Not all birds migrate. Migration is a response to ecological conditions, and birds that migrate do not differ much physically from those that do not migrate. Before northern winters begin many birds travel south to warmer climates. Some animals are impelled to travel for similar reasons, and their trips, too, are often annual and linked to the seasons. These traveling animals are called migrants and their trips, migrations.
Ducks migrate south in the winter.
Although the majority of these insects pass the winter in a resting state, some migrate southward. Each species of insect usually passes the winter in one particular phase of development. Some butterflies winter as pupae, caterpillars, or eggs. Butterflies are in their cocoons. In the winter some insects may come out of hibernation during brief periods of mild weather.