Cryptochromes, blue-light absorbing proteins involved in the circadian clock, 
have been proposed to be the receptor molecules of the avian magnetic compass. 
In birds, several cryptochromes occur: Cryptochrome 2, Cryptochrome 4 and two 
splice products of Cryptochrome 1, Cry1a and Cry1b. With an antibody not 
distinguishing between the two splice products, Cryptochrome 1 had been detected 
in the retinal ganglion cells of garden warblers during migration. A recent 
study located Cry1a in the outer segments of UV/V-cones in the retina of 
domestic chickens and European robins, another migratory species. Here we report 
the presence of cryptochrome 1b (eCry1b) in retinal ganglion cells and displaced 
ganglion cells of European Robins, Erithacus rubecula. Immuno-histochemistry at 
the light microscopic and electron microscopic level showed eCry1b in the cell 
plasma, free in the cytosol as well as bound to membranes. This is supported by 
immuno-blotting. However, this applies only to robins in the migratory state. 
After the end of the migratory phase, the amount of eCry1b was markedly reduced 
and hardly detectable. In robins, the amount of eCry1b in the retinal ganglion 
cells varies with season: it appears to be strongly expressed only during the 
migratory period when the birds show nocturnal migratory restlessness. Since the 
avian magnetic compass does not seem to be restricted to the migratory phase, 
this seasonal variation makes a role of eCry1b in magnetoreception rather 
unlikely. Rather, it could be involved in physiological processes controlling 
migratory restlessness and thus enabling birds to perform their nocturnal 
flights.