Figure 3. Boxplot of normalised tectal thicknesses of the four bird species.

The log₁₀ of the thickness of the tectum was normalised to the log₁₀ of the width of the midbrain hemisection; N =  22 for kiwi, 40 for Emu, 26 for Barn Owl and 49 for Pigeon. Data was analysed using Mann-Whitney non parametric pairwise comparisons (against Kiwi). *: p≤0.005; **: p≤0.001.