Many birds use short overnight torpor, including hummingbirds and swifts. The poorwill goes further: individuals have been recorded torpid for periods of weeks, with body temperature dropping to around 5 degrees Celsius and oxygen consumption falling to a small fraction of resting levels. Rousing takes hours.
The first scientific record came from Edmund Jaeger, who located a torpid poorwill in a rock crevice in the Chuckwalla Mountains in 1946 and found a bird in the same site in following winters. The behaviour was already known to Hopi people, whose name for the bird refers to sleeping. Later fieldwork confirmed torpor in wild birds elsewhere in the range.
Whether this qualifies as hibernation in the strict mammalian sense is partly a definitional argument, and how widespread prolonged torpor is across poorwill populations is not well quantified. Poorwills also use torpor outside winter, including on cold nights and during incubation lulls.