Are external head measurements a reliable predictor of brain size in the Common Quail (Coturnix coturnix)?

Comparative research conducted during the past two decades revealed ecological and evolutionary consequences of interspecies differences in relation to brain size. However, relatively much fewer studies have focused on intraspecific variation in brain sizes. This may arise from the lack of a reliable and universal methodology to estimate brain size that can be employed in wild populations in vivo and in a minimally invasive manner. Here, we assessed whether variation in brain mass of Common Quails (Coturnix coturnix (Linnaeus, 1758)) was predicted by external measurements of the head. Contrary to previous work, we found that the height of the head and not the volume of the head was the best predictor of brain mass in the Common Quail. However, we found that the height of the head explained only a relatively small proportion of variance in brain masses (i.e., 74.4%). Our data suggest that the external measurements of the head may not represent a universally applicable methodology to estimate brain sizes in birds and should, therefore, be used cautiously and validated for the studied species.