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Migrating birds, especially those that fly unerringly to small islands in the Pacific, may use the same navigating trick. Just where they keep their instruments is still unknown, but Professor Yeagley says that birds have a mysterious organ in their eyes, at the end of the optic nerves. This organ may contain the delicate electrical nerve fibers that pick up variations of magnetism, and the even more delicate sense that measures the earth's slow turning.