Figure 2. Head volume of birds (mm³) in relation to level of background radiation (µSv/h), after controlling for species and body mass.

The line is the linear regression line with the equation log₁₀(Head volume)  = 3.3918−0.0045 log₁₀(Background radiation). Residuals from a model that included species as a predictor were added mean log₁₀-transformed head volume 3.3934 to facilitate interpretation.