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.
Birds can get caught in storms and other bad weather. Sometimes birds fly into tall buildings or towers. Safe places for birds to stop, rest , and eat are disappearing. Some birds fly at night and use the stars to help them. Birds that fly during the day use the position of the sun as a guide.