American Treecreepers

I always think of brown creepers as winter birds but, depending where you live in Pennsylvania, they may be summer residents, spring and fall migrators, and/or winter visitors. Because we live on the westernmost ridge in the ridge-and-valley province, they are migrators as well as winter visitors. But along the Allegheny Front and in the densely forested areas in northern Pennsylvania, they breed, and in southeastern Pennsylvania they are merely passing through on their way north to breeding grounds that extend from Newfoundland west to northern Saskatchewan and Minnesota.

Pennsylvania is near the southern edge of their breeding grounds in eastern North America, and our brown creepers are part of the oldest subspecies, first named in 1838—Certhia americana americana. Nine other subspecies range from Alaska to New Mexico in mostly old-growth coniferous forests and three more subspecies live from Mexico to Nicaragua.

Once brown creepers were thought to be a subspecies of the Old World Eurasian treecreeper, hence their alternate name “American treecreeper,” but based on their calls and especially their song, they were declared a separate species. I’ve only heard their song twice—once here in March and the second time in the Poconos in June—but in tone it reminded me of an eastern meadowlark. Described as “peepee willowwee,” “seetidie swee,” or “see, seeedsee sideeu,” by a variety of observers, our eastern subspecies song is more complex, quavering, and ends on a high note whereas the western subspecies is more rhythmic and ends on a low note.

Even though their song is distinctive, their high “tsee” calls are so close to those of golden-crowned kinglets that I have to see the bird before I can positively identify it.

Seeing this small, brown-backed bird, which looks like a tree-climbing mouse, is not easy. It resembles a “fragment of detached bark defying the law of gravitation,” according to observer Winsor Marrett Tyler, as it hitches up a tree, using its decurved bill to search bark crevasses for insects, spiders, and pseudoscorpions and its long, stiff tail as a prop. When it flies to another tree, it looks like a dry leaf blowing in the wind.

Its brown head, back and tail broadly streaked with white give it excellent camouflage when threatened by a possible predator. Flattening its body against the tree trunk, it spreads its wings and ducks its head, holding that position for as long as 10 minutes according to Maine ornithologist William Brewster, who added in his account, “It was one of the prettiest demonstrations of the effectiveness of protection coloration that I have ever witnessed.”

I’ve never seen more than one brown creeper at a time on our property, but sometimes it is not alone. Most often a golden-crowned kinglet or two is nearby. Occasionally, it is part of a larger wintering flock consisting of black-capped chickadees, tufted titmice, white-breasted nuthatches, dark-eyed juncos and a selection of woodpeckers—red-bellied, downy and/or hairy. But even though a brown creeper may feed near them, it doesn’t always pay them much attention or follow them as they wander.

A brown creeper may come to a bird-feeding area, but only if it contains a suet cage with a selection of peanut hearts or hulled sunflower seeds the experts say. Still, it is almost totally insectivorous most of the year. The closest one has come to my feeding area was last December 10 when it briefly landed on the trunk of a large ash tree infested with emerald ash borers below our back porch bird feeders. In my almost three decades of participating in Project FeederWatch, I’ve never recorded a brown creeper on or below my feeders.

Assembling my notes on brown creepers for the last five years, I discovered that I have seen them on every part of our square mile of forest. They used to be more common along our stream among the hemlocks, but as they have thinned, due to hemlock woolly adelgids, this coniferous-loving bird has moved into our hardwood forest. During a study done by Brian Rollfinke and Richard Yahner in 1991 in central Pennsylvania, they found brown creepers foraging on pines, oaks, and maples, especially white and chestnut oaks.

Sometime in April our brown creepers head north to breed. Both the Alaskan and our eastern subspecies are strongly migratory and move not only latitudinally but altitudinally, breeding up to 4,500 feet in the East and 11,000 feet in the West. Here in Pennsylvania they are most common breeders in the Allegheny National Forest and Sproul State Forest, but according to the Second Atlas of Breeding Birds in Pennsylvania, brown creepers have shifted 10 miles north between the first and second atlasing periods. Furthermore, the number of occupied blocks in the southern half of the state has decreased by 57% in the Piedmont and 34% in the ridge-and-valley province, although it has increased by 10% in their core range within the northern Appalachian Plateau. But even as their range has contracted here, it has expanded farther north in New York and Ontario.

Once a male brown creeper arrives in his breeding area, he engages in intense singing bouts with rivals for a territory he claims of five to 15 acres. In one case, two males countersang 53 times in less than 10 minutes. During territory establishment, a male sings from a high, vertical perch, but once he has won his territory, he whistles while he works, singing from a lower tree trunk while he forages for food.

During courtship, Tyler writes, “he has thrown off his staid creeper habits and has become for a time a care-free aerial sprite, giving himself up, so it seems, to an orgy of speed, wild dashes, and twists and turns in the air,” although both male and female also silently chase, spiraling around tree trunks and then landing one above the other on the trunk, and sometimes fluttering their wings. Usually, this ends with the male feeding the female, a practice he continues while she builds the nest, lays five or six white eggs often splotched with pink or reddish-brown, and sits on the eggs for 13 to 17 days.

The nest of a brown creeper is unique. Both bird artists Alexander Wilson and John James Audubon in the early 19th century, thought they nested in woodpecker holes. But in 1879, based on information from Dr. Thomas Brewer regarding reports to him of brown creepers using detached tree bark as a nest base, just as the European treecreepers were known to do, William Brewster searched in the region of Lake Umbagog in Maine, where he found several nests looking like “loosely hung hammocks” or “new moons.” Nearly always, the female suspends her nest between the trunk and a loose piece of bark on a large, dead or dying tree.

Built in two parts, anywhere from a couple feet to 40 feet from the ground, the base has twigs and strips layered together and stuck to the rough inner surface of the loose bark by insect cocoons and spider egg cases and is hammock-shaped with points that extend above it and are attached to the bark. Into its center a nest cup is constructed of a wide variety of natural materials, for instance, wood chips and fibers, pieces of leaves, bark, lichens, and mosses, feathers, and spider egg cases. Researchers found a nest in Michigan that held 872 pieces.

The female occasionally nests in other, more creative sites—behind a wooden grill or window shutters, in fence posts, nest boxes, or, where large trees are scarce, in tree knotholes.

Once the eggs hatch, the female broods the almost naked nestlings, but both parents feed them for the 14- to 20 days they mature in the nest, and at least a further 17 days as they perfect their flying and foraging skills.

Brown creepers are big tree birds that use mature and old-growth hemlock, white pine and northern hardwood forests in the East for foraging, nesting and even during the winter. Development, clearcutting, and partial logging often eliminate brown creeper habitat, so it is essential to keep as many unfragmented, mature and old-growth forests as possible and to allow more previously logged forests to regrow into old-growth, which is what we’ve been doing with our forest. Although hemlock woolly adelgid-killed trees may provide more nesting habitat for them for a short time, once they fall, the habitat will be gone. It will be interesting to see if they can adapt to other tree species here and in northern Pennsylvania as the climate warms and the hemlocks disappear from most, if not all, of our forests.