Migratory animals use different mechanisms to sense the Earth's magnetic field for homing and navigation. Writing in Current Biology, a team led by Wolfgang Wiltschko shows that bright coloured light makes migratory birds switch to a magnetic orientation strategy that is also used by lobsters and salmon. In a companion paper, the researchers report the intriguing finding that chickens can be trained to orient using magnetic cues.

There are two main types of magnetic compass found in certain animals. An 'inclination compass', which is used by birds and sea turtles, defines 'poleward' as the direction along the Earth's surface, in which the angle between the vertical component of the magnetic field and the gravity vector is smallest. By contrast, a 'polarity compass', which is used by lobsters and salmon, determines north using the horizontal component of the magnetic field.

The inclination compass is thought to involve magnet-sensitive chemical reactions triggered by specialized photopigments on the retina on light absorption. Changes in magnetic fields can affect these chemical reactions, which, in turn, alter neuronal activity.

In the first study, the researchers analysed how the ability of migratory robins to orient could be affected by light intensity. Under low-intensity turquoise light in the local geomagnetic field, the robins showed normal migratory behaviour, with southerly headings in autumn and northerly headings in spring. However, under high-intensity turquoise light the robins preferred northerly headings in both seasons.

This unexpected finding prompted the researchers to characterize the birds' orientation mechanism under high-intensity light in a manipulated magnetic field. Interestingly, inverting the horizonal, but not the vertical, component of the magnetic field reversed the robins' heading, which indicates that the birds might have switched to a polarity compass.

This is further supported by the fact that an oscillating magnetic field, which is known to affect inclination compasses, disoriented the robins under low-intensity light, but did not change their fixed-direction headings under high-intensity light. The authors conclude that birds might have at least two magnetoreception mechanisms that are used under different conditions.

Despite birds' innate orientation systems, it has not been possible to train them to move in a certain direction in the laboratory using a food reward. Wiltschko et al. conjectured that, in nature, birds do not use magnetic signals to find food, and tests involving such a stimulus might be alien to them. Instead, the researchers trained domestic chicks to locate a hidden social reward, and found that their ability to orient in the test arena was affected by magnetic fields.

This second study is the first demonstration of a conditioned magnetic compass response in an avian species, and suggests that the ability to orient using magnetic cues has been retained despite thousands of years of domestication.