: Dryocopus pileatus – The generic name is probably derived from the Greek word drus, which is an oak tree and kopos, which means laborious toil; a testimonial to the arduous and incessant effort of holing a tree. The species name pileatus is the Latin word meaning ‘wearing the felt cap,’ which the rakish red crest resembles.

Potpourri:  The pileated woodpecker in particular and woodpeckers in general are a study in evolutionary dynamics. Had not Darwin noted the transformations of the finches of the Galapagos Islands during the famed HMS Beagle expedition, he surely would have had a second chance with the birds in his own backyard. The order Piciformes, which is dominated by the woodpeckers of the Picidae family, (both names derived from the Latin picus, meaning woodpecker) is distinct from the order Passeriformes (the perching birds more commonly, though incorrectly, called songbirds) in a number of adaptations to promote pecking at trees for both food and shelter. The ubiquity of the woodpeckers is testimony to the success of this approach; they occupy and have exploited a niche in which they have virtually no avian competition. There are about 220 species worldwide. That this is a variation on an evolutionary theme is best evidenced by the almost universal combination of feather coloration that the members of the woodpecker family share: dichotomous black and white plumage that extends from beak to tail adorned with a splash of red, almost always on the head or neck. First appearing in the Oligocene 25 million years ago (but perhaps earlier, the fossil record is scant) it is almost certain that the first bird that took to pecking at wood had black and white plumage. Over time and as the evolutionary pressures would demand, the red was likely a favorable variation that accentuated mate identification. In the case of the pileated woodpecker, both male and female birds have the vibrant red crest, the male having an additional red malar (cheekbone) stripe just below the eye.

The Pileated Woodpecker is the largest of the 25 extant North American picine species; the more-or-less presumed extinct Ivory-billed Woodpecker (Campephilus principalis) that once thrived in the now depleted cypress forests of the deep south is larger by approximately three inches (every year or so someone claims to have heard the iconic call, which has been compared to the high-pitched blare of a toy trumpet, without the obligatory visual sighting – an avian Yeti). D. pileatus is the woodpecker nonpareil, rocketing through the forest in a series of strong hyperbolic loops broadcasting a piercing kee-kee-kee sound from one trunk to the next. While often heard and seen, its high trajectory and Gadarene pace directed at what always seems to be a distant objective affords only a fleeting glance. While generally perceived as a peripatetic and singular species, pileated woodpeckers are aggressively territorial and monogamous; a male-female pair generally occupies and defends a well-defined area. The aggression is manifest when the territory is violated by another bird; the offender is attacked by its sexual counterpart – the male driving away the male and the female the female. The visual identification of the mate is quite strong. Experiments with the closely related Northern Flicker (Colaptes auratus) involved the artificial application of the male marking on the female of a mated pair. The male attacked the female mercilessly until the experimenter removed the disguise. Since woodpeckers do not communicate with the varied tones of the songbirds, they evolved another signaling method known as drumming, pecking on a resonant hollow object with a rate and frequency peculiar to the species. Pileated woodpeckers drum of 15 beats per second for a period of 1 to 3 seconds. The function of the plangent drumming is, like birdsongs, species identification for mating purposes and for territorial establishment.

Woodpecker Tree_BuckRidge_120401
Woodpecker holes

Pecking at wood is an arduous task; imagine trying to fell a tree with an icepick for a tool. With a measured speed of 20 feet per second, a woodpecker’s jabbing impact due to deceleration when the beak hits wood is estimated to have a force of about 1,000 G’s; humans can withstand just over 45 G’s (one G is the generally taken to mean the forceexerted by gravity – actually, since Force = Mass x Acceleration – in this case gravitational acceleration – the G-force is really the weight on the mass due to the acceleration of 32 feet/second squared). The incessant pecking needed to excavate a hole proceeds at a rate of about 100 strikes every minute which adds up to about 12,000 neck-jerking strikes per day (the equivalent of 12 years of professional football). The woodpeckers have evolved to undertake this enormously demanding task over the millennia with four separate adaptations that relate not only to the removal of wood but also to the extraction of the carpenter ants that serve as their primary food: (1) Physiology of the head and neck; (2) Configuration of the feet; (3) Stiffness of the tail feathers; and (4) Tongue length and structure.

Physiologically, woodpeckers have unusually thick skulls that contain relatively small brains; the dura matter of the brain and the osseous skull are separated by minimal space to offset any long-term slamming effects (now recognized as a problem in human contact sports). Furthermore, the skull curves inward where it is joined to the beak to provide for the flexure of a shock absorber; the muscles of the neck and head are specially configured to optimize the power of the thrust. The jack hammer effect is accentuated with the leverage effect of a long neck and a heavy, chisel-shaped bill. The remarkable cephalic adaptation extends to sensory protection from flying wood chips: the eyes are protected by a nictitating membrane – an inner and transparent eyelid; the nasal openings are reduced to thin slits and are covered with protective plumage. Woodpeckers have zygodactylous feet; they have two toes that extend to the front and two toes that extend to the back. It was originally thought that this adaptation, which is a variation of the three toes forward and one backward that the passerine or perching-song birds use for holding onto branches, was to promote the vertical ascent of tree trunks. However, woodpeckers climb trees with all toes forward. The zygodactylous fore and aft configuration is actually a pecking adaptation to firmly anchor the body in position for the beak- hammer extensions. The rigid rectrices or tail feathers accentuate the stability of the pecking posture by providing for a counterpoise at the rear to establish a solid tripod-like configuration (the two feet and the tail). Taken together, the head, neck, beak, feet and tail act in synchrony – an evolutionary design for a boring machine.

While woodpeckers eat a variety of foods including fruits and berries, their main food is insects extracted from the trees they work so assiduously to extract. They are adapted to this peculiar practice with an exceptionally long, bony tongue that is barbed, sticky and sensory. The tongue is robustly supported by a bony shaft known as the urohyal that extends its considerable length – which is typically twice as long as the beak. The physiology of the skull to both contain and extend this prodigious organ in the small confines of the head requires complex biological engineering. The tongue is held in place by two parallel bony structures called the hyoid horns that spiral around the top of the head. The extension and retraction of the tongue is affected by sliding the horns in muscular tubes that are connected at the base of the bill. The tip of the tongue is equally complex, a spearhead with rearward facing bristles instrumented with proprioceptive nerve endings to provide sensory information to the brain about the nature of the surface contacted – a soft impaled insect for extraction and consumption or the hard surface of impenetrable wood. In essence, the woodpecker is a highly adapted spear-fishing bird that extracts its prey from wood and not from water. A team led by M. Zhang at the Hong Kong Polytechnic University used slow motion cameras and computed tomography to calculate the forces that woodpecker skulls were subject to when pecking. They concluded that there were three primary factors that minimized impact trauma: (1) The loop of the tongue’s hyoid bones acted as a protective strap; (2) The asymmetry of the beak had a dampening effect in lowering the overall load transmission; and (3) The skull consisted of spongy plates that distribute the load evenly to alleviate high stress points.

Woodpecker Holes AT Blackburn 140222
The tell-tale holes of woodpeckers

Pecking for food is important but not crucial as woodpeckers have other means of sustenance. However, pecking is the sine qua non for the construction of a safe, secure and sheltered nest. It is likely that the evolution of the tree-hole nesting behavior followed food pecking practices – the appearance of a hole after pecking in a concentrated area for formic food would have provided the necessary suggestion. Nesting holes are constructed on an annual basis by both the male and the female and can take up to 6 weeks to construct. Once the initial hole is excavated, the bird eventually climbs inside to do the finishing work and to line the egg laying areas with wood chips. The pecked nesting holes are rarely reused by the woodpeckers that made them, a fact which contributes strongly to the ecological health of the area by providing a valuable resource for other birds. The size and shape of the hole varies according to species; Pileated woodpeckers make large rectangular shaped holes.

The combination of the striking appearance of the red-capped woodpeckers and the cacophony of their prandial pursuits was duly noted by ancient cultures; they figure prominently in both Greek and Roman mythology and in the legends of the Native Americans. The Greek ipne was the sacred bird of Ares and the Latin picus was the sacred bird of Mars. Plutarch offers that the courage and the élan of the woodpecker in assaulting the mighty oak tree elicit its association with the god of war. The mercurial behavior of the woodpecker suggested that it was possessed of a powerful spirit – one that was able to foretell the future. Picus was the first king of Latium in Roman mythology, a character noted for his oracular abilities. The Cherokee, like the Romans and Greeks, considered that woodpecker to be a symbol of war, using the red bonnet as a battle ornament. But the quintessential American icon of the pileated woodpecker is Walter Lantz’s Woody Woodpecker whose Mel Blanc laugh will forever echo in the halls of cartoon history. Presumably inspired by an interruption in honeymoon nuptial activities, the mad-cap character epitomizes the seeming restless abandon of its namesake.