Woodpeckers are unique among North American birds because they are primary cavity excavators, meaning they create the holes they nest in using their own bills. This process is distinct from species that rely on natural hollows or abandoned nests. The purpose of this excavation is to create a secure home for raising a clutch of eggs and protecting their young until they fledge. Locating and preparing a suitable site directly influences the reproductive success of the pair.
Preferred Habitats and Tree Selection
Woodpeckers search for a location that offers a balance between structural integrity and ease of excavation. Most species favor dead trees, known as snags, or portions of live trees that have begun to decay internally. The soft, punky wood of a snag makes the chiseling process less energy-intensive than working with solid timber. The chosen tree must be structurally sound enough to remain standing throughout the nesting period. Trees with heartwood decay are often selected because the outer layer provides strength while the core is soft.
The species of tree and its size are important selection criteria. For instance, the large Pileated Woodpecker often selects trees with a diameter at breast height (DBH) greater than 40 centimeters to accommodate its deep cavity. Certain species, like the Red-cockaded Woodpecker, prefer excavating live pine trees infected with a fungus that softens the heartwood. In environments where trees are scarce, such as the Sonoran Desert, Gila Woodpeckers carve their nests into the thick, fleshy trunks of saguaro cacti. Woodpeckers may also use human-made structures like utility poles or fence posts if natural snags are unavailable.
Anatomy of a Nest Cavity
Once the location is chosen, the pair excavates a chamber carefully shaped for breeding. The entrance hole is typically circular and sized just large enough for the adult bird to pass through, which helps deter larger predators. The Pileated Woodpecker is an exception, as its entrance hole can be more oblong or triangular. The entrance leads to a vertical shaft that opens into a wider nesting chamber at the bottom, creating a flask or gourd shape.
The depth of the cavity varies significantly by species. A small Downy Woodpecker may excavate a chamber about a foot deep, while a large Pileated Woodpecker’s chamber can descend up to 75 centimeters. Woodpeckers do not bring in external materials like moss or feathers to line the nest. Instead, the fine wood chips created during excavation are left in the bottom of the chamber to serve as a soft, insulating substrate for the eggs and young. These cavities are typically only used for a single breeding season before the birds excavate a new home the following year.
Distinguishing Nesting Holes from Other Cavities
Not every hole a woodpecker makes is a nest; key visual cues distinguish a true breeding cavity from holes created for other purposes. Woodpeckers make holes for three main reasons: nesting, roosting, and foraging. A true nesting hole is characterized by a single, clean, and symmetrical entrance with smooth edges, leading to a deep chamber designed for a clutch of eggs.
A roosting cavity, used for sleeping outside the breeding season, is also deep but may have a less refined entrance. It is sometimes accompanied by multiple entrances on the same tree, providing escape routes from predators. Foraging holes, made in the search for insects, are the most common and look very different. These feeding cavities are shallow, irregular in shape, and have rough, torn edges where the bird chipped away wood to expose prey. Yellow-bellied Sapsuckers create distinct, shallow, organized rows of small holes known as sap wells to feed on tree sap and the insects it attracts.