Outside my window, birds and squirrels and a presumable variety of other critters are pairing up in my backyard — bluebirds and house sparrows have commenced their annual fight over the bluebird houses that, if history is prognosticator, will soon be home to a clutch of sparrow chicks.

House Sparrows are the definition of “backyard bully.” I once watched a house sparrow carefully remove a set of three bluebird eggs from the latter’s nest and place them carefully, punctured and drained, together on the ground below.

The warm days, though scattered, invite wandering in the Big Woods, which I did one day with a favorite companion. I followed along with my camera, capturing images of rhododendron buds and last fall’s Puffballs. In one section of the trail, she stopped every several yards to stomp the leaves and watch clouds of brown smoke swirl around her foot.

Puffball mushrooms, I am told, are tasty in spring, but by fall, when they are most recognizable, they turn brown and their interior meat turns to powder. Their shells become leathery bellows for critters and playful humans to stomp, puffing spores out across the ground to begin this year’s crop.

Then I saw it, just off the trail, a girdled Gray Birch, its inner layers exposed where the two-foot-wide cummerbund had been stripped from its waist, standing alone among its kin, begging attention. “I wonder what did that,” I asked.

The likely cummerbund-stripper turned out to be a Hairy Woodpecker, busy prying chips from the exposed birch like a carpenter with a wood chisel. With a few taps and a pry, another piece of wood joined the growing carpet of compost chips on the ground.

The weakened trunk, about four inches in diameter, apparently had broken off in the wind; two more sections lay strung out on the ground. The bird, working on the still standing section, had stripped away the bark and set to work on the exposed body layers, carving dimples into the still-standing section of tree trunk.

The exposed innards were clean and not obviously rotten, the fibers still bright yellow and slightly bendy, like “green” wood exposed by a camper’s hatchet. All around it was marked with tiny holes, perfectly cylindrical, about a sixteenth-inch in diameter. On closer inspection, the holes were not empty.

Adult bark beetle is about the same size as the end of a ballpoint pen.

A adult bark beetle removed from its hole in the trunk of a birch tree. The pen indicates the size of the tiny bug. (©John Messeder photo)

Bark beetles — so called because they set up their nurseries just under tree bark — lay their eggs on the tree’s bark, April through September. When they hatch, the larvae gnaw their way into the tree to feast in the layer of wood that carries sap — the tree’s blood — from the ground to the rest of the tree.

Then they pupate, become adults and winter over within the host tree until spring, when the set to work on the next generation.

The good news in most forests: the beetles (estimated 600 species in the U.S., alone) do not generally attack a healthy tree, but prefer trees already suffering from stressors such as drought, woodpeckers, ravenous insects and tree-killing fungi.

Adult bark beetle on a chipped birch ledge

A adult bark beetle in early spring, a little sluggish where it is chipped out of its winter lair. Soon it will be time to lay eggs for the next generation of woodpecker food. (©John Messeder photo)

The bad news: once bark beetles take an interest in an already stressed tree, they often kill it, increasingly so in some areas because milder winters are not killing as many wintering bark beetles, which results in larger populations ready to attack in the spring.

The good news; it’s a smorgasbord for woodpeckers and, later, forest floor compost for fungi and new generations of trees.

It’s the cycle of life in one tiny spot in the forest. The tree draws sustenance from the ground and the sun. Bugs feed on the distressed and dying tree, then become fodder for the woodpecker, which chops the tree into compostable chips, which become part of the ground and sustenance for the next tree.

It’s not pretty if one thinks of it in individual steps, but as a whole, it’s the perfect perpetual motion machine.