FIG. 45.12

Does a biological clock play a role in birds’ ability to orient?

BACKGROUND One suggestion for how pigeons home is that they use a sun compass. If you are in the Northern Hemisphere and you know the time is 12 noon, then the sun is due south of you. Orienting yourself by this method is possible only if you know the time, so the question arises whether homing birds have the ability to tell the time. One way to answer this question is to “clockshift” the birds. Researchers raise birds in an artificial day–night cycle that is out of sync with the actual one. When released into a sunlit environment, these birds’ sense of time is shifted by a set number of hours.

HYPOTHESIS If a bird’s ability to home is dependent on an internal clock, clockshifting should affect the bird’s homing ability in a predictable way. Given that the sun travels 360 degrees in 24 hours, a 6-hour clockshift will result in a 90-degree error in homing direction because 360/(24/6) = 90.

EXPERIMENT Birds were clockshifted by raising them in a chamber under an artificial light. Birds from a home loft in Ithaca, New York, were released on a sunny day at Marathon, New York, about 30 km east of Ithaca. Release on a sunny day made it possible for the birds to use the sun’s position to navigate.

RESULTS As expected, the control birds (those that were not clockshifted) were usually good at picking the direction of their home loft, heading approximately westward toward Ithaca. The results for the clockshifted birds were very different: They miscalculated the appropriate direction. These birds headed approximately northward, as shown by the positions of the red triangles on the compass in the figure.

INTERPRETATION Assume the birds are released at 12 noon, when the sun is due south. The control birds know to fly in a direction 90 degrees clockwise from the direction of the sun, but the clockshifted birds “think” it is 6 p.m., so they expect the sun to be in the west. Their 90-degree clockwise correction, then, has them flying due north.

CONCLUSION The clear difference between control and clockshifted birds in the experiment shows that an internal time-based sun compass is an important component of the birds’ homing abilities. However, the scatter of points (for both experiment and control) suggests that other factors are also important. This conclusion is reinforced by the observation that birds home well on cloudy days, when they cannot use a sun compass, suggesting that birds use multiple cues and navigational systems when they are homing.

SOURCE Keeton, W. T. 1969. “Orientation by Pigeons: Is the Sun Necessary?” Science 165:922–928.