Day length is one of the most important cues for migration. Birds have an internal clock that is sensitive to changes in day length, and they use this clock to track the seasons. As day length increases in the spring, birds start to produce hormones that trigger them to migrate north. In the fall, as day length decreases, birds start to produce hormones that trigger them to migrate south.
Temperature is another important cue for migration. Birds are generally more active in warm weather, and they may stay in one area longer if the weather is mild. However, when the weather starts to get too hot or too cold, birds may be forced to migrate in order to find more favorable conditions.
Food availability can also influence when birds migrate. Some birds, such as hummingbirds, are highly dependent on flowers for nectar, and they will migrate to areas where flowers are in bloom. Other birds, such as waterfowl, are more dependent on aquatic plants for food, and they will migrate to areas where these plants are abundant.
In addition to these environmental factors, birds may also be influenced by social cues when deciding when to migrate. For example, some birds migrate in flocks, and they may follow their flock mates to their destination. Other birds may learn about migration routes from their parents or from older birds in the flock.