Birds of prey are largely taking their cue to migrate from the length of the day. Other species of birds have more variable migration "triggers"--many flycatchers, for example, will come north early if insects are to be had there. All birds, however, are coordinating their movements to the availability of food in the place where they're going. Sometimes this happens by instinct, sometimes by year-to-year weather patterns.
Birds of prey are largely taking their cue to migrate from the length of the day. Other species of birds have more variable migration "triggers"--many flycatchers, for example, will come north early if insects are to be had there. All birds, however, are coordinating their movements to the availability of food in the place where they're going. Sometimes this happens by instinct, sometimes by year-to-year weather patterns.