Carolina Chickadee Poecile carolinensis Photo: Ken Thomas (via Wikipedia)
Cavity-nesting chickadees and titmice (Paridae), if disturbed while excavating a nest or incubating eggs, display a defensive reaction of mimicking a hissing snake. The hissing may resemble that of a threatened Copperhead or Black Snake, and is accompanied by rapid fluttering of the wings against the sides of the nest cavity. Various observers have described this form of Batesian mimicry in Carolina Chickadees, Mountain Chickadees, Oregon Black-capped Chickadees, Chestnut-backed Chickadees, Plain Titmice, and several European Paridae species. In addition, both adults and nestlings of other hole-nesting, but non-Paridae bird species (e.g. Wrynecks, Flickers, and Wood Warblers), will hiss when disturbed. The hissing sounds, as well as the accompanying swaying and gaping behaviors, are thought to be most convincing as a threat to small mammalian predators.
Sibley, C.G. (1955). Behavioral mimicry in titmice. Wilson Bull. 67, 128-132.