(a) Scatter plot of LOG brain volume to LOG body mass (present study data together with published strigid data21). O. murivorus (orange circle) exhibits the lowest ratio brain volume/body mass of all owls. Dashed arrow shows the evolutionary trajectory from O. sunia (green–blue triangle) to O. murivorus, which strongly deviates from the allometric trend (Y/X regression line; r2 = 0.91). Slope = 0.55. The translucid grey zone represents a 95% confidence interval around the regression. Dashed arrow symbolizes evolutionary trajectory from O. sunia to O. murivorus. (b)–(d), Transverse section of 3 skulls in 3D volume at level of foramen magnum, showing the minimal thickness of cranium wall as measured (red arrows) in the wulst (1) and cerebral hemisphere (2) regions, relative to cranium width. (b), O. murivorus; (c) O. sunia; (d) O. scops. Not to scale.