Chipmunks build burrows with complex tunnels leading to nest areas and food storage, which they fill in the summer and autumn. Squirrels bury food. Occasionally a squirrel will forget where it stashed nuts or cones and a tree will sprout from that spot in the spring. Bears, deer, and raccoons all build up their fat reserves to keep from starving when food is scarce. The big brown bat, a common sight over lakes in the summer, is known for hibernating in buildings, but will also use caves and mines.
Amphibians brumate in existing holes created by roots or small mammals.
Due to our mild winter weather many animals in the Pacific Northwest, such as squirrels and bears, do not technically hibernate, but become inactive in their dens or burrows, and sleep for extended lengths of time. Some animals overwinter in different forms from their summer appearance. Many butterfly species, for example, overwinter as eggs or pupae. The adults only appear during the milder summer months when nectar producing flowers are plentiful. Other animals migrate to warmer climates.