Hibernation and estivation are caused by different environmental conditions; low food supplies trigger hibernation, while hot and dry weather triggers estivation. Both conditions induce the same physical changes in an animal.
Both hibernation and estivation lower an animal's body temperature and metabolic rate. This reduced rate of metabolism allows the animal to survive for long periods without food. Many bird species undergo a similar process called torpor.
Hibernation is always a reaction to reduced food supply, but estivation may occur in response to either lowered availability of food or conditions too hot and dry for the animal to survive. Animals burrow to cooler underground locations prior to estivation to escape these conditions. Hibernation is associated with cold temperatures because food is scarcer in cold climates during the winter months.
Birds that use torpor do so nightly during cold weather. During the night, the bird's metabolic rate and body temperature drop just as they do in a hibernating animal. This reduces the number of calories that the bird burns overnight. The hummingbird is a unique example of a bird that uses nightly torpor in a warm climate due to its very high metabolic rate. A hummingbird's body temperature can drop up to 50 percent during torpor.