Gadlfy Petrels: skull

Cranial structure of the Gadfly Petrels

After Warham 1990


The skulls of the Gadfly Petrels have the lachrymal bones fused to the frontals. A feature shared with the fulmarine petrels, the prions and the Blue Petrel. Only in young birds this fusion might not be completed. In most species the bill is rather stubby, but some - such as The Cook's Petrel - have rather slender bills. The bill and cranial structure of this group has been studied intensively.
The skull is bulky and short, rounded downwards above the foramen magnum, thus resembling that of the fulmars and not the more delicate and elongate skull of the shearwaters. The depression marking the temporal fossa is shallow (deep in shearwaters), and the openings in the frontal and orbito-olfactory septa from the orbit of the nasal chambers are small - big in shearwaters.
Unlike Puffinus the skulls of adult Pterodroma, Lugensa and Pseudobulweria  have fused pre-frontal (lachrymal) bones, a feature separating gadfly petrels from Bulweria, whose skull is otherwise very pterodromine. Fledglings often have free pre-frontal bones and a perforated anti-orbital wall. Fusion of the pre-frontals is completed before or shortly after fledging.

In Lugensa fenestrated extensions of skull bone lateral to the supra-orbital depressions may help to protect the large eyes.


Gadfly petrels differ from other Procellariids in their deep bills, always black in color. The bill is laterally compressed, particularly in Lugensa. The maxillary nails are strongly hooked, convex above and concave below, the tomia of the upper mandible following a sinous line when viewed from the side. The tomia of the lower mandible are almost straight, the gonys a little concave. Well marked sulci separate the lateral plates of both mandibles. The nasal tubes are short, rather elevated, with a thin central septum and apertures that usually open upwards. There is a little culmen exposed between the nasal openings and the upper nail.

Jouanin drew attention to differences in bill structure among Pterodroma. He noted that the bill of the Reunion Petrel Pterodroma aterrima was shaped differently from that of P. macroptera and the Trinidade Petrel P. arminjoniana, and so were the nostrils. In macroptera the openings lie in a vertical plane, in aterrima they open on an inclined plane which contains the anterior edge of the median septum.
Another distinctive feature is the notch in the rhamphoteca at the junction of the latericorn and the maxillary unguis of P. atterima. Joanin noted that the bill of the Tahiti Petrel P. rostrata had a similar structure. Both species belong to the Pseudobulwerias.

Bill and details of nostrils of (top) P. macroptera, (centre) P. arminjoniana, and (bottom). P. atterima. Note rhamphotecal notch of aterrima (circled).
From Joanin (1970) in Warham 1990.

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