Fig. 2. Testing for the physical principle underlying the chickens' magnetic compass: pooled number of choices of the four screens in the control field (left column of diagrams) and various treatments, with the direction of the magnetically correct choice (mcc) projected upward. Arrowheads indicate samples with a significant preference of an axis; rounded ends and a ring around the centre indicate random choices. Top row, test under different light regimes; centre row, oscillating field of 1.566 MHz of two different intensities added; bottom row, iron-containing structures in the upper beak deactivated by local anesthesia (for numerical data, see Table 2).