Superparamagnetic magnetite in the upper beak tissue of homing pigeons

Homing pigeons have been subject of various studies trying to detect magnet ic material which might be involved in magnetic field perception. Here we f ocus on the upper-beak skin of homing pigeons, a region that has previously been shown to contain nerves sensitive to changes of the ambient magnetic field. We localized Fe3+ concentrations in the subcutis and identified the material by transmission electronmicroscopy (TEM) as aggregates of magnetit e nanocrystals (with grain sizes between 1 and 5 nm). The particles form cl usters of 1-3 mum diameter, which are arranged in distinct coherent elongat ed structures, associated with nervous tissue and located between fat cells . Complementary low-temperature magnetic measurements confirm the microscop ic observations of fine-grained superparamagnetic particles in the tissue. Neither electron-microscopic nor magnetic measurements revealed any single- domain magnetite in the upper-beak skin tissue.