Migrating birds appear to have a 'sixth sense' which means they always manage to find their nesting grounds, - a talent that has long mystified scientists. Now researchers have found the secret to this skill. It is down to a protein in the bird's eyes that is sensitive to blue light. This protein lets them 'see' Earth's magnetic field as an overlay on their normal field of view. Experts looked at robins and zebra finches finding Cry4 protein was responsible. For a bird to know roughly where it is in the world, and correct itself if it goes off course, it needs what is known as 'true navigation'. They do this by using the magnetic field to plot their migratory routes.