Which North American bird is commonly cited as the only bird known to enter true hibernation? The common poorwill. This nocturnal nightjar can remain in prolonged torpor for weeks or even months, a feat not documented in other birds in the same way.
The common poorwill spends these inactive periods concealed in rocky shelters, especially in the southern part of its range. Its body functions slow dramatically, allowing it to conserve energy when cold weather reduces the supply of flying insects. Many other birds use short daily torpor or migrate, but the poorwill’s extended seasonal inactivity is the reason it is linked with “true hibernation.”
The behavior was definitively described by naturalist Edmund Jaeger after he found a torpid poorwill in California in 1946 and published his account in the late 1940s. Earlier observations existed: Meriwether Lewis recorded apparently hibernating poorwills during the Lewis and Clark Expedition in 1804.
Strictly speaking, ornithologists often describe the state as extended torpor rather than insisting on the mammalian label hibernation. That terminology explains why the common poorwill is best phrased as the bird “commonly cited” as the only known avian hibernator.