Boobies and gannets
Boobies and gannets
Boobies and gannets are nine species of marine birds that make up the family Sulidae, in the order Pelecaniformes, which also includes the pelicans, cormorants, anhingas, tropic birds, and frigate birds.
Boobies and gannets have a narrow, cigar-shaped body, a longish, pointed tail, and long, narrow wings. Their feet are fully webbed, and are used in swimming. The beak is strong, pointed, has a serrated edge for gripping slippery prey, and is brightly colored in some species. Unlike some of the other groups in the order Pelecaniformes, boobies and gannets have fully waterproof plumage.
Gannets and boobies feed on fish, which they find by flying over the surface of the ocean at an altitude of up to 98 ft (30 m). These birds then catch their prey by spectacular, head-long, angled-winged plunges into the surface of the sea, seizing their quarry in their beak, and swallowing it underwater. During the breeding season, gannets and boobies are found in near-coastal waters. In their non-breeding season, however, these birds may occur far out to sea. Almost all species of gannets and boobies are colonial nesters.
Gannets (Morus spp.) are three species of birds of temperate and subarctic oceans, and they breed in colonies on rocky cliffs and ledges. Both birds of a mated pair incubate their single egg, which they cover with their webbed feet before snuggling down to brood. After the chick develops its flight feathers and is ready to fledge, it is abandoned by its parents. It soon leaps into the sea from its cliff-top nest and begins to fish for itself.
The six species of boobies (Sula spp.) are all tropical and subtropical birds. Boobies breed in nests built on near-shore shrubs, or on coastal cliffs.
Species of gannets
The northern gannet (Morus bassana ) breeds in north-temperate and subarctic waters on both sides of the Atlantic Ocean. In North America, the largest colonies of these birds occur at Cape Saint Mary’s on Newfoundland and on Bonaventure Island in the Gulf of the Saint Lawrence River. There are another four smaller colonies of northern gannets in the western Atlantic Ocean and another 28 in the eastern Atlantic.
Adult northern gannets have a white body, with black wing-tips. The head is a bright lemon-yellow. During the first year after birth gannets are a dark-brown color, while older sub-adults have a dirty-white plumage and lack the yellow head of the sexually mature adults. The tail of gannets is pointed, as is the profile of their head, giving the bird a double-ended shape in flight.
The populations of northern gannets in some of their breeding colonies can be quite large. These birds are aggressively territorial, and their nests are therefore spaced at about twice the distance that a sharp beak can be thrust towards a neighbor. Other displays involve birds engaging in ritualized posturings to impress their neighbors, or to infatuate a potential mate.
In healthy colonies, all of the suitable space may be covered with nests. At Cape Saint Mary’s, population growth in recent decades has resulted in all of the prime nesting habitat on cliffs to be fully utilized. This has forced many birds to nest on adjacent coastal meadows, an accessible habitat in which they are vulnerable to land predators.
After their breeding season, northern gannets occur widely in waters of the continental shelves. During the winter, gannets range as far south as the northern Gulf of Mexico and the southeastern United States.
Other species of gannets include the Cape gannet (Morus capensis ) of South Africa, and the Australian gannet (M. serrator ) of Australia. These species are rather similar to the northern gannet, and some taxonomists consider all of these taxa to be subspecies of Morus bassana.
Species of boobies
Three species of boobies are relatively widespread in tropical waters. The brown booby (Sula leucogaster ) is the most common species, breeding in all of the tropical oceans. This species has dark-brown upper parts and breast, and a white belly. Male birds have a dark-blue face, while that of females is yellow. Immature birds are more uniformly brown.
The blue-faced or masked booby (Sula dactylatra ) is the largest species. This species breeds in the tropics of the Pacific and Indian Oceans. The blue-faced booby has a mostly white body, but the flight feathers are all black, resulting in a black stripe running the length of the back of the wings. The “mask” is an area of black feathers just behind the beak.
The red-footed booby (Sula sula ) is a species of the tropical Pacific, Indian, and Atlantic Oceans. This species is named after its bright-red feet and legs. The red-footed booby nests in shrubs and trees.
Other, less widespread species are the Peruvian booby (Sula variegata ) of offshore islands and coastal headlands of Peru, the blue-footed booby (S. nebouxii ) of western Mexican and Central and South American waters, and Abbott’s booby (S. abbotti ), which only occurs in the vicinities of Assumption and Christmas Islands in the Indian Ocean. Peruvian boobies can nest in huge colonies, which can contain as many as one million pairs of birds.
Boobies do not breed in North America, but several species are regular visitors to coastal waters during their non-breeding season. The blue-faced booby occurs most frequently in the vicinity of the Dry Tortugas off extreme southern Florida, and also in the Caribbean Sea, the Gulf of Mexico, and Baja California, as well as farther south of all of those places. The brown booby and blue-footed booby are also occasional visitors to the extreme southern United States.
Boobies, gannets, and people
Guano is a commercially important product obtained by digging the surface of the huge colonies of Peruvian boobies and other seabirds off northern South America, and at colonies of Cape gannets off South Africa. Guano is a natural, phosphorus-rich compound derived from the excrement of seabirds that is used as a fertilizer.
KEY TERMS
Overfishing— Harvesting of fish at a rate that is greater than their productivity, leading to a collapse in the size of the stocks.
Plunge-diver— A bird that dives head-long into the water to catch prey swimming fairly close to the surface.
For many years, gannets, and to a much lesser degree boobies, were considered to be serious competitors with humans for commercially important marine fish. For this reason, gannets were often killed, and only a few decades ago their numbers were perilously small. This sort of indiscriminate killing is not much of a problem anymore, except in a few remote places.
In some regions, gannets and boobies may be killed for their meat and feathers, and where they are accessible, their eggs may be collected for eating.
Boobies and gannets are also vulnerable to collapses in the populations of the fish that they feed upon. For example, Peruvian boobies and other seabirds have suffered precipitous population declines when their most important prey of anchovies collapsed as a result of oceanographic changes associated with El Niño. El Niño is a warm-water phenomenon that impedes nutrient cycling, greatly reducing the productivity of phytoplankton, and ultimately, causing a collapse of fish stocks.
Since the beginning of the 1990s, there has also been a collapse of many fish stocks in coastal waters off eastern Canada. The reasons for this ecological change are not known for certain, but the leading hypotheses include the effects of overfishing and climate change. The collapse of the fisheries of the northwest Atlantic has led to severe economic hardship for many people who are dependent on that natural resource for their livelihood. However, there have also been severe effects on northern gannets and other sea-birds, which depend on those fish stocks as a source of food, particularly when they are raising their young. Consequently, these birds have experienced unsuccessful reproduction, and this may pose a threat to the longer-term health of their populations in that region.
Resources
BOOKS
Ehrlich, P.R., D.S. Dobkin, and D. Wheye. Birds in Jeopardy. Stanford, CA: Stanford University Press, 1992.
Forshaw, Joseph. Encyclopedia of Birds. 2nd ed. New York: Academic Press, 1998.
Harrison, P. Seabirds: An Identification Guide. Rev. ed. New York: Houghton Mifflin, 1985.
Nelson, J. B. The Sulidae: Gannets to Boobies. Oxford: Oxford University Press, 1978.
PERIODICALS
Alten, Michelle. “A Tale of 3 Boobies: How Three Similar Bird Species Lead Dissimilar Lives In the Galapagos.” International Wildlife 28 (January-February 1998): 28–35.
Bill Freedman