Things that creep during the day

By BILL CHAISSON
Of a Feather
I feel like I’m always surprised to see a brown creeper. I don’t think they are uncommon, but they are such low-profile birds that they go unnoticed much of the time. They do sing, but I have never heard the song. Their tawny brown and white coloration is wonderful camouflage against the tree trunks where they constantly forage. And they spend most of their time clinging to the bark and looking for insects and larvae in the crenulations, mostly silent, but occasionally muttering a reedy note.

My most recent sighting of a brown creeper (Certhia americana) was on the nearly dead ash tree to which the wire that holds up my bird feeder is attached. I did not see the bird visit the feeder, although these insect eaters are known to consume the occasional seed during the winter. They are permanent residents here, but in the more northern parts of their range (Canada, higher altitudes) they retreat to warmer climes. There is a large area of the United States that includes the Southeast and much of the Midwest where they are seen only in winter. 

Interestingly, they prefer to forage in conifers, but to nest in deciduous trees, so they require a mixed forest. They are fun to watch because they are so methodical and geometrical in their habits. You will often initially catch sight of a creeper as it moves from tree to tree because they habitually emit a ‘titip’ note as they fly, according to Sibley. (I have more often heard the thin ‘see’ that is ascribed to them by Peterson.) In addition, they have a buffy band on their wing feathers at the base of the primaries and secondaries that is more eye-catching than the rest of their plumage. When a bird alights, it will be at the base of the tree. Clinging to the bark, they will then ascend the trunk in a spiral, appearing and disappearing from view until they reach the top third of the tree. They then fly to a nearby tree and begin the whole thing again. I don’t know if there are clockwise and counterclockwise birds in the sense that we are left- or right-handed.

Bird guides place the creeper after the nuthatches (Sittidae) and before the wrens (Troglodytidae). While bird guides don’t necessarily follow taxonomic order, in this case they do. Nuthatches and treecreepers split first and then the ancestor of wrens and gnatcatchers (Polioptilidae) split from the treecreeper line. (The families are all members of the superfamily Certhoiidae.) Although the brown creeper is the only North American species, it is part of a Holarctic group that includes very similar looking species in northern Europe and Asia.

The resemblance to nuthatches is clear, both physically and behaviorally. Nuthatches, however, have evolved to be able to advance both sideways and upside down along a tree trunk, while the creepers only advance upward. In an example of convergent evolution, the treecreepers have stiffened tail feathers than allow them to brace themselves against the trunk in manner similar to the unrelated woodpeckers. Nuthatches have slender beaks in order to probes crevices in tree bark, but their bills are more robust and allow them to “hack” at seeds and nuts (hence their name). Creeper bills are slender and decurved (curved downward), more specifically adapted to probing for insects in tight places.

As is the case with many widespread species that are not highly mobile, the brown creeper shows a lot of variation in plumage across its continent-spanning range. David Sibley is of the opinion the gray, reddish, and brown morphs within populations are as different from one another as those that have been used to define different subspecies. But he characterizes the western birds as relatively small, dark, and long-billed, while the eastern birds are larger (over 5 inches long), paler, buffier, and shorter billed. This is the reverse of the plumage difference for most birds with western and eastern populations.

Their quietness and methodical way of foraging sets creepers apart. They don’t flit about and squabble like a lot of small birds. And indeed they have some unusual habits. Rather than flee at the approach of a predator, they flatten themselves against the side of a tree trunk with their wings and tail feather splayed out. In other words, they become as much like a limpet as possible and are very difficult to get a grip on. 

These birds also roost for the night in some sort of confined space (most birds just go to sleep on a tree limb) and they may also do so as a group during the winter in order to say warm and because they aren’t maintaining separate territories outside the breeding season. The young in a brood also have the habit of forming a circle with their head together when they go to sleep.

The brown creeper is a sort of provisional member of the feeding guild that includes the chickadees, titmice, nuthatches, and downy woodpeckers. ‘Provisional’ in the sense that it isn’t as much of a seed eater as the others and so does not benefit from bird feeders as much. However, its presence among its unrelated brethren is a sign that this guild very much pre-dates the existence of bird feeders. Heck, it may predate the existence of our species.