ThebirdfamilySulidae comprises thegannets andboobies. Collectively calledsulids, they are medium-large coastalseabirds that plunge-dive forfish and similarprey. The 10species in this family are often consideredcongeneric in older sources, placing all in thegenusSula. However,Sula (true boobies) andMorus (gannets) can be distinguished viamorphological,behavioral, andDNA 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.[3][4]
Sulids measure about 60 to 85 cm (24 to 33 in) in length and have a wingspan around 140 to 175 cm (4.59 to 5.74 ft). They have long, narrow, and pointed wings, and a quite long, graduated, and ratherlozenge-shaped tail whose outer feathers are shorter than the central ones. Their flight muscles are rather small to allow for the smallcross-section required for plunge-diving, as an adaptive trade-off relative to some sacrifice in flight performance. Consequently, they are verystreamlined, reducingdrag, so their bodies are "torpedo-shaped" and somewhat flat.[5]
They have stout legs andwebbed feet, with the web connecting all four toes. In some species, the webs are brightly colored and used incourtship displays. The bill is usually conspicuously colored, long, deep at the base, and pointed, with saw-like edges. Theupper 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 ofbinocular vision than in most other birds.[5]
Theirplumage 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 their heads. The face usually has some sort of black markings, typically on thelores. Unlike their relatives (thedarters andcormorants), sulids have a well-developedpreen gland whose waxy secretions they spread on their feathers for waterproofing and pest control. Theymoult their tail feathers irregularly and theflight 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 ofstress has been recorded.[5]
The sulids are distributed mainly intropical andsubtropical waters, but they, particularly gannets, are found intemperate regions, too. These birds are not trulypelagic seabirds like the relatedProcellariiformes, and usually stay rather close to the coasts, but the abundant colonies of sulids that exist on manyPacific islands suggest that they are not infrequently blown away from their home range bystorms, and can wander for long distances in search of a safe place to land if need be.[5]
Allspecies feed entirely at sea, mostly on mid-sizedfish and similarly sizedmarineinvertebrates (e.g.cephalopods). Many species feed communally, and some species followfishing boats toscavenge discardedbycatch andchum. The typical hunting behavior is a dive from midair, taking the bird a 1–2 m under water. If prey manages to escape the diving birds at first, they may give chase using their legs and wings for underwater swimming.[5]
As noted above, thebehavioral traits of gannets and boobies differ considerably, but the Sulidae as a whole are characterized by several behavioralsynapomorphies: Before taking off, they point their bills upwards (gannets) or forward (boobies). After landing again, they point downwards with their bills. In response to a threat, they do not attack, but shake their heads and point their bills towards the intruders.[3]
All sulids breed incolonies. Males examine the colony area in flight and then pick a nest site, which they defend by fighting and territorialdisplays. Males then advertise to females by a special display and call. Their display behavior is characteristic, though not as diverse as the numerous variations found among thecormorants; it typically includes the male shaking his 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.[3][5]
Abbott's booby fledgling still get fed by their parents several months after being able to fly and stay in the same tree, mostly even on the same branch, where the nest was situated.
Theclutch is typically twoeggs. The eggs are unmarked (but may become stained bydebris in the nest), whitish, pale blue, green, or pink, and have a coating that resembleslime. Egg[verification needed] weight ranges from 3.3 to 8.0% of the female's weight. Incubation lasts 42 to 55 days, depending on the species. Both sexesincubate; like their relatives, they do not havebrood patches, but their feet becomevascularized and hot, and the birds place the eggs under the webs. Eggs lost during the first half of incubation are replaced.[5]
At hatching, parents move the eggs and then the hatchlings to the tops of their webs. The young hatch naked, but soon develop whitedown. They beg by touching the parent's bill and takeregurgitated food straight from itsgape. At first, at least one parent is always in attendance of thealtricial young; after two weeks, both parents leave the nest unguarded at times while they go fishing. The times for the chicks to fledge and become independent of their parents depend greatly on the food supply. Rarely does more than one chick survive to maturity, except in thePeruvian booby (Sula variegata), which has the biggest clutch (two to four eggs), and less often in theblue-footed booby (S. nebouxii).Siblicide by the stronger of two chicks is frequent.[5]
Skull (above) and life restoration (below) of the freshwaterMasillastega rectirostris, one of the earliest known fossil sulids
Sulids are related to a number of otheraquatic birds, which all lack externalnostrils and abrood patch, but have all four toes webbed and agular sac. The closest living relatives of the Sulidae are thePhalacrocoracidae (cormorants and shags) and theAnhingidae (darters). The latter are somewhat intermediate between sulids and cormorants, but (like many cormorants) they arefreshwater birds in aclade containing otherwiseseabirds, and alsosymplesiomorphic with sulids butsynapomorphic with cormorants in some other respects. Thus, the Sulidae seem to be the oldest and most distinct lineage of those three, which are united in asuborderSulae. Therein, the Sulidae are typically placed simply as afamily; sometimes, a superfamilySuloidea is recognized, wherein some of the primitive prehistoric forms (e.g.Empheresula,Eostega, andMasillastega) are placed asbasal lineages distinct from the living Sulidae. However, the proposed familyPseudosulidae (orEnkurosulidae) is almost certainly invalid.[2][6][7][8]
The Sulae were traditionally included in thePelecaniformes in its obsoleteparaphyletic circumscription, butpelicans, the namesake family of the Pelecaniformes, are actually more closely related toherons,ibises and spoonbills, thehamerkop, and theshoebill than to the sulids and allies. In recognition of this, the Sulae have been proposed for separation in a neworderPhalacrocoraciformes, which also includes thefrigatebirds (Fregatidae), as well as one or moreprehistoric lineages that are entirelyextinct today.[7] The IOC World Bird List usesSuliformes as the proposed order name.[10]
Within the family itself, three living genera—Sula (boobies, six species),Papasula (Abbott's booby), andMorus (gannets, three species)—are recognized. A 2011 study of multiple genes found Abbott's booby to be basal to all other gannets and boobies, and likely to have diverged from them around 22 million years ago, and the ancestors of the gannets and remaining boobies split around 17 million years ago. The most recent common ancestor of all boobies lived in the late Miocene around 6 million years ago, after which time the boobies steadily diverged. The gannets split more recently, only around 2.5 million years ago.[9]
Tropical oceans between the 30th parallel north and 30th parallel south. In the Indian Ocean it ranges from the coastlines of the Arabian Peninsula and East Africa across to Sumatra and Western Australia
Thefossil record of sulids is quite extensive due to the manyMiocene/Pliocene forms that have been recovered, but the lineage of sulids extends back to theEocene, and all things (such as theEarly EocenefrigatebirdLimnofregata) considered, the sulids seem to have diverged from the lineage leading to cormorants and darters around 50 million years ago (Mya), perhaps a bit earlier. The initialevolutionary radiation formed a number ofgenera that are now completely extinct, such as thefreshwaterMasillastega (which, as noted above, might not have been a modern-type sulid) or the bizarreRhamphastosula (which had a bill shaped like anaracari's). The modern generaevolved (like many other living genera of birds) around theOligocene-Miocene boundary about 23 Mya.Microsula, which lived during that time, seems to have been a primitive booby that still had manysymplesiomorphies with gannets. Like the other Phalacrocoraciformes, the sulids originated probably in the general region of theAtlantic or westernTethys Sea – probably the latter rather than the former, given that their earliest fossils are abundant inEurope, but absent from the well-studied contemporaryAmerican deposits.[6][8]
Prehistoric sulids (or suloids) only known fromfossils are:
Masillastega (Early Eocene of Messel, Germany) – may belong inEostega
Eostega (Late Eocene of Cluj-Manastur, Romania) – may includeMasillastega
Sulidae gen. et sp. indet. (Thalberg Late Oligocene of Germany) –Empheresula?[11]
Sulidae gen. et sp. indet. (Late Oligocene of South Carolina, United States) –Microsula?[12]
Empheresula (Late Oligocene of Gannat, France – Middle Miocene of Steinheimer Becken, Germany) – including"Sula" arvernensis,"Parasula"[13]
Microsula (Late Oligocene of South Carolina, United States – Grund Middle Miocene of Austria) – may belong inMorus orSula, includes"Sula" avita,"S." pygmaea,Enkurosula,"Pseudosula"[1]
Sulidae gen. et sp. indet. (Late Pliocene of Valle di Fine, Italy) –Morus?[15]
For prehistoricspecies of the extantgenera, see the genus articles.
TheEarly OligoceneProphalacrocorax ronzoni ofRonzon, France, was variously placed in theseaduck genusMergus, inSula, and after a distinct genus was established for it, in the Phalacrocoracidae. While it is quite likely to belong in the Sulae and may have been an ancient sulid (or suloid), of the three placements explicitly proposed, none seems to be correct.[6][2][16][8]
^abcKennedy, Martyn; Spencer, Hamish G. & Gray, Russell D. (1996): Hop, step and gape: do the social displays of the Pelecaniformes reflect phylogeny?Animal Behaviour51(2): 273-291.doi:10.1006/anbe.1996.0028 (HTML abstract)Erratum:Animal Behaviour51(5): 1197.doi:10.1006/anbe.1996.0124
^Friesen, V.L.; Anderson, D.J.; Steeves, T.E.; Jones, H. & Schreiber, E.A. (2002): Molecular Support for Species Status of the Nazca Booby (Sula granti).Auk119(3): 820–826. [English with Spanish abstract]DOI: 10.1642/0004-8038(2002)119[0820:MSFSSO]2.0.CO;2PDF fulltextArchived 2016-03-03 at theWayback Machine
^abcdefghNelson, J. Bryan (2003): Gannets and Boobies.In:Perrins, C. (ed.):The Firefly Encyclopedia of Birds: 82–87. Firefly Books, Oxford.
^Benson, Richard D.; Erickson, Bruce R. (2013). "A new genus and species of booby (Sulidae: Aves) from the Pliocene of South Carolina, with a new corollary to the nature of sister taxa".Science Museum Monographs in Paleontology.7. St. Paul, MN: Science Museum of Minnesota.
^Aproximalhumerus fragment somewhat similar to a gannet's: Lambrecht (1933: p.286)
^Göhlich, Ursula B. (2003): The avifauna of the Grund Beds (Middle Miocene, Early Badenian, northern Austria).Annalen des Naturhistorischen Museums Wien A104: 237-249 [English with German abstract].PDF fulltext
Lambrecht, Kálmán (1933): Familia Sulidae.In: Handbuch der Palaeornithologie: 284-287 [German]. Gebrüder Bornträger, Berlin.
Mlíkovský, Jirí (2007): Taxonomic identity ofEostega lebedinskyi LAMBRECHT, 1929 (Aves) from the middle Eocene of Romania.Annalen des Naturhistorischen Museums in Wien A109: 19-27 [English with German abstract].PDF fulltext
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