A bird uses a variety of cues to navigate accurately. One cue is the direction of the sun. However, because the sun is continually changing its position in the sky, a bird also needs other information—such as the time of day—to determine what the sun's position means.
In the accompanying animation, we describe an experiment that tests whether pigeons use what is called a time-compensated solar compass to determine compass directions. That is, does a pigeon use an internal clock, called a circadian rhythm, to assess the time of day and thereby interpret the sun's position in the sky?
Biologists set up an experiment to determine whether birds can use an internal sense of time, called a circadian clock, along with the sun to determine compass directions. The experiment began by placing birds in cages in which the birds could see the sun and sky, but no landmarks to which they could orient.
The experiment was performed in the Northern Hemisphere, such that at 6 AM the sun was in the east, at noon it was in the south, and at 6 PM it was in the west. The 12 hours of light during the day's normal light/dark cycle began at 6 AM.
A number of food bins were arranged around the outside of the cage. In the first phase of the experiment, the birds were offered food only in the south-facing bin. Here, the trained bird successfully learned to find food in the south-facing bin, even though the cage was periodically rotated.
Each dot represents an attempt by a bird to get food from a bin in a particular direction. The trained birds were then moved to the next stage of the experiment. The birds were removed to a light-controlled environment, and their circadian rhythms were phase-shifted by six hours. Their 12-hour period of light began at midnight rather than 6 AM.
The birds were exposed to two weeks in the phase-shifted light/dark cycle, and then returned to the circular cages. These birds, which were trained to look for food in the south bin, now predominately looked for food in the empty east bin. They did this even though the south-facing bin was still the only bin with food.
The six-hour clock shift resulted in a 90-degree error in orientation, showing that animals can orient by means of a time-compensated solar compass. At sunrise, the birds' internal clocks were striking noon, a time at which the birds had previously found food in the bin located toward the sun. Now this direction is to the east.
It appears that pigeons and perhaps other birds use their internal clocks (circadian rhythms) to determine compass directions from the position of the sun. In the Northern Hemisphere, if their internal clocks say it is noon, the sun should be located in the south. Likewise, if their clocks say it is late afternoon, the sun should be located in the west.
Clock phase-shifting experiments provided the evidence for this conclusion. Pigeons were put in a room where the light/dark schedule could be shifted with respect to real time. Exposure for 2 weeks to a light/dark cycle in which lights came on at midnight caused a 6-hour phase advance of their internal clocks. Thus, when they were returned to the outdoor cages, the birds saw the sun rise when their internal clocks indicated it was noon. As a result, these birds, trained to seek food in the south, oriented to the food bins in the east. A 90-degree error in their orientation was induced by a 6-hour phase shift of their internal clocks.