Does avian magnetoreception rely on both magnetite and maghemite?
  Recently, a putative magnetoreceptor in the upper-beak skin of homing pigeons was chemically characterized using X-ray flourescence and absorbtion studies [Fleissner et al., 2007. A novel concept of Fe-mineral based magnetoreception: histological and physiochemical data from the upper beak of homing pigeons. Naturwissenschaften 94 (8), 631-642]. In this short communication we point out that the novel concept propagated is based on conclusions that are not supported by data and, in fact, stand in contradiction to previously published crystallographic and magnetic data. 
Michael Winklhofer, Joseph L. Kirschvink