How do birds know where they are going when the migrate over long distances?

Many factors contribute to bird navigation during long distance migrations, including learning, instinct, their solar compass (perception of sun orientation and day length), landmarks, olfactory cues, and a curious ability still being researched called magnetoception (or magnetoreception) which enables them to detect the orientation of the earths magnetic field. The mechanism is not fully understood but some tests indicate that homing pigeons, for example, might exploit magnetite contained in small deposits around the beak. Another theory suggests that a complex protein called cryptochrome might affect bird vision by altering the sensitivity of retinal neurons such that, in effect, birds might be able to 'see' the earth's magnetic field.