Why Do Woodpeckers Peck Wood?

Some central North America bird species, including chickadees, woodpeckers, nuthatches, and sapsuckers chisel cavities in dead or dying trees to create shelter and to find food.  Many of these birds are year-round inhabitants, and their efforts are easier to observe during late autumn and winter when there are few leaves to obscure our view.  Today we will investigate how beak sizes, beak shapes, and several other adaptations contribute to woodpecker woodworking skills.

^{_{Pictures above clockwise from top left: Yellow-bellied sapsucker by Keith Williams, May 2015; White-breasted nuthatch by Canopic, Feb 2021; Red-Breasted nuthatch by Doug Greenberg, Sep 2018; Black-capped chickadee by CCPoor, Mar 2012}}

Woodpecker beaks are thick and sturdy, with a chisel-like tip to cut away wood in small chips.  The hard-hitting tip exerts about 1200 Gs of force up to twenty times each second.  This amount of force is equivalent to a human hitting their head against a brick wall at 16mph.  A human brain floats inside the skull surrounded by cerebrospinal fluid.  When our head stops suddenly the brain continues moving  forward until it is stopped by the skull.  If the force is hard enough, this may result in a concussion.  A woodpecker’s brain fits snugly inside its skull and moves and stops in conjunction with skull movement.  Inside the skull, the bone is spongy, absorbing energy from the brain moving after the skull stops, managing the impact without harm.

Other bodily adaptations also help to control and dissipate the energy resulting from pecking activities. A woodpeckers top beak overhangs the bottom beak, forcing pecking vibrations downward, away from the skull and through the muscles to the rib cage.  Additional ribs are present at the top of the rib cage, attaching strong muscles to the skull that assist in managing the vibrations.

A woodpecker’s body is held very steady by feet that are adapted to gripping in a vertical position, pulling the body tight to the trunk.  At the same time, the pygostyle, a set of fused bones located at the base of the backbone with strong muscles running the length of the tail, allow the bird to stiffen the tail and wedge it against the tree trunk.  Claws, feet, and tail provide strong structural support as the bird works to chisel out a cavity from solid wood.

A woodpecker’s eyes contain two additional organs not usually found in other bird species.  The pecten and choroid surround the sides and back of the eye socket.  They are filled with fluid right before pecking is started to compress the eyeball into the socket and minimize all vibrations affecting the eyes as the bill strikes wood.  Additionally, an extra thick nictating membrane, the clear, third eyelid present in all birds, helps protect the eyes from flying chips.

Smaller woodpecker species, including the downy woodpecker, have beaks with less of a chisel shape and more of a point.  The beak is used to punch through thin bark layers and pick out small insects from tunnels running under the tree bark.  Insects living inside grass, weed stems, and galls can be easily extracted by a finely pointed beak.

Woodpeckers provide a vital role in forest habitats.  Whether a species migrates or not, most will likely change locations for better food sources, warmer weather, or breeding at one or more times during the year.  Each time they move, they create a new nesting cavity.  As old cavities are abandoned, there are 40 or more other species waiting to use these cavities for shelter, protection, or hunting.  As you take your winter walks, be on the lookout for active cavities among the woodland trees in your area – and the species that are actually using them!