Migrating birds use the sun's location and a built-in biological clock to know which way to fly
Published: September 1, 2005
|Checking a compass is probably the last thing birds do before migrating. They want to make sure they leave in the right direction, and it's easy for them to do because their compass is built in. |
Only a small number of species have been studied in detail, but we know birds have at least three different kinds of compasses: solar, stellar, and magnetic. Birds that migrate by day likely determine their direction from the sun. Birds that migrate by night utilize the stars. And many, if not all, of both groups probably utilize the earth's magnetic field as a back-up system.
The sun compass was discovered in the 1950s by Gustav Kramer, who studied European Starlings kept in an outdoor pen in Germany. (Remember that the starling is migratory in Europe.) Kramer found that birds, when physiologically ready to migrate, gathered in the part of the pen that corresponded to the correct compass bearing for their impending flight. In order to select the correct direction, however, they had to see the sun. If the sky was overcast, the starlings spread randomly around the pen.
In an experiment designed to explore the role of the sun, Kramer built an enclosed pen with large windows in the walls. When the starlings perceived the sun through the windows, they oriented in the appropriate direction. But when Kramer closed windows in line with the sun and placed large mirrors adjacent to open windows off to one side, the birds reacted as if the sun in the mirrors was the true sun. That is, they oriented the same number of degrees from the "mirror sun" as they would have from the true sun.
The altitude of the sun was not important, as the birds responded to the sun azimuth, a point on the horizon located directly below the sun.
How birds tell time
Kramer and his co-workers made another important observation: They noted that birds orienting correctly in a cage or pen maintained the correct bearing throughout the day, even though the sun appeared to move continuously across the sky. To compensate for this movement, the scientists said, the birds needed some kind of biological clock.
Of the birds studied so far, those with a sun compass also have a biological clock. And they are genetically programmed with a compass bearing for their southern migration. Because migration in North America is generally north-south, this bearing is close to south and is measured about 90 degrees from the position of the sun at sunrise at the time of migration.
To compensate for the sun's apparent movement, birds must know how much it moves. Presumably, birds "learned" the rate over the eons of time in which they experienced the earth's rotation of 360 degrees every 24 hours. The rotation produces the sun's apparent movement of 15 degrees per hour. But that by itself is not enough, as birds must still be able to measure time.
The mechanism of the biological clock has been studied through a series of "clock-shift" experiments in which birds are kept in windowless aviaries and subjected to different combinations of light and darkness. It has been demonstrated that birds kept in aviaries in which the lights are turned on at true sunrise and switched off at true sunset orient themselves the same as wild birds. These birds serve as controls.
The experimental birds experience the same number of hours of light and darkness as the controls, but the lights are switched on and off earlier. Sufficient time is allowed for the birds to acclimate to the new conditions, then they are tested by being placed in an outdoor pen at true sunrise. Having experienced "dawn" (lights switched on inside the aviary) earlier than the controls, will they respond as the controls do or differently?
Degrees of separation
Consider what happens when the aviary lights come on three hours before true sunrise: When birds from the aviary are placed in an outdoor pen at true sunrise, they orient southeast rather than south. Experiencing three hours of light before sunrise causes the experimental birds to adjust their position 45 degrees to the east of the position selected by the controls.
Dividing the amount of their adjustment by the number of hours yields the magical correction of 15 degrees per hour. Somehow, the birds are able to measure the time elapsing after sunrise, or the total time spent in the light.
How diurnal migrants have learned that they must orient a particular number of degrees from the rising sun and compensate for the apparent movement of the sun throughout the day is astonishing, and another example of the amazing behavior of birds.