Context: The timing of migration seems to be controlled primarily by changes in day length. Migrating birds navigate using celestial cues from the sun and stars, the earth's magnetic field, and probably also mental maps.

Question: What causes the timing of migration?
Answer: changes in day length

Question: How do migrating birds navigate?
Answer: celestial cues from the sun and stars

Question: How else to migrating birds navigate?
Answer: the earth's magnetic field

Question: What might help migrating birds navigate?
Answer: mental maps