  The rotations of microscopic magnetic particles, magnetosomes, embedded into
the cytoskeleton are considered. A great number of magnetosomes are shown to
possess two stable equilibrium positions, between which there occur transitions
under the influence of thermal disturbances. The random rotations attain the
value of order of radian. The rate of the transitions and the probability of
magnetosomes to stay in the different states depend on magnetic field direction
with respect to an averaged magnetosome's orientation. This effect explains the
ability of migrant birds to faultless orientation in long-term passages in the
absence of the direct visibility of optical reference points. The sensitivity
to deviation from an `ideal' orientation is estimated to be 1-2 degrees.
Possible participation of magnetosomes in biological effects caused by
microwave electromagnetic fields is discussed.
