The eyes and brain are the main components of an avian head. Previous studies have suggested that eye size influences the brain shape. However, the correlation between them has not been quantitatively confirmed. It is also known that feeding behavior and diurno-nocturnality are associated with eye size relative to brain size. Accordingly, we assumed that behavioral characteristics of birds can be determined from their brain shape. Hence, we assessed the variations in brain shape relative to changes in brain size and eye size using CT images of avian skulls. As a result, the angle of brain posture (the angel between the plane of the foramen magnum and lateral semicircular canal) tended to be larger with increase in the brain size and eye size. Furthermore, the birds with relatively smaller eye size compared to brain size had an inclined brain and those with relatively larger eyes had a relatively more upright brain. Thus, a correlation between the brain shape (posture) and eye size was confirmed. We therefore conclude that in Aves, brain shape is influenced by eye size and is associated with feeding behavior and diurno-nocturnality. This relationship between brain shape and behavioral characteristics can be useful tool to reconstruct the palaeoecology of extinct avian species such as Archaeopterx.