Courtesy Kenn Kaufman
In the United States, the Magnificent Frigatebird breeds only on the Dry Tortugas, off the tip of the Florida Keys. Breeding also occurs at many scattered sites throughout the Caribbean and south to coastal Brazil, and in the Pacific from Ecuador north to Baja California. The majority of Magnificent Frigatebirds nest on the Mexican coast at three colonies. The non-breeding range extends further to sea from the Pacific coast of Ecuador north to central California. In the Atlantic, non-breeding frigatebirds frequent the southern coasts of Florida southward through the Gulf of Mexico, the Caribbean, and coastal South America to southern Brazil.
These birds breed on islands near warm waters with shrubby plants like barna (Crataeva tapia
) and mangroves (particularly red mangroves), upon which they build their nests. Nesting adults forage over both shallow and deep water. When not breeding, the Magnificent Frigatebird frequents coastal waters, lagoons, deep ocean waters, and woody islands, but rarely roosts on the water.
Like a soaring hawk, the Magnificent Frigatebird patrols the sky looking down at the surface for food, then swooping or gliding down to the water to snap up flying fish and squid. Because its feathers soak up water, the birds dive only briefly in pursuit of prey or a drink. Sometimes working in groups, frigatebirds steal food from other seabirds, like boobies, gulls, and terns, by chasing them until the food is dropped or regurgitated. A skillful flyer, the frigatebird then nabs the item from the air before it hits the water. The diet is supplemented by fishery waste, immature seabirds, young turtles, and small crabs.
Unique features of the Magnificent Frigatebird's breeding include plumage differences between sexes; a year-long breeding cycle; and male group displays. From September to April, colonies form around groups of displaying males perched atop shrubby plants. As females wheel above, males bend backwards, open their fluttering wings, inflate their bright red throat sacks, and drum the sacks with clattering bills. When a female perches near a male, other males join the display. A monogamous pair bonds with ceremonies like head shaking, bill sparring, and neck crossing.
Males gather or steal sticks, which females arrange into a loose, fairly flat nest, built where the pair bonded. Some nests are covered with grasses or vines. The female lays only one smooth white egg, which is incubated by the pair for about eight weeks. Blind, helpless, and naked, the chick develops very slowly and takes pre-digested food from both parents. The male departs after about a month, and the young frigatebird is fed by the female for another 13 months. Fledging occurs in about five months. The frigatebird's long breeding cycle and habit of stealing food are probably adaptations to the low productivity of tropical waters.
Because the breeding cycle lasts about a year for Magnificent Frigatebird females, they are probably non-migratory. Immature frigatebirds and post-breeding males have been observed far from their breeding grounds, as indicated by leg bands, but no migration pattern has yet been described.