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Blane Klemek column: Cuckoos feast on forest tent caterpillars

The black-billed cuckoo, top, and a yellow-bellied sapsucker.

While recently hiking on my favorite of Lake Bemidji State Park's many scenic trails high above and overlooking beautiful Lake Bemidji, I stopped to gaze into the canopy of the mature maple-basswood forest. So thick was the greenery of the forest that only bits of the bright blue sky and dappled sunlight were visible.

The afternoon was almost completely calm. It was just the kind of day where hearing the most minute of sounds was possible — a slithering garter snake across dried leaves, the high-pitched calls of brown creepers ascending the trunks of nearby trees, as well as the distant vocalizations of waterfowl and gulls far out on the lake.

Another interesting and clearly audible sound was the sound — actually many sounds — that I've heard before and find, in a way, somewhat unnerving. I was surrounded by the sound of caterpillars munching on leaves.

Indeed, all about me were caterpillars busily feeding on newly formed leaves everywhere high and low. And as I took the time to search for the true sources of the munching music I soon discovered the likely species — forest tent caterpillars.

I no sooner identified the insects when I heard the call of an obscure and rarely observed bird of the forest that made me smile. For those of you familiar with this bird, its habits, and amusing vocalizations, you already know that the black-billed cuckoo has made its way back to the Northland just in time to feast on the abundance Mother Nature provides, particularly on soft-bodied insects such as caterpillars.

To some, black-billed cuckoos might appear to be unremarkable looking birds that are easily overlooked. Slim and long-tailed, these otherwise large 12-inch long songbirds spend most of their time within the dense foliage of thickets and forest and woodland canopies hunting for food — and as already mentioned, mostly caterpillars.

Males and females are indistinguishable from one another. Off-white below and a soft brownish-grayish above, these bi-colored birds' most distinguishing features — aside from their unique call — are their black, decurved beaks, white tail-spots, and red orbital rings. The juveniles of the species are drabber overall and have greenish eye rings instead of red.

The cuckoo's call is somewhat similar to a handful of other species of birds — although perhaps a stretch — such as the saw-whet owl, mourning dove, and pied-billed grebe. That withstanding, the cuckoo's whistle, which is frequently delivered in repeated triplicate arrangement, "po-po-po ... po-po-po ... po-po-po," is unmistakable and quickly put to permanent memory once heard for the first time.

Preferred habitats for cuckoos are forests of dense deciduous trees and shrubs. It is within these environments where they actively hunt and capture their primary sources of food. And while the range of another species of cuckoo, the yellow-billed cuckoo, includes southern Minnesota, chances are good that any cuckoo bird heard or observed here in northern Minnesota will be a black-billed cuckoo.

Cuckoos seem to disappear into obscurity during most of their summer stay, but in years when caterpillar populations are especially high, you may also notice an abundance of cuckoos. During years when hoards of forest tent caterpillars work to defoliate forests and woodlots by consuming the leaves of trees, particularly quaking aspen leaves, black-billed cuckoos will be present as they hungrily and happily gorge themselves with fat, juicy, and nutritious caterpillars.

Black-billed cuckoos consume many different species of caterpillars, as well as other insects, but they favor moth and butterfly larvae above all other invertebrates. In fact, cuckoos eat both hairy and hairless caterpillars, including the spiny ones too.

As strange as it sounds, and because the spines of these latter caterpillars tend to stick to the lining of a cuckoo's stomach, cuckoos have the astonishing ability to shed their stomach lining every so often in order to rid themselves of accumulating spines. Other foods that cuckoos consume are spiders, small mollusks, fish, and wild fruits and berries.

Another noteworthy fact about cuckoos is that the bird is a surprisingly rapid nester. Only 17 days pass from the time when an egg (usually two to four) is laid in the nest-bowl to the time the young nestlings fledge. Yet, despite their reproductive strategy, most pairs of cuckoos raise only one brood per year. To compare, many other species of neo-tropical migrants raise two or more broods a nesting season.

The cuckoo nest is normally built of twigs in the lower parts of deciduous trees and shrubs. Apparently because of their irregular nesting habits, some eggs tend to become partially incubated during the egg laying process before the entire clutch is deposited. Years ago, while working in the woods, I once stumbled upon one such nest and was surprised to find it occupied by a large, somewhat homely half-grown cuckoo chick. Docile acting and very watchful, the chick just looked at me, as if as curious about me as I was of it.

Black-billed cuckoos are remarkable birds that are frequently heard, but less seen. During some years, perhaps because of low caterpillar abundance, you might not observe a single cuckoo. Nevertheless, the black-billed cuckoo travels all the way from South America to Minnesota each spring in order to spend a few months raising their young and calling out their comical and charming "cuckoo calls," as we get out and enjoy the great outdoors.