Many animals snuggle up in caves, burrows (holes in the ground), or hollow tree trunks and sleep during the cold winter months when food is harder to find and cold weather makes it harder for their bodies to work. This is called hibernating. When an animal hibernates, its heart beats very slowly and it also breathes less. The animal's body temperature goes way down also - to just a few degrees above freezing! The animal's brain also goes into a form of sleep. It takes a long time for a hibernating animal to wake up. Hibernating is great for animals because they do not have to try to find food and they don't have to even go outside in the cold at all! Because its body slows down and stops working the way it normally does and because the animal is not moving around and using up energy, its body is able to survive off of the fat that it stored up by eating a lot in the fall. Besides being protected from the cold while they hibernate, the sleeping animals are also usually well protected from any other animals who might bother them. They make sure their den, cave, or burrow is only big enough for them and their babies and that it is someplace that is hard for other animals to find.
Some animals that hibernate are reptiles and amphibians like frogs and snakes, insects like ladybugs and bees, some fish, and some mammals like bats, hedghogs, and prairie dogs.
Some animals sleep for long lengths of time in the winter, but they do not go into true hibernation. Their heart and breathing slows down, but most of the time their body temperature does not drop as much. They will wake up a few times during the winter to move around a little bit or to eat food they have stored in their den. It doesn't take them as long to wake up as true hibernators, but they will be sluggish for awhile. Animals that go into this type of hibernation (sometimes called torpor ) include bears, chipmunks, and raccoons. Even though they don't sleep all winter long, they still save lots of energy because their bodies slow down and make food last much longer than normal.
Other animals, like foxes and rabbits, do not hibernate at all - they stay active throughout winter. Their speed helps them find and catch food (or escape from predators) and their fur usually changes to white so that they blend in with the snow and are protected from other larger animals.
Lots of different kinds of birds fly south to warmer areas when it starts to get cold. This is called migrating. Cold weather makes it harder for them to find food, even if they can survive the cold. Birds like ducks and geese that get their food from lakes and ponds will usually fly someplace warmer when the water starts to freeze, because they can't catch fish and other things from the water anymore! Cold weather also means it is harder for birds to find shelter because many of the plants and trees they build nests in lose their leaves and become bare. When snow comes, birds will need to have shelter, and a tree without leaves won't do them much good.
Birds that migrate usually leave at about the same time each year and fly back home at around the same time each spring! How do they know when to come back to their homes? Well, that is something we don't know for sure, but birds (like most other animals) have strong instincts that tell them when it's warm enough to head back to their homes up north. Instincts also help birds learn how to fly, help mother animals know how to take care of their babies, and other important things that help them survive. When birds migrate, they fly in large groups or flocks and they often fly in the shape of the letter "v," which helps them glide along on the currents in the air so they don't have to work as hard! Some kinds of birds might fly very long distances, like from Canada down to Central America, while other kinds of birds only fly across a few states. It can take the flocks several weeks or even months to get to their winter home in the south, depending on how far they have to go and how fast they fly.
Birds aren't the only animals who migrate! Monarch butterflies live in Canada and the northern U.S. in the summer, but they fly all the way south to Mexico for the winter. Blue whales spend the winter in warmer waters down south where their babies are born, and then they travel north in the summer to colder water where there is a lot to eat.
When winter comes, plants also go through changes to make it through the cold. Many trees and plants lose their leaves in the fall and most plants stop making food and growing during the winter. This is called being dormant. Some plants produce seeds in the fall and then die. The seeds stay dormant all winter and then sprout in the spring. Other plants store food in their roots and then lose the stems and leaves that are above ground. In the spring the food that was stored in the roots will provide energy for the plant to sprout and grow again.
Dormant - something that is alive, but is temporarily not growing or having any physical activity. This usually happens in the winter to protect the living thing from the cold.
Hibernate - when an animal's body goes into a deep sleep for a long period of time and the animal's heart rate, temperature, and breathing all drop or slow down.
Torpor - a state of sleep when an animal's body slows down but their temperature does not drop and the sleep does not last as long as hibernation does.
Migrate - traveling to the same place at about the same time each fall or winter and returning each spring.