What are the components of a bird's skull? Do they have most of the reptilian skull components, or do they have lost them when it became fused? Can individual bones be distinguished? Also, I have read that their upper beak consists of the premaxilla only. Don’t they have maxillas at all? Do they have temporal and antorbital fenestrae? Surprisingly little can be found on the net about the transformations and the present bones of a bird skull, though there are tons of information about stem tetrapods, modern amphibians, reptiles, dinosaurs and mammals.

Googling "bird skull illustration" privides a number of labelled images for you to study.