The vertebrate skull operates under competing demands to facilitate a functioning feeding apparatus while housing and protecting sensory organs. These competing demands are both mechanical and spatial in nature. Cranial form and function therefore represents trade-offs between these demands. The evolution of the avian cranium involved dramatic increases in brain size as well as biomechanical shifts in the feeding apparatus. Most notably, the evolution of the avian skull represents the transition from the akinetic skulls of early dinosaurs to the highly kinetic skulls observed in modern birds. Using an integrated approach of morphometrics, muscle modeling, and free body force analysis, we quantified the muscle orientation change across the theropod-bird transition and its biomechanical effects on the moments and reaction forces of the joints necessary for cranial kinesis. We demonstrate that wider braincases arranged jaw muscles into more rostrocaudal positions in birds, creating net moments about cranial joints suggestive of mobility. Joint reaction forces shift to a more rostrocaudal orientation in birds, with highly kinetic birds having relatively low reaction forces from the otic joint. These data complement and expand upon our current understanding of the evolution of cranial kinesis in tetrapods and the role of intracranial joints in vertebrate feeding.