The photoreceptor hypothesis. An alternative hypothesis is that the primary process underlying the compass is instead a magnetically sensitive chemical reaction within the photoreceptors of tic bird's eyes. The alignment of photopigment molecules with the earth's magnetic field might alter the visual pattern in a way that could be used to obtain directional information.
Which hypothesis is correct? Experiments have shown that the a agnetic detector used by birds in blind cages is light sensitive, as - photoreceptor compass should be-but this would also be truc of a light-activated magnetite compass.
In 2004. University of California Irvine researchers devised a clever experiment to distinguish between the two hypotheses. They sodied the way in which migrating European robins held in orientation cages use the magnetic field as a source of compass information to ap in the appropriate migratory direction. They found that the robins oriented 16 degrees north during the spring migration, the appropriate direction. To distinguish between the magnetite and photoreceptor hypotheses, the robins in the cages were exposed to oscillating low-level radio frequencies (7 MHz) that would disrupt the energy state of any light absorbing photoreceptor molecules involved in sensing the magnetic field but would not affect the alignment of magnetite particles.
The chart shown here presents the results of this study. Each data entry is the mean of three recordings. For each recording, the bird was placed in a 35 -inch conical orientation cage lined with coated paper, and the vector (the directional position of first contact with the paper) recorded relative to magnetic north (north = 360 degrees; east = 90 degrees; south = 180 degrees: west = 270 degrees).
Analysis
I. Interpreting Data Plot each column on a circle. For birds orienting to the geomagnetic field without radio interference, what is the greatest difference (expressed in degrees) between recorded vectors and the mean vector of If degrees? For birds orienting with 7 MHz radio interference?
2. Making Inferences,
a. For birds orienting to the geomagnetic field without radio interference, how many of the 12 birds oriented with an accuracy of +/- 30 degrees relative to the mean vector of 16 degrees? For birds orienting with 7 MHz radio interference, how many were +'- 30 degrees?
b. If you were to select a bird at random, what is the probability that it would orient within +/- 30 degrees of the appropriate migration direction (16 degrees north) without radio interference? With radio interference?
3. Drawing Conclusions Is the ability of European robins to orient correctly with respect to geomagnetic fields disrupted by 7 MHz radio frequencies? Is it fair to conclude that the birds' compass sense involves a molecule sensitive to rudio disruption, such as a photoreceptor? That it does not involve particles not sensitive to radio disruption?
Do Birds Use Magnetic Particles as Compass Needles?
Although vision is the primary sense used by all vertebrates, birds also sense their environment using other cues. Some migrating birds use infrasound to orient themselves. Others may use visual cues, such as the angle of polarizing light or the direction of sunset.
Sensing cues such infrasound, polarizing light or direction of light might seem unusual to us, these cues all have one thing in common. They use familiar sensory receptors-eyes and ears, basically. There is, however. onses receptors-eycs to this rule: Many birds that migrate long distances somention use the earth's geomagnetic field as a solisten information!
This use of magnetism by birds was first demonstrated in the laboratory with a surprisingly simple experiment. A bird is placed in an "orientation cage" like the one ion the photo below. It is really just a cone of carbon paper. A bird attempting to escape the cone will peck at the paper wall where it is trying to get out, each peck leaving a black mark. just as a typewriter key does on carbon paper.
When a bird is placed in an orientation cage, it tends to peck primarily at one place, indicating that it knows where "north" is, even when it cannot sec its surroundings. It will peck at the same place, even in the dark.
Now if the researcher places an artificial magnet outside the cage that deflects the magnetic field by 120 degrees clockwise, the orientation of the bird's pecking will shift 120 degrees to the southeast! There is no escaping the conclusion of this experiment: Birds have a magnetic compass.
The sensory system underlying the magnetic compass of these birds is one of the great mysteries of sensory biology. There are the competing hypotheses.
The magnetite hypothesis. One hypothesis is that crystal's of the magnetic mineral magnetite within brain cells of migrating birds act as miniature compass needles. While trace amounts of magnetite are indeed present in some brain cells, intensive research has failed to confirm that information about the orientation of...
