Woodpeckers are a diverse group of birds, with nearly 200 species worldwide, and their lives are fundamentally connected to trees. They utilize woody plants for communication, shelter, and sustenance. The specific qualities of the wood, bark, and internal structure dictate whether a tree is suitable for a particular woodpecker activity. The relationship between a woodpecker and a tree is highly selective, with different species seeking unique conditions to meet their daily needs.
Structural Requirements for Nesting and Shelter
Woodpeckers are primary cavity excavators, requiring trees with soft interior wood for hollowing and hard exterior wood for structural integrity. They frequently target “snags” (standing dead or dying trees) or living trees where the heartwood has begun to decay. This internal softening is often caused by fungal infections, such as the white trunk rot fungus that affects aspens, or Phellinus igniarius, which creates conditions suitable for larger species like the Pileated Woodpecker.
The soft, decayed heartwood allows the bird to expend less energy when excavating the deep nest chamber. The remaining sound sapwood and bark surrounding the core provide insulation and protection from predators. Woodpeckers generally select large-diameter trees, often over 16 inches, because they are old enough to have developed significant heart rot.
Species like the Northern Flicker and Yellow-bellied Sapsucker commonly choose aspens or cottonwoods because these trees are naturally prone to heart rot. Larger trees offer a more secure and insulated environment for raising young. These excavated cavities also provide housing for many other species that cannot excavate their own nests.
Tree Species Preferred for Insect Foraging
Foraging is the most common interaction between a woodpecker and a tree, as the bird searches for wood-boring insects, larvae, and ants beneath the bark. Woodpeckers are drawn not to the tree species itself, but to the presence of an infestation or disease that attracts their prey. They target trees with loose bark or those that are stressed, dying, or recently dead.
A clear example of this selective foraging is seen in areas infested with the Emerald Ash Borer (EAB), an invasive insect that feeds on ash trees. Woodpeckers, including Downy, Hairy, and Red-bellied species, locate and consume a large percentage of EAB larvae. They strip away the outer bark to expose the larvae tunnels, a behavior known as “blonding” due to the lighter color of the exposed wood.
Woodpeckers have been recorded removing up to 95% of the EAB population in heavily infested ash trees, illustrating their role as a natural control agent. They focus their efforts on trees showing symptoms of canopy decline, which indicates a higher density of borer larvae. This foraging also occurs on other insect-riddled species, where the presence of beetles or ants signals a rich food source.
Specialized Trees Targeted by Sapsuckers and Nut-Caching Species
Some woodpeckers have specialized feeding strategies that require particular tree characteristics beyond common insect prey. Yellow-bellied Sapsuckers drill precise, uniform rows of shallow holes, known as sap wells, to access nutrient-rich tree sap. They favor thin-barked species with high sugar content, such as sugar maple, paper birch, hickory, and Eastern hemlock.
The sapsucker targets phloem sap, which flows just beneath the bark and can contain a sugar concentration of 20 to 30 percent. They often return to the same trees, maintaining the wells to ensure a continuous flow of sap. This sap provides sustenance and attracts insects for additional protein.
Nut-Caching Species
The Acorn Woodpecker relies on mast-producing trees, primarily oaks, for their winter food supply. These highly social birds drill thousands of custom-sized holes into a single “granary tree,” which can be a dead limb or a thick-barked trunk. They store acorns by hammering them tightly into the holes to keep them dry and safe from thieves.
The granary tree, maintained by family groups over generations, must be located where the nuts can dry out to prevent mold. The birds test each acorn, ensuring a perfect fit so the nut cannot be easily pulled out.