You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Bird_migration

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?

changes in day length