Dispersie na het broedseizoen, trek en overwintering van Grote Sterns Thalasseus sandvicensis uit de Voordelta

The (sea)bird skull differs very much from a mammal skull and by its built it reveals a reptilian origin. The skull consists of an assembly of over 30 bones, most of them fused to form the cranium that is connected tot the upper mandible by a flexible ' hinge' and the lower jaw. Only a few parts of the upper skull, quadatums en pterygoids, always stay separate to form – together with the palate and upper mandible - a flexible, sliding construction enabling the bird to swallow large prays. Most seabird skull show a depression above the eye cases that holds the salt gland. In the Pelecaniiformes this gland is located inside the eye case. In some Alcids, Divers, Penguins, albatrosses, petrels and skuas this depression develops a bony ridge at the outer edge. The upper mandible holds the nasal openings; large (covered by a horny sheath) in most seabirds, but relatively small in Pelicaniiformes and Procellariiformes with the typical tubes in the latter. Gannets don't have external nares as an adaption to high speed diving. The lachrymals fuse in many seabirds to the frontals, but not in all groups. The lower jaw consists of 5 pairs of partly flexible bones that have fused to one V-shaped bone. The lower jaw articulates to the quadratums of the upper skull. Is albatrosses, frigatebirds, cormorants and gannets the gonys shows ventrally a spiny outcrop, covered by the bill sheath. Its function is not clear. A few other small bones can be found in seabird skulls. A tiny bone, ossiculum lacrymopalatinum, is locarted between the lachrmals and the palate in albatrosses, frigatebirds, many petrels, some gulls and auks. Its function is not clear and probably of little taxonomic value. In terns a backwards pointing sesamoid bone is connected to the apex of the lachrymals. In cormorants and darters an occipital style is connected to the cerebellar bulb. This is a ossification within the large neck muscles and therefore no part of the skull proper. The tongue and eyes also have bony reinforcements.