Laura Erickson's For the Birds

Birds and humans expect trees to be lush and green in June, but this year in northern Wisconsin, forest tent caterpillars have defoliated enormous tracts of deciduous forests, leaving oak, aspen, and fruit trees as bare as December.

These caterpillars, descriptively if inaccurately called army worms, have an extraordinary population cycle. Most years they’re pretty much invisible, but in years like this one they’re almost unbelievably abundant. We read in the newspaper that a train couldn’t make it up a one-degree incline; the caterpillars’ slippery bodies left it spinning its wheels. In early June we tried to make recordings of warblers in the treetops, but kept picking up a loud background noise-a humming pitter patter like steady rain which we suddenly realized came from army worms, chewing and walking on the leaves, their frass raining down on the leaves below. As the millions and millions of army worms grow, so does the noise they produce. Of course, when we stand in an area being eaten alive by them, we’re not likely to be listening to the noise, so distracting are the caterpillars dropping down and crawling on us, strands of their webbing catching in our eyes and hair.

Birds-building nests count on leaves in the branches surrounding their nests to protect their babies from exposure to sun, rain, wind, and the prying eyes of predators. Even without army worms crawling all over their babies’ tender skin and wide open eyes, the loss of protective leaves can seriously reduce nesting success for many species. But one bird’s population actually increases right in step with the army worms. During caterpillar population highs, Black-billed Cuckoos also increase and multiply, because these caterpillars provide their primary food source.

Black-billed Cuckoos are beautiful, graceful birds that belie the phrase “you are what you eat,” considering that the rough, hairy skin of tent caterpillars is rather the antithesis of their own soft, sleek plumage. Cuckoos are 12 inches long, the size of Blue Jays, but more slender, with fully half of their length in their lovely rounded tail. They’re rich brownish gray on their backs, pure white beneath, with red rings around their eyes. They have big tail spots, not as huge as their close relative the Yellow-billed Cuckoo but noticeable and pretty nonetheless. When they fly across an open area, their unusually long tail makes their silhouette easy to recognize. One feature we’re not likely to notice when we see a cuckoo perched or flying is their zygodactylous feet, with two toes facing forward and two back, exactly like the roadrunners they’re closely related to. We’re more likely to notice their feet when a cuckoo crashes into our window and stuns or even kills itself Unfortunately, their habit of flying low makes them a frequent victim of window strikes. Many people have cuckoos nesting in their backyards this year, but tragically few will even notice these attractive birds unless one bonks into their window.

Despite their handsomeness, the fact that they often perch on open branches, and their habit of calling frequently, cuckoos are just not the kind of bird people pay attention to. They usually perch in a Zen-like state, motionless, paralleling the branch. If we’re using the birders’ trick of watching for movement, we simply won’t see a perched cuckoo. And even when we pan the branches, its silhouette often makes a cuckoo blend right in as if it were just another branch. Why should a cuckoo flutter from branch to branch searching for food when its food is literally crawling underfoot?

The “cu-cu-cu” call is fairly loud and easily within the sound frequency range that most humans can hear, but something about it not only doesn’t command attention, it actually seems to lull us into NOT noticing. Perhaps a cuckoo is actually saying, “Pay no attention to that bird in the trees,” or “Move on, folks. Nothing to see here.’‘ Whatever the meaning, most people have to consciously listen in order to even notice a cuckoo. But when we actually spot one, that Zen-like stillness often allows us nice long looks. I watched one calling at my mother-in-law’s recently. I was so taken with its beauty and fascinated by how its throat puffed out with every call that I didn’t notice dozens of caterpillars dropping down on and crawling over me. Black-billed Cuckoos not only help us by eating huge numbers of army worms; they also distract us from noticing them with their fascinating beauty.

By next year the army worm population will crash. I can’t say I’ll miss them, but I sure will miss seeing and hearing the birds cuckoo enough to thrive among them.