Not all birds fly in a V. Many birds migrate to where they can find food for the winter months. Geese, ducks, and other waterfowl will use the same flyway year after year. Birds have to eat alot of food to store fat for their journey. Owls and hawks may not have to fly very far south in search of food.
Birds fly at different times of the year.
This depends on how far north they live, and when their food supply is disppearing. It requires alot of energy to make the trips, so birds must find places to get food during their flights. Farmers do not want ducks and geese to settle on their fields and eat the grain. Hunters kill ducks and geese. Strong winds can blow birds off their course.