by Christine Hanrahan
Recently, an FWG volunteer asked if I knew what was causing the bark to be stripped from some of the ash trees in the ash woodlot. I unhesitatingly replied, “hairy woodpeckers,” then immediately qualified that by saying “more than likely.” I never like to state anything categorically, especially when it comes to the natural world. After all, bark can slough off on its own, and no doubt squirrels were inadvertently stripping some of the loose bark as they moved up and down the trees. Nonetheless, I had watched hairy woodpeckers working on these trees and flaking bark off rough-barked trees, such as ash, elm, maple, ironwood and so forth, in many other locations over the years.
A number of decades ago, I assumed that the bark scaling was the work of black-backed or three-toed woodpeckers, as they are known to strip bark rather than bore holes the way other woodpeckers do. But there was an awful lot of scaling going on and I was not seeing either of these fairly rare (to our region) woodpeckers. Then I realized that those two species typically work on conifers. It didn’t take long to find that hairy woodpeckers were the ones responsible for the work on deciduous trees. Since those early days, I have become so familiar with this bark scaling activity that until I was asked about it, I assumed the cause was well-known.
Hairy woodpeckers feed voraciously on wood-boring insects, which in our ash trees, would include the notorious emerald ash borer (Agrilus planipennis), a buprestid beetle. While these woodpeckers will eat seeds, nuts, and some fruit in season and are common visitors to feeders in winter, the primary component of their diet is insect matter, larvae in particular. Arthur Cleveland Bent, who wrote comprehensive life histories of North American birds in the 1930s, said of the hairy woodpecker’s diet: “More than 75 percent of their food consists of injurious insects…” Quoting Prof. F.E.L. Beal, he writes “The largest item in the annual diet of the hairy woodpecker consists of the larvae of cerambycid and buprestid beetles.” Cerambycids are long-horned beetles. Certainly, that is some of what they’d be feeding on in the ash woods.
Given that the trees are decaying, the wood revealed beneath the bark is usually semi-rotten and easier to probe for prey. How the woodpeckers find and extract their food has been written about extensively, and Googling the species should yield some information on this front. But essentially, it is thought that hearing, smell, and touch help them locate prey, which they then secure with their long, barb-tipped tongue. Bent reports that the stomach contents of some hairies reveal bits of inner bark, so not all the bark falls to the ground!
Today (21 January), while walking around Mile Circle and Hillsdale Park in Rockcliffe Park, I saw at least 4 hairy woodpeckers feeding on ash, maple, and elm trees – all but one of them scaling off the bark. As I stood under one tree, I was amused by the rapidity with which the bark was being stripped. A great shower of the stuff fell on me and the snow below the tree. In other cases, the bark was harder to flake off and took more time. Without exception, all of the many trees I saw with these marks – today and in the past – have been dying. A dying tree can carry on for some years before it truly dies and becomes a snag (a standing dead tree), thus the woodpeckers may, by extracting the destructive larvae, help extend the life of the tree somewhat. What is for sure is that they are not damaging the tree. Bent says, “Evidently the hairy does but little damage by denuding the trees of their bark.” Undoubtedly, some folk may not like the look of the stripped bark, but, as one of the photos below shows, in time, it is barely noticeable.
As best as I can recall, I’ve not seen hairy woodpeckers feed this way other than in winter months, and I also recollect reading somewhere that it is a behaviour more typically found in this season.
So the next time you see trees with their bark scaled off as in the photos below, look for hairy woodpeckers in the vicinity. If you examine the scaled areas closely, you’ll likely see insect (mostly beetle) exit holes, as well as rougher holes where the hairy woodpecker has probed the bark.
One of the photos below, although poor quality, shows the hairy with a piece of bark in his bill, ready to drop it to the ground, and an expanse of just-scaled bark.
Reference: Bent, A.C. 1939. Life Histories of North American Woodpeckers. Order Piciformes. Smithsonian Institution. United States National Museum Bulletin 174.