Why was the woodpecker pounding such a puny branch?
The Hairy Woodpecker could only be seeking food. The branch was too tiny to surround a roost cavity.
I could see why a woodpecker might choose this red oak to forage. It has several branch stubs: jagged wounds where fungi can enter. Many wood-boring insects prefer laying eggs on decayed and damaged trees. The hatchling larvae can easily chew their way inside, where depending on species, they may overwinter.
But this particular branch? It’s so skinny—maybe four inches in diameter—it must have been frozen to its core in yesterday’s 14-degree weather. Wouldn’t an insect mom want an egg-laying site with more mass? A trunk or a thick limb that would retain warmth to help her babies survive a Michigan winter. I was sure the woodpecker wouldn’t find food in this branch.
He pounded and chiseled. It took him just six minutes to prove me wrong.
The bird began twisting as he struck the tree. I could see him chewing when he removed his bill from the hole.
Here’s a snippet of what I filmed. See if you can spot the woodpecker’s flicking tongue as he slurps. After the video, I’ll share what I learned when I explored why I had so underestimated the bird’s foraging strategy.The tree, it turns out, has several ways to protect itself in winter. All trees have both living and dead cells. As ice forms inside the tree, it draws water from the living cells, concentrating the sugars left behind. The sap becomes thick and its freezing point drops. Trees also produce antifreeze proteins. These cold adaptations aren’t foolproof. In extreme cold, water in the sap can freeze. It expands and puts pressure on the bark, causing cracks or even an explosion. Fortunately, our weather this week isn’t that severe.
The insects have an equally interesting story. I can’t tell what kind of critter the woodpecker found in the oak. But entomologists I asked for help said the woodpecker’s prey might be in diapause—a state of suspended animation when their bodies generate sugary compounds that won’t freeze. Or, if they’re insects that do not enter diapause, they may be eating lignin inside the tree, which helps maintain body temperature. Or—they might in fact be frozen. Apparently, some insects can survive freezing for several years.
Frozen insect pops, anyone?
Resources I used to satisfy my curiosity include:
Facebook’s Entomology page—many thanks to the members who answered my question.
Cornell’s Birds of the World
National Forest Foundation article on winter tree survival.
A Let’s Talk Science article about trees in winter. This non-profit Canadian organization provides science programs for youth and educators.
Image Credits: Carol Doeringer.