Among the diversity of bird species we have on SḴŦAḴ | Mayne Island, there is one group that stands out in my mind – the woodpeckers (Picidae). We have five species of this family as year-round residents and they are known to many who have suet feeders. While they provide much joy to observe, they have a much more significant role in sustaining the forest and the health of other species on SḴŦAḴ | Mayne Island.

Our Local Woodpeckers

The five resident woodpeckers on Mayne (with SENĆOŦEN names where applicable) are:

• ȾETEṈ | Pileated Woodpecker (Dryocopus pileatus)
• ȾIḴT | Northern Flicker (Colaptes auratus)
• Red-breasted Sapsucker (Sphyrapicus ruber)
• Hairy Woodpecker (Picoides villosus)
• TEMEȽÁĆSEṈ | Downy Woodpecker (Picoides pubescens)

The Importance of Tree Cavities

Cavity-nesting vertebrates comprise a major component of many forest communities. About 25–30% of forest vertebrate species in the Pacific Northwest nest or roost in cavities, and most are obligate hole-nesters, meaning they will only nest in cavities (Bunnell et al.,1999).

Woodpecker excavations in dead and living trees. Most photos by Michael Dunn.

On SḴŦAḴ | Mayne Island, there are an estimated 27 vertebrate species that use excavated tree cavities for nesting, roosting, or feeding. These species may be classified into three groups according to how they acquire tree cavity sites:

• Primary cavity excavators create holes in trees for nesting, roosting and foraging for food (this includes pileated, hairy, sapsucker and flicker woodpeckers).
• Secondary cavity nesters include a variety of songbirds, ducks, birds of prey, and small mammals (on Mayne these could be wrens, swallows, owls, sparrows, rodents or bats) that require cavities for part of their life cycle, but cannot excavate them themselves. They rely on shelters created by primary excavators, or on a limited number of naturally occurring holes.
• Weak cavity excavators, the third group, may excavate their own small cavities in decayed trees, use naturally occurring holes, or use cavities created by another species (this group includes nuthatches, chickadees, and downy woodpeckers) (Aubry and Raley, 2002).

All together, these species —and their interactions with each other and the forest system they occupy — create an interdependent dynamic that must persist through time for their survival. Woodpeckers are the key to this dynamic.

Suitable nest cavities are essential for reproduction in most cavity-nesting species. As primary cavity excavators, woodpeckers create tree cavities to breed and survive, and in doing so, they provide a whole suite of benefits to other species that are cavity users. These benefits range from providing shelter, to providing easier access to food, to providing safe breeding sites.

Some of these cavity users are dependent on certain sized cavities produced by a specific species of woodpecker. For instance, the Northern Saw-whet owl prefers nesting cavities excavated by ȾIḴT | Northern Flickers, while the smaller Northern Pygmy Owl prefers those produced by the Hairy Woodpecker. Raptors such as the American Kestrel take to the larger, more spacious cavities produced by the ȾETEṈ | Pileated Woodpecker. Cavities are not only used for nesting. For example, the Red-breasted Sapsucker drills rows of shallow sap wells in certain trees. In the very early spring, migrating Rufous hummingbirds drink sap from these holes when other spring nectar sources are not available.

Pileated Woodpeckers as a Keystone Species

A primary cavity excavator may even function as a “keystone” species if it provides a critical resource in the community, such as cavities for hole-nesting vertebrates. A keystone is a relatively uncommon species that is linked to the survival of a variety of other species, and plays a critical role in the ecosystem (Aubry and Raley, 2002).

For the coastal dry forests of the Coastal Douglas Fir Zone, of which SḴŦAḴ | Mayne Island forests are a part, the main primary excavating species is the ȾETEṈ | Pileated Woodpecker. This species uses a variety of trees for excavation, mostly conifers in the form of dead snags, or live trees with early decay. All the trees it selects are large diameter and tall, with cavity nest sites set higher up on the tree than all other primary excavators. Many of the selected live trees have heart rot fungal infections, which the pileated woodpeckers access by pecking through the living outer wood of the tree. This softer rotted interior wood allows for easy excavation of large cavity chambers, as well as access to the woodpecker’s primary food of ants and wood boring beetle larvae (Easton 2015). The live wood surrounding the cavity ensures that the cavity will remain intact for a longer time, and so will be available for other cavity users.

These are just a few of the benefits that the ȾETEṈ | pileated woodpeckers provide for secondary cavity users:

• Excavation of nest cavities and cavity “starts” (pileated woodpeckers may start several nest sites and then leave them in various states of completion);
• Excavations of openings to  natural cavities in dead trees; and,
• Foraging excavations in living, decayed, and downed trees.

There are several factors that make ȾETEṈ | pileated woodpeckers particularly useful primary cavity excavators:

• They mate for life and defend a large territory. Being non-migratory, they create cavities in this territory throughout the year.
• They do not re-use their nest cavities, but create new ones in different trees each year.
• As they feed, the woodpeckers continue to find and excavate ant or beetle infested trees and logs.

Because of this relatively high level of activity, pileated woodpeckers add to the stock of quality suitable nest and roosting cavities for secondary users over time. In addition, with their strong beaks, while excavating they create large chips of both live and decaying wood that falls to the forest floor. These smaller pieces decompose faster and enter the forest nutrient cycle more quickly (Aubry and Raley, 2002).

Preserving Woodpecker Habitat

When it comes to long term conservation of the woodpecker species who inhabit our island forests, it is not just the types of trees (deciduous and coniferous) that we should look at, but the age, height, and the density of dead snags, decaying live  trees, and regenerating trees that are part of a healthy mature forest. By ensuring that suitable trees are present for woodpeckers, we also ensure that the range of species supported by woodpecker excavations can also persist.

What You Can Do:

If they are not a danger, consider leaving dead and dying standing trees on your property to allow foraging, roosting and nesting opportunities for woodpeckers and the species they support.

References

Aubry, K., and C. Raley. 2002. The pileated woodpecker as a keystone habitat modifier in the Pacific Northwest. DOI:10.1650/7482

Bunnell, F. L., L. L. Kremsater, and E. Wind. 1999. Managing to sustain vertebrate richness in forests of the Pacific Northwest: relationships within stands. Environmental Review 7:97–146

Easton, W.E. 2015. Pileated Woodpecker. In the Atlas of Breeding Birds of British Columbia. 2008-2012. Bird Studies Canada, Delta, BC.

Martin, Kathy, Kathryn Aitken, and Karen Wiebe. 2004. Nest sites and nest webs for cavity-nesting communities in interior British Columbia, Canada: Nest characteristics and niche partitioning. The Condor 106:5–19.