The role of many-to-one mapping of vertebral form to function in Psittaciform tripedal locomotion

Parrots highlight the functional diversity of the avian neck by contributing to a range of behaviors, including arboreal locomotion. The parrot neck is used alongside the beak and hindlimb to allow them to successfully navigate arboreal habitats via tripedal locomotion. Whether specific morphological characteristics of the neck enable this behavior are currently unknown. By combining geometric morphometrics with phylogenetic comparative methods we investigate the factors correlate with shape variation in the cervical vertebrae of parrots. We find that phylogeny, allometry, integration, diet and tripedal locomotion all have a significant influence on the morphology of psittaciform cervical vertebrae. However, the influence of diet and tripedal locomotion is weak, with a high degree of morphospace overlap existing between dietary and neck use groups. Additionally, we find no evidence of convergence in parrot neck morphology due to the incidence of tripedal locomotion or dietary specialization. We thus conclude that changes to the neuromuscular control of the neck, not morphological adaptations, are primarily responsible for tripedal locomotion in parrots. We argue that many-to-one mapping of form to function allows parrots with similar neck morphologies to participate in a range of behaviors, and this may be a common feature amongst all birds.