True hibernation denotes a lowering of body temperature, a near cessation of heart beat and breathing, a general low metabolism and other physiological changes. The state of winter inactivity and torpor of bears, skunks and raccoons is not considered a hibernation. They retire to shelter in winter when it becomes cold, but they remain warm blooded. It was left to the author of this paper to find by merest chance a torpid Nuttal's (sic) Poor-will in the Chuckawalla Mountains of California. For four winter seasons I carried out those observations and experiments which at last definitely proved that, in at least one avian species of the family Caprimulgidae, hibernation is indeed a fact.¹