Clusters of iron-rich cells in the upper beak of pigeons are macrophages not magnetosensitive neurons.
Treiber CD, Salzer MC, Riegler J, Edelman N, Sugar C, Breuss M, Pichler P, Cadiou H, Saunders M, Lythgoe M, Shaw J, Keays DA. Clusters of iron-rich cells in the upper beak of pigeons are macrophages not magnetosensitive neurons. Nature. 2012 Apr 11; 484(7394):367-70.