Skull

The Skull

It appears that the head is prone to convergence
as earlier workers surmised, and postcranial
characters are less prone to convergence. (Siegel-Causey 1986)

A bird skull is in fact an assembly of 30+ different bones of which the majority fuse after the juvenile stage. Some skull bones however never fuse and form a flexible construction.

Skull of a fledgeling Razorbill Alca torda

The process of fusion of may take several months to almost a year. In recently fledged or first winter seabirds the fusion and ossification of the skull bones has not yet completed in many species. With age seabird skulls and bills can become heavier and sometimes show different kinds of small bony protrusions.

Bills sometimes also need several years to become fully 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 paragraph) and has nothing to do with the bony part.

Cranium and upper mandible

The separate bones that form the cranium and the upper mandible fuse during the growth of the chick and juvenile bird. This process of fudsion and ossification may take up to a year. In some species, such as several Alcids, Divers and some Skuas the skull needs several years to become fully grown. As soon as the fusion is completed, outcrops from the lachrymal bone and the frontals form a solid supraorbital ridge that provides protection for the vulnerable eye in these species. Eventually the different parts of the skull form one single construction that holds the brain and the eyes and the upper mandible.
After fusion the upper mandible is connected to the forehead with a flexible bony 'hinge' (13). This hinge, the palate, the flexible jugals, quadratums (1) and pterygoids (2) form an ingenious, partly sliding construction enabling the upper jaw to move up and down to a certain extent. The pterygoids and the quadratums also can move little bit sideways.
These features are essential to the seabird to open its mouth widely to allow passage to large preys which are swallowed in one piece.
The lachrymal bones (6) are often fused tot the frontal bone, but not in all species. It is an important characteristic for identifying tubenoses. All Fulmars, Prions and Gadfly Petrels for instance have fused lachrimal bones, but Albatrosses and Shearwaters don't. In the Gannets, Boobies, Tropicbirds and Frigatebirds the lachrymal bones also don't fuse, but in the closely related Cormorants and Shags they do. In the Grebes also no fusion takes place. In Divers and all Gulls, Terns, Skuas and Alcids the lachrymals become fused within a short period after fledging.
On top of the cranium above the eye sockets a depression for the nasal or salt gland (9) is present in most seabirds, except in the Pelecaniiformes. In this group the nasal gland is located inside the eye socket.

Lower mandible

After fusion of the eight embryonal parts the lower mandible is a single and very flexible V-shaped bone. In Abatrosses, Cormorants, Gannetsanf Frigatebirds the lower mandible has a spiny outcrop at the gonys, where both sides of ther mandible meet.

Parts of the skull

Not all bones and parts of the seabird skull are mentioned in this table, only the most important for identifying purposes. For a comprehensive description of parts of skulls see Zusi & Livezey (2006), Denes & Silveira (2007), Baumel (1993)


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 Procellariiformes are all based on the same principle. The majority of the petrels and shearwaters 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 each 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)


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