Is there a field proxy for brain size in great-tailed grackles ( Quiscalus mexicanus )?

There is an increasing need to validate and collect data approximating brain size on individuals in the field to understand what evolutionary factors drive brain size variation within and across species. We investigated whether we could accurately estimate endocranial volume (a proxy for brain size) as measured by computerized tomography (CT) scans, using external skull measurements and/or by filling skulls with beads and pouring them out into a graduated cylinder for male and female great-tailed grackles. We found that while females had much stronger correlations than males, estimations of endocranial volume from external skull measurements or beads did not correlate with CT volumes at a standard that surpassed our strict criteria. We found no accuracy in the ability of external skull measures to predict CT volumes because prediction intervals from data points overlapped extensively. We conclude that we are unable to detect individual differences in endocranial volume using external skull measurements. These results emphasize the importance of validating and explicitly quantifying the predictive accuracy of brain size proxies for each species, and each sex, under consideration.