The Sixth Sense of Direction, or, Navigation Secrets of Migratory Birds
If humans had to migrate yearly from one spot on the globe to another thousands of miles away in order to find adequate food and to reproduce, and if the only navigational tools available were landmarks, the earth's magnetic field, the sun and the stars, most people would not have a strong enough sense of direction to make the trip. Yet migratory birds accomplish just this feat twice a year when they travel to their wintering grounds and back to their summer habitats. Each bird has the ability to utilize more than one of the above devices to navigate its way north and south. The urge to migrate at certain times yearly, and even the direction in which to start off in, may be programmed directly into the genes of migratory birds, but is affected by their environment and can be changed through evolution. The migratory "sense" that birds seem to possess seems like a sixth sense, difficult for humans to imagine. It turns out to be a combination of many different factors that birds have learned, throughout the evolutionary process, to interpret.
Evidence for the genetic nature of the migratory reflex and the ability to navigate has been demonstrated by captive birds. Right before the time when they would naturally migrate, caged birds demonstrate restive activity, called Zugunruhe ("migratory restlessness") (6). They display Zugunruhe even if not exposed to daylight or to seasonal temperature changes, which suggests that birds have circannual rhythms programmed into their genes that alert them when the time is right to migrate (6). Similarly, humans have circadian rhythms which guide our sleep patterns over twenty-four hour periods. Also, the trait for Zugunruhe can be passed down: birds with a strong urge to migrate, when bred with birds with a less strong urge, give birth to birds with an intermediate level of restlessness (1). In captivity, birds ready to migrate will not only sense when the time is right but also will orient themselves in the correct direction; when the wild birds of their species change course, captive birds even tend to orient themselves accordingly (6). That captive birds know when and where to migrate while totally unaffected by environmental influences strongly suggests that genes play a role in the migratory process. That baby birds abandoned in the wild will start out on their migration at the right time of year in the right direction without having been shown the way also points to genetic influences (5).
If genes account for the desire to migrate and for an initial starting direction, how do birds navigate from there? Their ability to sense the earth's magnetic field is part of the answer. The magnetic force alerts them to which direction is north, and they adjust their flight path from this information. It was recently established that some birds, including homing pigeons, navigate in this way by using a small amount of magnetite located in the beak underneath the nostrils (2). Pigeons with magnets attached to their beaks were unable to navigate.
Other birds, perhaps most birds, may rely instead on a "radical-pair mechanism" (2). Free radicals are unstable atoms or groups of atoms; in the presence of a magnetic field, their electrons are less disposed to bonding. Birds can detect when more or less energy is needed to bind the free radicals, and thus can sense the presence of a magnetic field (2). Blue-green and ultraviolet light produce the best conditions for free radicals; birds exposed only to yellow and red light have more trouble orienting themselves for migration than those exposed to light at the other end of the spectrum (2). Cryptochromes, the receptors of blue-green light, have been found in the eyes and neurons of migratory birds, and in one species specifically they were found in an area rich with magnetism-sensing neurons. Radical pairs might be an even better explanation than magnetite for birds' sense of direction because they can be used to detect magnetic fields 600 times weaker than the fields that magnetite can detect (2).
The stars and the sun also help to guide migratory birds. Orthinologist Gustav Kramer found that birds adjusted their preferred flight position in relation to the sun in accordance with his adjustment of the sun's position with mirrors (3). In the wild, he observed birds flying in a straight east to west direction despite the movement of the sun during the day; this suggests that birds can keep track of the sun as they navigate. The pineal gland, located on the tops of birds' brains, is light sensitive and has been linked with birds' time keeping behavior (3). Many migratory birds also use "celestial navigation" to guide themselves at night (4). Under a planetarium sky, birds have been shown to change their orientation when the star pattern shifts, and to become more and more confused as more and more stars in the planetarium go dim. Experiments suggest that they use the pattern of constellations in the night sky as a whole to orient themselves, not just the North Star (3).
Birds do use landmarks to navigate as they fly, but this tactic is only helpful after the bird has made the journey before. Mountain ranges, coastlines, rivers, and sometimes even large buildings aid birds in sighting their way (4). Migrating flocks have been observed drifting off their flyway in the presence of strong winds, but flocks flying above a river have reoriented themselves using that landmark (3). Bird watchers are well aware of birds' ability to recognize major landmarks; geographic features that are easy to pick out from above, such as Cape May in New Jersey and Point Pelee on Lake Erie, are popular spots for both birds and their observers (3). Birds' sense of smell may be one more tool they use to find their way. Pigeons with a blocked sense of smell are not able to home as well as others, and European Starlings with severed olfactory nerves returned to their spring grounds in fewer numbers than those who could smell their way (3). The smell of the sea may the directional cue that these birds use.
Migratory birds clearly do not rely only on one tactic to make their yearly journeys, but on a collection of methods. If not enough ultraviolet light is available to create free radicals, birds do not have to rely on the earth's magnetic field to guide them but can instead track the stars. If they must fly under cloud cover and cannot see the sun or stars, they can use landmarks to direct themselves. One orientation tool can even help birds use others; after being exposed to the night sky, birds can orient themselves better if they have to rely later only on cues from the earth's magnetic field (3).
The evolutionary adaptations of migratory birds are the results of millions of years spent developing their directional sense. Their innate ability to determine when and where to migrate, however, can be adjusted by environmental influences, just as genetically similar plants grow differently under varied conditions. Caged birds will display fewer symptoms of Zugunruhe if they are well fed, because the instinct to move in order to find food will be dulled; also, birds have been known to alter their flyway in order to better use energy by avoiding barren, mountainous regions (6). So, both the environmental and the genetic serve birds in their complicated navigation of the globe, and the evolutionary process continues. Humans will probably never be able to understand the experience of having a "sixth sense" for navigation, which is shared by all migratory birds. When a flock of birds flies overhead, bound for a very specific destination, with a precise plan for reaching it, one can only wonder at all the processes that birds orchestrate in order to navigate their flight.
1. Deinlein, Mary. "Have Wings, Will Travel: Avian Adaptations to Migration." Smithsonian National Zoolological Park, Migratory Bird Center. http://nationalzoo.si.edu/ ConservationAndScience/MigratoryBirds/Fact_Sheets/default.cfm?fxsht=4
2. Lindsay, Bethany. "The Compasses of Birds." The Science Creative Quarterly, Sept-Nov 2006: Issue 2. http://www.scq.ubc.ca/?p=173
3. "Migration of Birds: Orientation and Navigation." U.S. Geological Survey, Northern Praire Wildlife Research Center. http://www.npwrc.usgs.gov/resource/birds/migratio/orient.htm
4. Scanlan, Kerry, et.al. "Bird Migration Facts." Zoological Society of Milwaukee. http://www.zoosociety.org/Conservation/BWB-ASF/Library/BirdMigrationFacts.php
5. Tan, Ria. "The Mystery of Migration." http://www.naturia.per.sg/buloh/birds/migration.htm
6. Wikipedia: Bird Migration. http://en.wikipedia.org/wiki/Bird_migration