Cranial integration in the ring-necked parakeet, Psittacula krameri (Psittaciformes: Psittaculidae)

The study of integration and modularity aims to describe the organization of components that make up organisms, and the evolutionary, developmental and functional relationships among them. Both have been studied at the interspecific (evolutionary) and intraspecific (phenotypic and ontogenetic) levels to different degrees across various clades. Although evolutionary modularity and integration are well-characterized across birds, knowledge of intraspecific patterns is lacking. Here, we use a high-density, three-dimensional geometric morphometric approach to investigate patterns of integration and modularity in Psittacula krameri, a highly successful invasive parrot species that exhibits the derived vertical palate and cranio-facial hinge of the Psittaciformes. Showing a pattern of nine distinct cranial modules, our results support findings from recent research that uses similar methods to investigate interspecific integration in birds. Allometry is not a significant influence on cranial shape variation within this species; however, within-module integration is significantly negatively correlated with disparity, with high variation concentrated in the weakly integrated rostrum, palate and vault modules. As previous studies have demonstrated differences in beak shape between invasive and native populations, variation in the weakly integrated palate and rostrum may have facilitated evolutionary change in these parts of the skull, contributing to the ring-necked parakeet’s success as an invasive species.