How do birds know when it's time to fly South?

Not all birds fly South for the Winter, and some other animals also migrate, including butterflies and some fish. Most know it is time to start their journey because the amount of sunlight each day starts to decrease . Some have built in clocks that tell them it is time to leave, much like people have an internal clock that tells us when it's time to sleep. Scientists aren't sure how birds know they're going the right way, but some of the suggestions are that they navigate by the stars or the sun, that they learn the way from their parents, or that birds can use the earth's magnetic field to line themselves up.

You must have learnt at school that a magnetic compass needle always points to the North. This is because the earth itself is a magnet and its magnetic lines of force stretch from the magnetic North Pole to the magnetic South Pole. Somescientists say that birds are sensitive to this magnetic field, which enables them to fly in the right direction. No organ, that functions as a magnet, however has been found in the body of birds. Experiments performed on migratory birds indicate that when the sky is overcast, they find it difficult to fly. Some scientists are of the opinion that perhaps, during migration, birds are guided by objects in the sky like the sun and the moon. Nothing can be said with certainty though.

http://www.enchantedlearning.com/subjects/birds/

Magnetic Resonance of the earth changes. Days change, Food becomes scarce. Internal compass and timing system similar to salmon when they run.

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it feel fcold then go south