Sulidae – Gannets & Boobies

The bird family Sulidae comprises the gannets and boobies. They are Suliformes, as are Fregatidae (Frigatebirds), Anhingidae (Anhingas & Darters) and Phalacrocoracidae (Cormorants & Shags). Collectively called sulids, they are medium to large coastal seabirds that plunge-dive for fish and similar prey. The eleven species in this family are often considered congeneric in older sources, placing all in the genus Sula. However, Sula (true boobies) and Morus (gannets) can be readily distinguished by morphological, behavioural and DNA sequence characters. Papasula (Abbott’s Booby) is given its own genus as it stands apart from both in these respects. It appears to be a distinct and ancient lineage, maybe closer to the gannets than to the true boobies

Sulids measure about 60 to 85 cm in length and have a wingspan of about 140 to 175 cm. They have long, narrow and pointed wings, and a quite long, graduated and rather lozenge-shaped tail whose outer feathers are shorter than the central ones. Their flight muscles are rather small to allow for the small cross section required for plunge-diving, and thus their wing loading is high. Consequently, they are very streamlined, reducing drag, so their bodies are torpedo-shaped, as well as somewhat flat.

Peruvian Booby Sula variegata – ©Dubi Shapiro

They have stout legs and webbed feet, with the web connecting all four toes. In some species the webs are brightly coloured and used in courtship displays. The bill is usually conspicuously coloured, long, deep at the base, and pointed, with saw-like edges. The upper mandible curves down slightly at the tip and can be moved upward to accept large prey. To keep water out during plunges, the nostrils enter into the bill rather than opening to the outside directly. The eyes are angled forward, and provide a wider field of binocular vision than in most other birds.

The plumage is either all-white (or light brownish or greyish) with dark wingtips and (usually) tail, or at least some dark brown or black above with white underparts; gannets have a yellowish hue to the head. The face usually has some sort of black markings, typically on the lores. Unlike their relatives (the darters and cormorants), sulids have a well-developed preen gland whose waxy secretions they spread on their feathers for waterproofing and pest control. They moult their tail feathers irregularly and the flight feathers of their wings in stages, so that starting at the first moult, they always have some old feathers, some new ones, and some partly grown ones. Moult as a response to periods of stress has been recorded.

Cape Gannet Morus capensis – ©Dubi Shapiro

The sulids are distributed mainly in tropical and subtropical waters. However, some, particularly gannets, are found in temperate regions too. These birds are not truly pelagic seabirds like the related Procellariiformes, and usually stay rather close to the coasts. But the abundant colonies of sulids that exist on many Pacific islands suggest that they are not infrequently blown away from their home range by storms, and can wander for long distances in search of a safe place to land if need be.

All species feed entirely at sea, mostly on mid-sized fish and similarly-sized marine invertebrates such as cephalopods. Many species feed communally, and some species follow fishing boats to scavenge discarded by-catch and chum. The typical hunting behaviour is to dive from mid-air, which takes the bird a meter or two under water. If their prey manages to escape the diving birds at first, they may give chase using their legs and wings for underwater swimming.