According to Storrs Olson the Atlantic Lesser Frigatebird should be treated as full species.
The Atlantic Lesser Frigatebird (Fregata trinitatis) is elevated to a separate species from Fregata ariel of the Indo-Pacific based on plumage differences and greater robustness of the rostrum and wing bones. The species now occurs only on the remote South Atlantic island of Trindade (South Trinidad) over 1,100 km east of mainland Brazil, although fossils establish that it once also occurred at St. Helena. Fregata trinitatis no longer breeds on the main island of Trindade and is only known to nest with certainty on a small rocky islet on the southern coast, where there may be fewer than 20 breeding pairs.
The Wilson Journal of Ornithology 129(4):661-675. 2017
doi.org/10.1676/1559-4491-129.4.661
Species Rank For the Critically Endangered Atlantic Lesser Frigatebird (Fregata trinitatis)
Storrs L. Olson
Atlantic Lesser Frigatebird: a species on the verge of extinction?
19 Jan 2018 - 12:12 -- Chris Sharpe
Lesser Frigatebird Fregata ariel comprises two subspecies found in the Indo-Pacific, and a third, trinitatis, breeding on the Atlantic island of Trindade, 1100 km off the coast of Brazil. Plumage and structural differences suggest that trinitatis may merit recognition as a species, Atlantic Lesser Frigatebird, making it one of the world’s rarest and most threatened seabirds. Fewer than 20 breeding pairs were found on a small islet alongside Trindade in 1975/76, with just six birds recorded over 60 days of surveys in 2013. The decline of trinitatis may have been caused by a combination of factors, from direct consumption of eggs and overfishing, to the effects of introduced mammals: feral pigs, goats and cats have been extirpated, but house mice remain abundant and are likely to prevent recolonisation of the main island. Taxon trinitatis would be a Brazilian breeding endemic that is already considered Critically Endangered at the national level.
www.hbw.com/news/atlantic-lesser-frigatebird-species-verge-extinction