It is widespread in the tropical Atlantic, breeding colonially in trees in Florida, the Caribbean and Cape Verde Islands. It also breeds along the Pacific coast of the Americas from Mexico to Ecuador including the Galápagos Islands.
It has occurred as a vagrant as far from its normal range as the Isle of Man, Denmark, Spain, England, and British Columbia.
The Magnificent Frigatebird is 100 cm (39 in) long with a 215 cm (85 in) wingspan. Males are all-black with a scarlet throat pouch that is inflated like a balloon in the breeding season. Although the feathers are black, the scapular feathers produce a purple iridescence when they reflect sunlight, in contrast to the male Great Frigatebird's green sheen. Females are black, but have a white breast and lower neck sides, a brown band on the wings, and a blue eye-ring that is diagnostic of the female of the species. Immature birds have a white head and underparts.
This species is very similar to the other frigatebirds and is similarly sized to all but the Lesser Frigatebird. However, it lacks a white axillary spur, and juveniles show a distinctive diamond-shaped belly patch.
The Magnificent Frigatebird is silent in flight, but makes various rattling sounds at its nest.
This species feeds mainly on fish, and attacks other seabirds to force them to disgorge their meals. Frigatebirds never land on water, and always take their food items in flight.
It spends days and nights on the wing, with an average ground speed of 10 km/h (6.2 mph), covering up to 223 km (139 mi) before landing. They alternately climb in thermals, to altitudes occasionally as high as 2,500 m (8,200 ft), and descend to near the sea surface. The only other bird known to spend days and nights on the wing is the Common Swift.
A scientific study which examined genetic and morphological variation in Magnificent Frigatebirds found both expected and also highly unexpected results. As predicted by the flight capacity of the species, the authors found signatures of high gene flow across most of the distribution range. This included evidence of recent gene flow among Pacific and Atlantic localities, likely across the Isthmus of Panama. This geological formation is a strong barrier to movement in most tropical seabirds. However, the same study also found that the Magnificent Frigatebird on the Galápagos Islands is genetically and morphologically distinct. Based on this study, the Galápagos population has not been exchanging any genes with their mainland counterparts for several hundred thousand years.
Given these findings, the Galapagos population of this tropical seabird may be its own genetically distinct species warranting a new conservation status. This small population of genetically unique Magnificent Frigatebirds is a vulnerable population. Any catastrophic event or threats by humans could wipe out the approximate population of two-thousand Magnificent Frigatebirds that nest on the Galápagos Islands. Magnificent Frigatebirds are currently classified as Least Concern (LC)' by the International Union for Conservation of Nature, but the Proceedings of the Royal Society paper recommends that, because of the genetic uniqueness of those on the Galápagos, this status be revisited