Shearwaters: skull

Tubenose skulls can be classified in a few different types: Albatross-type, Shearwater-type, Fulmar/Pterodroma-type, Storm-petrel-type and Diving-petrel-type. (Kuroda 1953)

The Shearwater-type is characterised by non-ankylosed lachrymals. In most species the bill is long and slender bill with a prominent hook at the tip. The nostrils are rather small.
The large Calonectris species have robust skulls with long and strong bills. Only leucomelas has a more delicate bill. In edwardsii the skull is very similar to diomedea/borealis, but considerably smaller.
Within Ardenna the two closely related species creatopus and carneipes have comparatively short and stubby bills, but otherwise resembling Calonectris. According to Penhallurick and Wink (2004) gravis/creatopus/carneipes may form a superspecies with gravis but the skull of the latter is closer to Puffinus and the other Ardenna species in shape, but larger.large and small shearwater skull compared
The two other clusters: Ardenna bulleri/pacifica and grisea/tenuirostris have delicate skulls but the bill of tenuirostris however is short compared with the others.
The skulls of the so called Manx' group are also delicate with slender bills but very similar in shape and proportions. In the small Puffinus species the cranium becomes larger to the bill. In most of them bills are thin and delicate, but in elegans the bill is robust for its size.
In all species there can be considerbale individual variation in bill length and robustness, especially in the large Atlantic en Mediterranean Calonectris species.

Anatomy and development of the skull

Skull development is very similar in all shearwaters. At the time of fledging ossification is not fully completed, but soon after the cranium gets fully closed sutures and fontanelles and fused maxillary bones. The frontonasal hinge may take several months to reach its mature stage in which the lengthwise parallel splits fuse for the greater part and leave just two to three short splits in the nasal band, a small flat section across the frontal bone (between the distal tips of the lachrymals) that forms the actual hinge of flexible bone (Sugimori et al. 1985)