  In the present letter we suggest a new theoretical model for a quantitative
description of the magnetoreception mechanism in birds. The considered
mechanism involves two types of iron minerals (magnetite and maghemite) which
were found in subcellular compartments within sensory dendrites of the upper
beak of several bird species. The analysis of forces acting between the iron
particles shows that the orientation of the external geomagnetic field can
significantly change the probability of the mechanosensitive ion channels
opening and closing. The performed theoretical analysis shows that the
suggested magnetoreceptor system might be a sensitive biological magnetometer
providing an essential part of the magnetic map for navigation.
