Brown Creeper (Certhia americana)

Birds of North Carolina – Brown Creeper (Carolina Bird Club)
In winter, this bird almost always occur in mixed species flocks led by chickadees and titmice, foraging through a variety of forests and woodlands, favoring a mix of pines and hardwoods. Creepers spiral upwards along tree trunks, usually staying on the shady side of the trunk, and then after a minute or two on one tree, fly down toward the base of another tree.

Audubon Guide to North American Birds – Brown Creeper
Looking like a piece of bark come to life, the Brown Creeper crawls up trunks of trees, ferreting out insect eggs and other morsels missed by more active birds. It is easily overlooked until its thin, reedy call gives it away.

All About Birds – Brown Creeper (Cornell Lab of Ornithology)
Brown Creepers are tiny woodland birds with an affinity for the biggest trees they can find. They are well camouflaged and inconspicuous against tree bark in a shady forest, but if you keep your eyes peeled for movement, you may find a creeper zigzagging upward as it gleans insects from the trunk, or see the small shape of one dropping from high on a trunk to the base of a nearby tree. Once learned, the high, insistent call note can alert you to the presence of these birds.

Brown Creeper Song and Calls (Cornell Lab of Ornithology)
Only the male sings, and usually only on the breeding grounds, though sometimes during migration as well. Males and females make high, wavering call notes that sound like a small chain being dropped into a heap; these notes are noticeably longer than the very short call notes of many other birds. Creepers give these calls all year long and especially while foraging.