THE ANATOMY OF THE SEABIRD SKULL

A bird  skull is in fact an assembly of about thirty bones of which the majority fuse after the juvenile stage.  In first winter seabirds the fusion and ossification of the skull bones has not yet completed in many species. Cleaning these skulls often ends up in a jigsaw puzzle of small bones. With age seabird skulls and bills can become heavier and sometimes show different kinds of small bony protrusions. 

In some species, such as several Alcids, the skulls need several years to become fully grown. After a few years outcrops from the lachrymal bone and the frontals eventually form  a solid supraorbital ridge that provides good protection for  the vulnerable eye. Their bills also need some years to become mature. The difference in winter and summer bills of some species is the result of partly 'molting' the bill sheath or ramphotecal coating (see next chapter) and has nothing to do with the bony part.

Some bones however stay 'loose'. These bones play a role in opening the bill. 

 

The upper mandible is connected to the forehead with a flexible but bony 'hinge' (13). This hinge and the loose connection of the quadratums (1) and pterygoids (2) form a flexible construction enabling the upper jaw to move up and down to a certain extent. The lower jaw  is built  out of two pairs of partly fused bones and is even more flexible. 

These features enable the seabird to open its mouth extremely wide to provide a passage to large preys which are swallowed in one piece. Pelicans are the most extreme in this respect.

The lachrymal bone (6) is often fused tot the frontal, but not in all species. It is an important characteristic for identifying tubenoses. All fulmarine petrels for instance have fused lachrimal bones, but albatrosses and shearwaters don't.

The depression for the nasal gland (9) is present in most seabirds, except the Pelecaniiformes. 

Not all bones and parts of the seabird skull are mentioned in this table, only the most important for identifying purposes. 

The skull shown here is of a Cory's shearwater (Calonectris borealis).

  1. Quadratum. Forms a flexible joint with the lower mandible
     

  2. Pterygoid. Connects the quadratum to the palatinum and the base of the skull.
     

  3. Jugale. Three fused bones that form the jugal arch.
     

  4. Palatum. A set of fused bones forms the palate.
     

  5. Maxillary. Fused with the nasale it forms the upper mandible.
     

  6. Lachrymal or Prefrontal. Fused or not fused to the frontals. Forming the frontal end of the orbital ridge of the eye case.
     

  7. Frontal. The major bone forming the forehead
     

  8. Nasale. Fused to the maxillare it forms the upper mandible
     

  9. Fossa glandula nasalis. A depression that contains the nasal or salt gland in most seabirds.
     

  10. Processus postorbitalis. The rear end of the orbital ridge.
     

  11. Processus zygozomaticus. The 'corner' of the brain case. A measuring point.
     

  12. Fossa temporalis. A depression that contains the large muscles of the lower mandible.
     

  13. 'Hinge'

 

 

THE EXTERNAL STRUCTURE OF A SEABIRD BILL

The bills of all seabird species are covered with a horny sheathing, the ramphothecal coating. The basic principle of this arrangement is rather similar most species, though there are differences depending on the family and the genus.

The ramphothecal coatings of the birds belonging to the petrel family are all based on the same principle. The majority of the petrels have nostrils at the base of the upper mandible which are covered by a single horny sheath, the naricorn. Albatrosses form a distinct group, having two separated nostrils on either side of the bill.

Other seabird groups have a more or less similar coating of the bill. In cormorants and allies such as gannets, boobies, frigatebirds and, darters the arrangement consists of less plates and there are no or hardly visible nostrils. This group also has a small separate horny plate at the gape of the upper mandible, which is absent in all other bird species.

Gulls, terns and most alcids have bill sheaths that are in one piece for both mandibles, but in skuas and jaegers the plates are still recognizable. Puffins and a few auklets molt their bills every year and have 'smaller' winter bills.

  1. Culminicorn (up to the feathering of the forehead in albatrosses, but to the nostrils in most other petrels. Absent in Macronectes)
     

  2. Latericorn
     

  3. Ramicorn (fused in some albatrosses, two separate plates in other petrels)
     

  4. Naricorn or nostril Separate in albatrosses, fused in other petrels with an internal septum.
     

  5. Maxillary or superior unguis/unguicorn
     

  6. Mandibular or inferior unguis/unguicorn
     

  7. Inter-ramicorn (albatrosses, cormorants and allies)
     

  8. Fleshy membrane (some albatrosses)
     

  9. Sulcus between the plates
     

  10. Tomia or cutting edges
     

  11. Fleshy bar (some albatrosses)
     

  12. Post-ramicorn (gannets, cormorants and allies)

    13.

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