Climatic Factors - ultimate factor that determines food supply and survivability on the breeding or wintering grounds, but also a proximate factor that influences the timing of migration. Bad weather, rain, and heavy winds may cause birds to fly faster or slow them down. In the Northern Hemisphere, high pressure systems have winds that blow clockwise around them and low pressure systems have counterclockwise winds. Photoperiod Clues - this is a proximal cue that tells the birds when to migrate. So they depend on the photoperiod for timing as photoperiod is very predictable and, on the average, is most beneficial for migration.
Species vary in their navigational abilities.
Some birds have the ability to find their way back if they are displaced. Most early studies on navigation tried to explain it by one mechanism. But now we know that birds use several mechanisms. There is some indication that birds use landmarks, at least sometimes. And, of course, birds that navigate at night cannot see landmarks.