The Réunion swamphen (Porphyrio caerulescens), also known as the Réunion gallinule or oiseau bleu (French for "blue bird"), is a hypothetical extinct species of rail that was endemic to the Mascarene island of Réunion. While only known from 17th- and 18th-century accounts by visitors to the island, it was scientifically named in 1848, based on the 1674 account by Sieur Dubois. A considerable literature was subsequently devoted to its possible affinities, with current researchers agreeing it was derived from the swamphen genus Porphyrio. It has been considered mysterious and enigmatic due to the lack of any physical evidence of its existence. This bird was described as entirely blue in plumage with a red beak and legs. It was said to be the size of a Réunion ibis or chicken, which could mean 65–70 cm (26–28 in) in length, and it may have been similar to the takahē. While easily hunted, it was a fast runner and able to fly, though it did so reluctantly. It may have fed on plant matter and invertebrates, as do other swamphens, and was said to nest among grasses and aquatic ferns. It was only found on the Plaine des Cafres plateau, to which it may have retreated during the latter part of its existence, whereas other swamphens inhabit lowland swamps. While the last unequivocal account is from 1730, it may have survived until 1763, but overhunting and the introduction of cats likely drove it to extinction.
Taxonomy Visitors to the Mascarene island of Réunion during the 17th and 18th centuries reported blue birds (oiseaux bleus in French). The first such account is that of the French traveller Sieur Dubois, who was on Réunion from 1669 to 1672, which was published in 1674. The British naturalist Hugh Edwin Strickland stated in 1848 that he would have thought Dubois' account referred to a member of the swamphen genus Porphyrio if not for its large size and other features (and noted the term oiseau bleu had also been erroneously used for bats on Réunion in an old account). Strickland expressed hope that remains of this and other extinct Mascarene birds would be found there. Responding to Strickland's book later that year, the Belgian scientist Edmond de Sélys Longchamps coined the scientific name Apterornis coerulescens based on Dubois' account. The specific name is Latin for "bluish, becoming blue". Sélys Longchamps also included two other Mascarene birds, at the time only known from contemporary accounts, in the genus Apterornis: the Réunion ibis (now Threskiornis solitarius); and the red rail (now Aphanapteryx bonasia). He thought them related to the dodo and Rodrigues solitaire, due to their shared rudimentary wings, tail, and the disposition of their digits. The name Apterornis had already been used for a different extinct bird genus from New Zealand (originally spelled Aptornis, the adzebills) by the British biologist Richard Owen earlier in 1848, and the French biologist Charles Lucien Bonaparte coined the new binomial Cyanornis erythrorhynchus for the oiseau bleu in 1857. The same year, the German ornithologist Hermann Schlegel moved the species to the genus Porphyrio, as P. (Notornis) caerulescens, indicating an affinity with the takahē (now called Porphyrio hochstetteri, then also referred to as Notornis by some authors) of New Zealand. Schlegel argued that the discovery of the takahē showed that members of Porphyrio could be large, thereby disproving Strickland's earlier doubts based on size. The British ornithologist Richard Bowdler Sharpe simply used the name Porphyrio caerulescens in 1894. The British zoologist Walter Rothschild retained the name Apterornis for the bird in 1907, and considered it similar to Aptornis and the takahē, believing Dubois's account indicated it was related to those birds. The Japanese ornithologist Masauji Hachisuka used the new combination Cyanornis coerulescens for the bird in 1953 (with the specific name misspelled), also considering it related to the takahē due to its size.
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