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How birds navigate
To return to previous years’ nesting sites and wintering areas, migrating birds rely on Earth’s magnetic field
Contributed by Eldon Greij
Published: February 24, 2012  PRECISION: Mechanisms in the eye and beak let birds gain information from Earth’s magnetic field.
Illustration by Elisabeth Kelly
Checking the data sheets revealed that last year’s nest site was about 16 feet from this year’s. Remarkable, I thought. For the first time, the old question of how birds navigate so precisely became personal.
For a long time, our amazement at bird navigation was largely restricted to birds returning to nests in the Northern Hemisphere. Later, when researchers increased their work in the Southern Hemisphere, they discovered that many birds were just as faithful in returning to their wintering sites.
Adding to the wonder of navigation was the uncanny ability of homing pigeons: After being removed great distances from their lofts, they still return with surprising accuracy — even from areas never before visited.
How do they do it?
For eons, migrating birds have oriented, or determined directions, from environmental cues, including the position of the sun, star patterns, polarized light from the setting sun, odors, and Earth’s magnetic field. Mechanisms to select directions from environmental cues are called compasses.
All birds studied so far have at least two different compass systems. Generally, birds that migrate by day have a sun compass; those that migrate at night have a star compass; and probably all migratory birds have a magnetic compass.
While a compass is a great navigational tool, it isn’t enough to allow birds to navigate, or to seek out particular
sites. For this, birds need reference points, like a map.
Finding the avian navigational map has proved elusive. It’s sort of the holy grail for migration researchers. In recent years, new data have supported a working hypothesis about a navigational map that involves Earth’s magnetic field.
Poles and angles
The magnetic field at Earth’s surface resembles that of a bar magnet. Remember in fifth grade when you sprinkled iron filings on a sheet of paper lying over a bar magnet? The magnetic lines radiated out from the poles and connected between the poles. The angles of inclination between the field lines and the bar were progressively larger from the equator to the poles.
Imagine a huge version of this bar magnet in the center of Earth. (See diagram.) Three aspects of the magnetic field available to birds are the direction of the field lines, which course north-south like meridians; inclination angles, which increase with higher degrees of latitude; and the intensity or strength of the magnetic field, which is weaker at the equator and stronger at the poles.
A fourth aspect that is obvious to us but not used by birds is a polarity compass that points to the magnetic north pole. We use a simple compass — a free-spinning magnetized needle that points to the magnetic pole. Birds probably don’t use a polarity compass because magnetic poles move and reverse over geological time, making magnetic north a poor long-term choice.
Biologists have discovered two different mechanisms that birds use for obtaining magnetic information: a light-dependent molecular reaction and magnetite (iron oxide).
The light-dependent mechanism occurs in the eye and is initiated when light of a particular wavelength stimulates a pigment molecule in the retina, raising its energy level and giving directional information. The optic nerve transmits the information to a center in the brain, where specialized brain cells interpret the meaning.
Working with European Robins, Katrin Stapput of J.W. Goethe University in Germany and her co-workers have shown that it isn’t just the amount of light, as a frosted lens will prevent proper response; the bird must also have clear vision. It’s as if the bird can actually “see” directional information superimposed on the visual field. A current hypothesis is that this mechanism is the magnetic compass for determining directions.
The second mechanism involves magnetite, which is found under the outer layer of the beak. Changes in the magnetic field cause the magnetite to move, which helps initiate a nerve impulse in a branch of a large facial nerve. The impulse is transmitted to a region near the stem of the bird’s brain, where specialized cells process the magnetic information.
The magnetite mechanism differs from the molecular mechanism in that it can detect changes of much lower intensity. Magnetic intensity varies — it’s lower at the equator and stronger at the poles — and irregular changes (anomalies) occur throughout Earth’s crust.
If a bird could remember the spatial pattern of the differing intensities, it would have a true map (like a road map) to facilitate migration and homing. It is presently hypothesized that the magnetite mechanism is involved with the formation of a map based on the pattern of spatial intensity differences.
The ability of birds to select directions and find particular locations over great distances boggles the mind. And it shows again the amazing behavior of birds.
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Eldon Greij is professor emeritus of biology at Hope College, located in Holland, Michigan, and the founding editor of Birder's World (now BirdWatching) magazine.
Read more by Eldon Greij.
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