Relative skull size as one of the factors limiting skull shape variation in passerines.

Despite a considerable interest among researchers in understanding the variation in skull shapes of birds and the factors influencing it, some of the drivers associated with the design features of an entire bird body, which are important for both successful terrestrial locomotion and flight, have been overlooked. One such factor, in our opinion, is relative skull size (skull length in relation to body mass), which can affect the position of the body's center of gravity. We tested the effects of relative skull size, allometry (i.e., absolute skull size), and diet on variation in skull shape. The study was conducted on 50 songbird species representing a wide range of body masses (8.3–570 g) and dietary preferences (granivores, insectivores/granivores, insectivores, omnivores). Skull shape was analyzed using two-dimensional geometric morphometrics. We found that similar patterns of skull shape occur among passerines with different body sizes and diets. Relative skull size predicted skull shape to a similar extent and with a similar pattern as the absolute size. The effect of relative skull size on skull shape variation is likely due to biomechanical constraints related to flight. [ABSTRACT FROM AUTHOR]