Has fall come to your part of the world yet, or is summer seeming to linger or perhaps to arrive late? It is technically fall in the northern hemisphere. It's time to head south; time to migrate. How do animals know when, where and how to go? The usual explanation is that the trip is driven by instinct, hard-wired into every migrating animal and sculpted by an ancient logic. But something has gone wrong. Many birds are not embarking on their seasonal migration.