The Masked Booby breeds throughout the world's tropical oceans and is locally abundant near hundreds of oceanic islands. The name "booby," originally coined by sailors, referred derogatorily to these birds' lack of fear around humans. Its feeding tactics are memorable: these dazzling white birds make high velocity plunge-dives in search of fish.
Masked Boobies measure 29 to 34 inches in length, with a wingspan of 62 inches; they weigh about 3.3 pounds (1,550 grams). These large waterbirds are white with black along the backs of the long, pointed wings, a long, pointed, black tail, a long, pointed, yellowish bill, and a dark face mask, for which the bird is named. Sexes are similar, but the female is slightly larger. The very similar Nazca Booby has an orange, rather than a yellow bill, and is smaller, with a significantly shorter, thinner bill.
Masked Boobies breed in the Caribbean, across the Pacific Ocean, to Hawaii, Australia, and Indonesia. Although its only U.S. breeding records are from the Hawaiian Islands and the Dry Tortugas, Florida, the species is regularly sighted along the Gulf of Mexico and as far north as North Carolina. In winter, they forage throughout tropical oceans, The Masked Booby population breeding along the Pacific coast of northern South America, including the Galapagos, was recently recognized as a separate species, the Nazca Booby.
Masked Boobies nest on low, flat areas, on small, tropical islands, especially those that are flat and unforested, where they commonly perch on the ground. Outside of the breeding season, these birds spend all their time at sea.
The Masked Booby diet consists primarily of fish and squid, particularly flying fish. These seabirds plunge-dive from heights of up to 100 feet into schools of fish below. Prey items are swallowed upon returning to the surface. Masked Boobies are known to associate with dolphin/tuna schools. The tuna probably drive this association by concentrating and chasing prey fish to the sea surface. Masked Boobies spend significant amounts of time resting on the ocean surface in feeding areas, rising into the air to join feeding flocks when prey is detected. These feeding flocks can range in size from one to several hundred birds. A minor percentage of foraging occurs at night.
Masked Boobies first breed at four years of age. Male courtship displays include neck stretching and bill pointing; paired birds may present pebbles or feathers to one another. Colonial nesting is typical for this species, with distances between nests ranging from two to three meters to over 100 meters. Each Masked Booby nest consists of a slight depression on the ground, surrounded by a circle of pebbles or other debris, often near a breezy cliff edge or slope suitable for taking off. Although the Masked Booby female regularly lays two light blue eggs, both young are never raised. The first egg is laid four to nine days before the second. Both parents incubate the eggs under the webs of their feet for 38 to 49 days. Upon hatching, the chicks are nearly helpless, with sparse white down. Both parents regurgitate food for the chicks. However, the older chick soon attacks and ejects the younger from the nest. The parents do not intervene, and the ejected chick is quickly scavenged by crabs, landbirds, or frigatebirds. The remaining chick makes its first flight at 109 to 151 days, but continues to return to the nest site, where it is fed by the parents for another month or two.
No regular migration has been clearly delineated for Masked Boobies. Birds in some areas vacate colonies when not breeding, but birds from Ascension Island in the central Atlantic and the Seychelles in the Indian Ocean are thought to be year-round residents. Galápagos birds have been recovered from the Pacific coasts of Guatemala and Mexico—over 1,000 kilometers from their breeding site—during the non-breeding season. Elsewhere in the Pacific, non-breeding birds are also known to visit sites 1,000 to 2,000 kilometers from their breeding sites.
Anderson, D. J. 1993. Masked Booby (Sula dactylatra). In The Birds of North America, No. 73 (A. Poole and F. Gill, Eds.). Philadelphia: The Academy of Natural Sciences; Washington, D.C.: The American Ornithologists' Union.
Kaufman, Kenn. Lives of North American Birds. Houghton Mifflin Company, New York. 1996.
Pitman, R. L., and J. R. Jehl, Jr. 1998. Geographic variation and the reassessment of species limits in the "Masked" Boobies of the eastern Pacific Ocean. Wilson Bulletin 110: 155-170.
Sibley, David A. The Sibley Guide to Birds. Alfred A. Knopf, New York. 2000.
