To us it seems miraculous: a migrating bird can embark from its wintering grounds and successfully return to the breeding site that it may have left months before. Guided only by their internal compass, birds and many other migrating species sometimes cover thousands of kilometres before arriving home. Yet, how these intrepid voyagers detect the earth's magnetic field for navigation is a topic of hot debate. Roswitha Wiltschko, Wolfgang Wiltschko and Christine Nießner from the Goethe-Universität Frankfurt, Germany, explain that some birds, such as chickens and migratory robins, are thought to ‘see’ the magnetic field superimposed on their vision when the Earth's magnetic field interacts with a specially activated form of a protein called cryptochrome (Cry1a), which absorbs UV to green wavelengths in cone cells in the retina. However, it was not clear which of the two possible activated forms of the Cry1a protein is essential for the navigators to detect magnetic fields.
Wiltschko explains that plants absorb blue–green light using a form of cryptochrome where the chromophore – the part of the protein (flavin adenine dinucleotide, FAD) that is responsible for the protein's light sensitivity – is partially reduced to a semiquinone by UV and blue light. However, in a second step, the semiquinone can be further reduced by UV, blue and green light to produce FADH− and it is this form that can then be reoxidised to produce a pair of electrons (a radical pair) that is essential for the detection of magnetism.
Knowing that exposure to different light colour combinations can produce the semiquinone and FADH− forms of Cry1a and that the Cry1a protein changes shape depending on whether it is carrying the semiquinone or FADH− chromophore, the Wiltschkos and Nießner produced an antibody that could distinguish between the two incarnations of the protein to discover more about which form of FAD is used by magnetism-sensitive birds for steering (p. ). However, instead of testing their theories on a migratory species, the team first investigated which forms of the Cry1a protein chickens produce under white and coloured lights.
Explaining that chickens orient naturally, Nießner and Susanne Denzau took birds that had been kept in normal daylight and tested whether they were able to produce the fully reduced form of FADH−. Exposing the birds to UV (373 nm), blue (424 nm), turquoise (502 nm) or green (565 nm) light, the duo then used the antibody to test whether the birds had been able to produce FADH− and found that all of the animals did. Then – reasoning that chickens that have been exposed to green light alone could only produce FADH− if they had access to a supply of semiquinone produced during earlier exposures to blue and UV wavelengths – the team isolated the birds in the dark for 30 min before exposing them to the four test wavelengths of light. If the team's ideas were correct, the birds that had been bathed in green light could not produce FADH−, as the supply of the essential semiquinone intermediate would have already run out. Using the antibody to test chickens' eyes, Nießner and Denzau could see that the birds that had been exposed to the blue and UV wavelengths had produced FADH−; however, as predicted, there was no FADH− in the retinas of the birds that had been kept in green light.
So, the chickens were capable of producing FADH− Cry1a as well as the semiquinone form of Cry1a; but which form of Cry1a do migrating birds use to set their bearing? This time, the Wiltschkos turned to a well-established migratory species, the European robin (p. ) to test how well birds that had been exposed to combinations of light colour that produced either the FADH− or semiquinone forms of Cry1a were able to set their bearings.
Plunging the birds into darkness for an hour, the team recorded which directions they wanted to fly under blue, turquoise or green light. Sure enough, the birds that were attempting to take off under the blue and turquoise lights set the correct northerly bearing as they were able to produce FADH−. However, the birds that were attempting to take off under green light were completely disorientated, trying to head in easterly and westerly directions. Without a supply of semiquinone, the birds were unable to produce the FADH− that is essential for magnetism detection. However, when the team monitored the birds' attempts to migrate under the three colours of light after spending the day in white light, even birds that had been exposed to green light successfully set northerly bearings, although by the second hour, the green light birds had run out of semiquinone – and FADH− in turn – and become disorientated again.
The robins clearly required FADH− to produce the radical pair that is essential to detect the Earth's magnetic field, as they lost the ability to navigate under green light when their supply of semiquinone Cry1a ran out. The team is now keen to find out how radio frequency fields, which disrupt birds' magnetic compasses, affect how Cry1a detects magnetic fields.
