Clusters of iron-rich cells in the upper beak of pigeons are macrophages not magnetosensitive neurons by Treiber, C, Salzer, M, Riegler, J, Edelman, N, Sugar, C, Breuss, M, Pichler, P, Cadiou, H, Saunders, M, Lythgoe, M, Shaw, J, Keays, D