Imagine you’re an albatross, a large seabird that spends months aloft over the open ocean. Now it’s the breeding season: time to head back to your favorite island, do some amusing courtship dances, and lay an egg. But you’re in the middle of the vast, featureless open ocean. How do you find your way back?
Homing pigeons, taken from their roosts and driven up to 800 km away, can fly home. (Several pigeons have received the animal version of the medal of honor for doing this while carrying messages in wartime.) Arctic shorebirds like the Red Knot will fly from the Arctic to southern South America, over 13,000 km, twice a year. Birds are very good at navigation. How do they do it?
Birds have several tricks up their sleeves. They will use landmarks if they can, like we do. At least some of them use smell. But they seem to primarily rely on the stars, the sun, and Earth’s magnetic field.
Stars: Many birds migrate at night, so using the stars to navigate is sensible. If you raise birds in a planetarium, wait until they develop their zugunruhe (“migratory restlessness,” the frustrated activity of a caged bird that knows it ought to be migrating) and look at what direction they want to migrate, you see that they orient in the correct direction. And if you rotate the “night sky”—which you can do because you’re in a planetarium—you find that their attempted migratory direction rotates too, exactly with the stars. This is how you show that birds navigate using the stars.
But there’s a problem with this. The night sky isn’t constant: it moves. It rotates over the course of the night, and over long time periods, it changes. If a bird evolves to navigate by the North Star (the one star that doesn’t rotate during the night), he’ll be fine, but his great-great-great-great-…-grand-chicks will get lost.
So raise some more birds in a planetarium. (Doesn’t that sound awesome? I wish my research involved raising birds in a planetarium!) Show them a “wrong” night sky, one that rotates, not around the North Star, but around Betelgeuse. When it comes time for zugunruhe, these birds orient by Betelgeuse. They have evolved not to use a certain star, but to use whichever star moves the least—a strategy that will work forever.
Sun: Using the sun to orient may seem unremarkable—even humans can do that!—but birds can do something with sunlight that we can’t: they can see polarized light. At sunset, there is a band of strongly polarized light running north-south, which nocturnally-migrating birds use to orient and to recalibrate their magnetic compasses.
Geomagnetism: Although the phrase “birds’ magnetic compasses” conjures up a mental image of a peacock with a pocketwatch-style compass, birds don’t carry their compasses on fobs, but in their eyes and brains. Magnetic photopigments in the eye and magnetite receptors in the brain allow at least some birds to navigate even when they can’t see the sun or stars—as long as sunspots or areas of iron-rich soil aren’t causing a fluctuation in the magnetic field.
An ornithologist named Yeagley reported in 1947 that magnets impaired pigeons’ navigation. Unfortunately for him, when others tried this out, the pigeons navigated just fine, so everyone thought he was crazy. Much later Keeton (1972) discovered that pigeons do use magnetism to navigate (and consequently, putting magnets on their heads disrupts their compass and confuses them), but only on days when they can’t see the sun. Yeagley had done his experiment on a cloudy day, his replicators had done theirs on sunny days, and it took 20+ more years to understand that birds really do use magnetism.
Source: Ornithology, 3rd ed. by F. B. Gill. 2007. New York: W.H. Freeman and Company.