The product of head height and head width is a strong predictor of brain mass in birds

Animal behavioral studies are often combined with research concerning cognitive abilities. Larger brains usually mean more complex neural networks and advanced cognitive functions. By measuring the brain size of different individual animals, we can explore differences in behavioral complexity between populations or species. However, obtaining accurate measurements of brain size is challenging both in field and laboratory environments, especially for rare and endangered species. Therefore, there is an urgent need to develop reliable methods for performing cranial brain mass. This study tests which external structures of the avian head can most accurately predict brain size. We selected five bird species from four orders, categorized external head measures into three types of parameters (direct, calculated and composite measurements), and analyzed these in relation to brain mass. The results showed that while head size can partially explain brain mass, the parameters of head height × head width were the most accurate predictors of brain mass in birds (90.4%). In addition, the positive correlation between endocranial volume and brain mass once again confirmed that avian endocranial volume can, to a certain extent, serve as a valid proxy for brain mass. Our study demonstrates that in the future we can more conveniently perform non-invasive measurements to better understand the relationship between bird brain size and behavior, ecology, and evolution.