Sulidae

The bird family Sulidae comprises the gannets and boobies. Collectively called sulids, they are medium-large coastal seabirds that plunge-dive for fish and similar prey. The ten 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, behavioral, and DNA sequence characters. Abbott's booby (Papasula) 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 (24 to 33 in) in length and have a wingspan of about 140 to 175 cm (4.59 to 5.74 ft). 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, as an adaptive trade-off relative to some sacrifice in flight performance. Consequently, they are very streamlined, reducing drag, so their bodies are 'torpedo-shaped' as well as somewhat flat. They have stout legs and webbed feet, with the web connecting all four toes. In some species the webs are brightly colored and used in courtship displays. The bill is usually conspicuously colored, 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. The sulids are distributed mainly in tropical and subtropical waters, but they, 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 (e.g. cephalopods). Many species feed communally, and some species follow fishing boats to scavenge discarded bycatch and chum. The typical hunting behavior is a dive from mid-air, taking the bird a meter or two under water. If prey manages to escape the diving birds at first, they may give chase using their legs and wings for underwater swimming. As noted above, the behavioral traits of gannets and boobies differ considerably, but the Sulidae as a whole are characterized by several behavioral synapomorphies: Before taking off, they will point the bill upwards (gannets) or forward (boobies). After landing again, they point downwards with the bill. And in response to a threat, they will not attack but shake their heads and point the bill towards the intruder. All sulids breed in colonies. Males examine the colony area in flight and then pick a nest site, which they defend by fighting and by territorial displays. Males then advertise to females by a special display and call. The display behavior is characteristic, though not as diverse as the numerous variations found among the cormorants; it typically includes the male shaking its head. Females search the colony in flight and on foot for a mate. Once they select males, pairs maintain their bonds by preening each other and by frequent copulation.