Bite force can provide an insight into the feeding biomechanics and ecology of vertebrates. There has been an increasing interest in bite force in birds with data being collected using force transducers although bite forces have also been calculated from the mass of the jaw musculature. Studies have sought to find ecological correlates between bite force and diet or feeding style as well as with morphology. This study collated data reported in the literature for bite force and mass of the jaw musculature in birds and explored the relationships between these variables and their relationships with body mass to test two hypotheses. First, bite force and mass of the jaw musculature would scale with body mass irrespective of the phylogeny. Second, bite force and mass of the jaw musculature would be directly related to each other and be unrelated to phylogeny. Phylogenetically controlled analyses showed that in relation to body mass there were different relationships with bite force, and with the mass of the jaw musculature, for non‐passerine (isometry and negative allometry, respectively) and passerine species (both positive allometry). By contrast, a single significantly positively allometric relationship described the relationship between jaw muscle mass and bite force. These relationships were driven in part by the diet of the species concerned but also may reflect morphological differences in jaw musculature. The few studies that compare measured and calculated bite force for the same species show that values derived from muscles were higher. Detaching muscles from their points of origin and insertion to calculate physiological cross‐sectional area may be biasing bite force calculations.

Functional characteristics of the jaw apparatus, for example bite force, in vertebrates is a combination of the skeleton and the musculature. In birds, bite force has been measured directly or calculated using various methods including summation of forces generated by the different elements of the jaw musculature. However, there have been no reports of the relationships between body size with the mass of the different muscle groups in a closely related group of birds. This study explored allometry in the different jaw muscle masses from parrot (Psittaciformes) species differing in body mass by 40-fold. It was hypothesised that the different muscle masses would exhibit isometry with body mass and skull size. Parrot heads were dissected and the masses of the individual muscle complexes were recorded. Data were subjected to phylogenetically-controlled regression analysis to document scaling effects with body mass and skull size. Most, but not all muscles, exhibited positive allometry with body mass but most were isometric with skull size. Consequently, as parrots get bigger, their skulls get proportionally longer, but that the muscles within the head isometrically scaled relative to the size of these proportionally larger skulls. The large muscles imply greater bite forces in parrots than have been reported to date, which seems to be associated with an increase in skull size to accommodate more muscles. It is unknown whether this pattern is applicable to other birds within specific orders or even across birds as a whole. There needs to be further investigation into the allometry of the morphological and functional properties of the avian jaw musculature.