Ontogeny of the cranial skeleton in a Darwin's finch (Geospiza fortis).

Darwin's finches are a model system in ecological and evolutionary research, but surprisingly little is known about their skull morphology and development. Indeed, only the early beak development and external variation in adult beak shape has been studied. Understanding the development of the skull from embryo up to the adult is important to gain insights into how selection acts upon, and drives, variation in beak shape. Here, we provide a detailed description of the skeletal development of the skull in the medium ground finch (Geospiza fortis). Although the ossification sequence of the cranial elements is broadly similar to that observed for other birds, some differences can be observed. Unexpectedly, our data show that large changes in skull shape take place between the nestling and the juvenile phases. The reorientation of the beak, the orbit and the formation of well-developed processes and cristae suggest that these changes are likely related to the use of the beak after leaving the nest. This suggests that the active use of the jaw muscles during seed cracking plays an important role in shaping the adult skull morphology and may be driving some of the intra-specific variation observed in species such as G. fortis. Investigating the development of the jaw muscles and their interaction with the observed ossification and formation of the skull and lower jaw would allow further insights into the ecology and evolution of beak morphology in Darwin's finches.