Magnetite-based magnetoreception: the effect of repeated pulsing on the orientation of migratory birds.

Previous studies have shown that a magnetic pulse affected the orientation of passerine migrants for a short period only: for about 3 days, the birds' headings were deflected eastward from their migratory direction, followed by a phase of disorientation, with the birds returning to their normal migratory direction after about 10 days. To analyze the processes involved in the fading of the pulse effect, migratory birds were subjected to a second, identical pulse 16 days after the first pulse, when the effect of that pulse had disappeared. This second pulse affected the birds' behavior in a different way: it caused an increase in the scatter of the birds' headings for 2 days, after which the birds showed normal migratory orientation again. These observations are at variance with the hypothesis that the magnetite-based receptor had been fully restored, but also with the hypothesis that the input of this receptor was ignored. They rather indicate dynamic processes, which include changes in the affected receptor, but at the same time cause the birds to weigh and rate the altered input differently. The bearing of these findings on the question of whether single domains or superparamagnetic particles are involved in the magnetite-based receptors is discussed.