Finding the way to wintering sites thousands of miles away is easy for animals -- they just put the coordinates into their GPS systems and follow the turn-by-turn directions. No problem.
Actually, the methods animals use to navigate their migration routes are even more amazing than an animal that could program a GPS device. Some of their navigation methods are so weird we don't really understand them.
The sun - This seems pretty simple. You can judge roughly what direction you're heading in by where the sun is. But factor in the time of day, time of year and cloud cover, and you're left with a pretty tricky navigation system. Yet starlings and ants navigate this way. Some birds can even travel at night using the sun -- theories suggest they take a "reading" from where the sun sets and use that to set their course. Others think that the polarization of light coming from the sun plays a role [source: Purves].
Landmarks - This is another pretty basic navigation system. Fly toward those mountains, head to the left a little when you see the ocean, and make a nest in the first nice-looking tree you can find. Whales traveling in the Pacific Ocean near the North American west coast use this method -- their landmark is hard to miss, because it's the entire continent of North America. They keep it on their left on the way south, and to their right when they head north.
Moon and stars - Planetarium experiments have proved that many birds rely on stellar cues to figure out which way to migrate. We can even tell which star they are orienting from (Betelgeuse, in the case of indigos - [source: Purves]).
Scent - Once an animal is in the general area, scent can pinpoint specific locations. Scent won't get an animal from Saskatchewan to Mexico, but it probably helps salmon find their exact spawning ground, for instance. The scent of rain might shape wildebeest migrations.
Weather - Wind conditions are often used as supplementary navigation aid by birds. When deprived of other cues, such as the sun or stars, birds chose to fly downwind in an experiment [source: Purves]. When the birds could see the sun and stars, they flew in the right direction regardless of wind direction.
Magnetic field - The earth has a magnetic field that's usually undetectable to humans who aren't holding a compass. Some animal species do have the ability to detect the magnetic field, however, and they use it to make their migrations. Bats and sea turtles use magnetic information to find their way [source: PhysOrg]. Some species of bacteria even rely on the magnetic field to orient themselves [source: SAO/NASA].
We're not 100 percent sure how animals detect the magnetic field, but small particles of a magnetic mineral called magnetite have been found in the brains of some species. Those particles may be reacting to the magnetic field and activating nerves in such a way as to send orientation information to the animal's brain.
If you'd like to learn more about animal migration and other topics like it, you might be interested in the links below.
Related HowStuffWorks Articles
More Great Links
- Arctic National Wildlife Refuge. "Caribou Migration." http://arctic.fws.gov/carcon.htm
- BBC (Alastair Fothergill, dir.). Planet Earth, Episode 1: "Pole To Pole." Original air date: April, 2006.
- Bolen, Eric G. & Robinson, William L. Wildlife Ecology and Management (5th Edition). Benjamin Cummings, 2002.
- British Trust for Ornithology. "Birds of Britain: Terns (Family Sternidae)." http://blx1.bto.org/birdfacts/grptexts/terns.htm
- Cornell Lab of Ornithology. "All About Birds: Whooping Crane." http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Whooping_Crane.html
- Drickamer, Lee C., Vessey, Stephen H., & Meikle, Doug. Animal Behavior: Mechanisms, Ecology, Evolution (Fourth Edition). Wm. C. Brown Publishers, 1996.
- Kirschvink, J. L.; Walker, M. M. "Magnetite-based Magnetoreception in Animals: 25+ Years of Theory & Experimentation (Abstract only)." http://adsabs.harvard.edu/abs/2005AGUFM.B31C1001K
- Laymon, Stephen A. "Altitudinal Migration Movement of Spotted Owls in the Sierra Nevada, California." http://elibrary.unm.edu/sora/Condor/files/issues/v091n04/p0837-p0841.pdf
- McGrath, Susan. "Spawning Hope." Audubon Magazine, Sept. 2003. http://www.audubonmagazine.org/features0309/hope.html
- National Park Service. "Migration Basics." http://www.nps.gov/akso/ParkWise/Students/ReferenceLibrary/general/MigrationBasics.htm
- Purves, William K., Orians, Gordon H., & Heller, H. Craig. Life: The Science of Biology (Fourth Edition). Sinauer Associates, Inc., 1995.
- Roach, John. "Longest Animal Migration Measured, Bird Flies 40,000 Miles a Year." National Geographic News, August 8, 2006. http://news.nationalgeographic.com/news/2006/08/060808-bird-migration.html
- Seaworld. "Habitat and Distribution (of Baleen Whales)." http://www.seaworld.org/infobooks/Baleen/habdisbw.html
- University Of North Carolina At Chapel Hill. "Baby Sea Turtles Use Earth's Magnetic Field To Navigate Across Atlantic Ocean And Back." ScienceDaily. Oct. 16, 2001. http://www.sciencedaily.com/releases/2001/10/011012073954.htm
- University of Washington NatureMapper Program. "Monarch Butterfly Facts." http://depts.washington.edu/natmap/facts/monarch_712.html
- U.S. Fish and Wildlife Service. "Whooping Crane Recovery." http://library.fws.gov/Pubs4/whoopingcrane_recovery01.pdf
- Zyga, Lisa. "Bats may use magnetic polarity for navigation." Physorg Biology News, Sept. 20, 2007. http://www.physorg.com/news109511798.html