Animal Diversity Web

Sulidae comprises one genus (Sula) and nine species.

Gannets and boobies are found in all but Antarctic oceanic regions. Gannets are distributed throughout tropical and subtropical oceans whereas boobies inhabit temperate waters.

Some sulids are pelagic while others are inshore marine birds. Breeding areas include offshore islands and continental coastlines.

Eggs are incubated for 42-55 days. Chicks are altricial and remain at nest for about 14-22 weeks. Post-fledging care may last from five to nine months. Adult plumage and sexual maturity occur at about two to six years. Flight feathers are molted almost continuously. Sulids may live for 10-20 or more years.

Sulids are medium to large birds (724-3600 g; 64-100 cm). Boobies exhibit sexual size dimorphism with females larger and heavier, whereas gannets appear monomorphic in size. Plumage in most species is white ventrally with brown-black present on primaries and/or dorsal or head feathers. Juveniles are generally darker than adults. Wings are long and set far back on the body. The stout bill is conical, with serrated edges in some species. Desmognathous palate, nostrils obsolete (no exposed external nares), nasal groove runs along bill. Adults have bare facial and gular skin, which along with the feet, may be black, red or blue in color. The eyes are situated beside the bill giving the birds binocular vision. Iris is often pale in color. Sulids lack brood patches. Legs are short and set far back on the body. Feet are totipalmate (webbed between all four toes) with middle toenail pectinate.

Sulids may breed in mixed colonies with other sulids or sometimes with frigatebirds or cormorants.

Common sulid prey items include mackerel (Scomber), pilchard (Sardinops), anchovy (Engraulis) and sandeels (Ammodytes).

Avian predators of sulids include: Short-eared Owl (Asio flammeus), goshawks (Accipitridae), gulls (Laridae), Sea eagle (Haliaeetus albicilla), Andean Condor (Vulture gryphus), and frigatebirds (Fregata). Other predators include: rats, cats, wild pigs, land crabs, sharks, and humans

Sulids are minimally seasonally monogamous, but mate and nest-site fidelity may last for many successive years. Complex displays are commonly exhibited in pair bonding and nest-site establishment and defense. Highly colonial species exhibit some of the most ritualized types of behaviors. Gannet displays are elaborate and stereotyped, including: headshakes, bill menacing, sky pointing, and greeting ceremonies. Boobies use their wings and feet frequently in displays and in aerial greetings.

Most sulids breed in colonies consisting of under 1000 pairs, although the Peruvian Booby (Sula variegata) colonies are estimated at nearly a million pairs. Breeding can be seasonal (annually or biennially) or varied depending on species and local conditions. Nests may be located on cliffs, slopes, trees, or ground. Ground nests are shallow depressions in accumulations of guano. Nests in trees are constructed of twigs. Boobies lay two to three eggs whereas gannets lay one egg. Eggs are pale blue-green to white.

Both sexes take turns incubating for 41-57 days with incubation stints lasting from 12-60 hours. Because sulids lack brood patches, the webs of the feet are used to transfer heat to eggs and chicks. Parents take turns feeding and brooding young continuously for about a month, and thereafter more sporadically. During the post-fledging period adults will feed the flying chicks, for up to nine months in some species.

Gannets are migratory whereas boobies are more sedentary. Sulids spend large amounts of time at sea foraging. Sulids fly higher than most seabirds and use their binocular vision to locate prey. When prey is located, sulids stall from 10-100 m above the water surface, dive with wings pushed backwards, and penetrate the water as deep as ten meters from the force of the dive alone. Once underwater the birds may use their wings in a swim pursuit of prey to depths of 15-25 m. Usually prey is captured and ingested as the bird returns to the surface. Some species also surface-dive or forage on foot in shallow water. Sulids spend considerable time on feather care, distributing oil from the preen gland which prevents plumage from becoming waterlogged. Heat-regulation entails several interesting behaviors including exposing the webs on the feet or excreting on them for cooling. Gular fluttering and wing extensions also help to dissipate heat.

Sulids are considered highly social. Most species breed in dense colonies. Some species forage cooperatively, flying in small groups and diving in unison to capture prey items.

Booby males and females have very different calls. Male vocalizations include plaintive whistles while females elicit trumpeting honks. The vocalizations of young boobies are similar to those of the female. Gannets are loud and raucous and the sexes' vocalizations are nearly identical. Adults may utter harsh screeches while fishing, rasping when arriving at the nest, or sighs that accompany sky pointing.

In the tropics sulids are collected for eggs, meat, and feathers. Boobies are renown for guano production, which is collected as an economically important resource for countries such as Peru. At the turn of the century, the Northern Gannet colony in the Gulf of St. Lawrence (100,000 pairs) was nearly eradicated by human culling. The colony was reduced to 500 pairs by 1932. Fortunately, by 1984 the colony had recovered to 6,700 pairs.

Two sulid species are included in the IUCN Red List of Threatened Species, Abbott's Booby (Sula abbotti) is listed as 'Critically Endangered' and Cape Gannet (Sula capensis) is listed as 'Vulnerable'. The main threats include: habitat destruction; human collection of eggs, chicks, and adults; over-fishing by commercial industries.

The evolutionary relationships of sulids remain unclear. Traditionally sulids have been considered related to other totipalmate birds (tropicbirds, frigatebirds, anhingids, cormorants and shags, pelicans) and taken together form Pelicaniformes. However, a hierarchy based on DNA hybridization includes sulids within a diverse group, Ciconiiformes. Several hypotheses of sulid sister group relationships include: sulids as sister to a group comprising cormorants, shags, and anhingids; sulids as sister to anhingids forming a group sister to cormorants and shags; sulids as sister to cormorants and shags thereby forming a group sister to anhingids. Within Sulidea, skeletal differences have been cited to support splitting gannets and boobies into two genera, Sula and Morus respectively. Recent analyses have suggested that gannet and booby similarities support a single genus, Sula.

The earliest fossil sulid (Sula ronzoni), from France, has been dated to the early Oligocene. The large gannet (Morus magnus) from California dates from the late Miocene. Other fossil sulids are dated from throughout the Tertiary period

Campbell, B., and E. Lack, editors. 1985. A Dictionary of Birds. Buteo Books, Vermillion, SD.

Cracraft, J. 1985. Monophyly and phylogenetic relationships of the Pelecaniformes: a numerical cladistic analysis. Auk 102: 834-853.

del Hoyo, J., Elliott, A. & Sargatal, J. (eds.) 1992. Handbood of the Birds of the World. Vol. 1. Lynx Edicions, Barcelona.

Feduccia, A. 1999. The Origin and Evolution of Birds, 2nd edition. Yale University Press New Haven.

Nelson, J.B. 1978. The Sulidae Gannets and Boobies. Oxford University Press, Oxford.

Sibley, C. G. & J. E. Ahlquist. 1990. Phylogeny and Classification of Birds, A Study in Molecular Evolution. Yale Univ. Press.

Contributors

Laura Howard (author), Animal Diversity Web.