Dear Straight Dope:
How do woodpeckers keep from bashing their little brains out when they peck the trees in my yard? Don't they get killer headaches?
A woodpecker’s job isn’t as big a headache as it might seem. Several adaptations allow these birds to get their grubs without knocking themselves out.
A woodpecker’s bill is stoutly built, composed of strong bones covered by a tough horny beak. Species that habitually excavate hard wood (some glean bark or even forage for ants on the ground) have bills with chisel-shaped tips and longitudinal ridges for added strength. As protection against flying chips the nostrils are reduced to slits and shielded by lateral ridges and feathers.
One factor in minimizing the impact is that, as in all birds, a woodpecker’s brain is much smaller (both absolutely and proportionately) than a human’s. Because surface area increases with the square of the linear dimension while mass increases with the cube, impact forces are spread over a relatively greater area, which alone makes the woodpecker 50 to 100 times less vulnerable to impact than a human would be. The blow itself is delivered straight on, reducing hazardous rotational and shearing forces. (However, on follow-through the bird will often twist its head to help dislodge a chip of wood.) Because the braincase is situated above the line of the bill, the force of impact is transmitted mainly below the brain. As in most birds, there is a movable hinge between the upper mandible (maxilla) and the frontal bone of the skull. The hinge in woodpeckers, however, is unusual in that it is folded inwards, so that when a blow is struck the joint undergoes tension rather than compression. Because of this, the tension can be counteracted by a special muscle that helps absorb the shock. The folding over of the frontal bone against the maxilla also forms a kind of locking device that prevents the bill from being forced open by the impact.
Woodpeckers have several other adaptations, including powerful neck muscles, strong legs with sharp claws, and stiffened tail feathers that serve to brace the body against tree trunks. But perhaps their most unusual feature is the tongue, one of the most remarkable adaptations found among birds. Many species use the tongue to extract wood-boring insects from their tunnels. The tongue tip is often barbed to harpoon its quarry, and the salivary glands exude a sticky, almost glue-like saliva to help secure the prey. When the tongue is retracted, the basal supports (hyoid apparatus) extend from the back of the mouth around the back of the skull all the way to the crown of the head. In some particularly long-tongued forms, they extend even further and originate inside the upper mandible. Some species with very strong bills don’t have this option because the inside of the upper mandible is taken up by structural supports, so the hyoid apparatus extends into the right eye socket and coils around the eyeball. See this site for an image:
Winkler, H., D.A. Christie, and D. Nurney. 1995. Woodpeckers: An Identification Guide to the Woodpeckers of the World. Houghton-Mifflen New York.
del Hoyo, J., A. Elliot, and J. Sargatal. (eds). 2002. Handbook of the Birds of the World. Volume 7. Jacamars to Woodpeckers. Lynx Edicions, Barcelona.
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