Migration is the regular movement of a whole population of animals from one place to another and back again when conditions change.
All kinds of animals including insects and birds migrate over thousands of miles. Whales swim across the vast oceans and herds of animals trek across the land.
The most common form of animal migration is the move to cooler areas in summer and back to warmer places in winter.
Interestingly, human societies migrate too. Bedouin tribes spend the dry season near oases in the Arabian desert but when the rainy season comes, they will wander with their sheep and camels to find new pastures.
To time their movements accurately, birds must be alert to changes in their surroundings.
The length of time between sunrise and sunset - day length - affects animals and plants the most.
