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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 and 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.〔Kennedy ''et al.'' (1996), Friesen ''et al.'' (2000)〕
==Description==
Sulids measure about in length and have a wingspan of about . 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.〔Nelson (2003)〕
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.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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