The Mangaia crake ("Porzana" rua) is an extinct species of flightless bird in the rail family, Rallidae. It was described in 1986 from subfossil bones of late Holocene age found in caves on the island of Mangaia, in the southern Cook Islands of East Polynesia.[1] It was placed in the then-loosely circumscribed genus Porzana, but it almost certainly does not belong to Porzana proper as understood in modern times. Rather, it most likely was one of the crakes which are now separated as genus Zapornia. While the species survived for hundreds of years of Polynesian settlement, even despite the establishment of introduced predators, at some point in the last millennium Mangaia suffered an ecosystem collapse with far-reaching consequences, the extinction of "P." rua among them.
| Mangaia crake Temporal range: Late Holocene
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|---|---|
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Gruiformes |
| Family: | Rallidae |
| Genus: | Porzana |
| Species: | †P. rua
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| Binomial name | |
| †Porzana rua Steadman, 1986
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The species name rua is Marquesan for a hole in the ground, such as a sinkhole, a grave, a chasm or a cavern. It alludes to the name of the type locality, Te Rua Rere ("The Flying/Jumping/Throwing Cave"), als well to the fact that the prehistoric deposits of Mangaia were metaphorical "graveyards" of extinct fauna, with this rail being one of the most numerous. The holotype specimen USNM 402876 is one humerus and two tarsometatarsus bones all belonging to the same bird. They were collected on April 13, 1984.[1]
Description edit
It was a good-sized crake, about as large as the similarly flightless and possibly very closely related Henderson crake (Zapornia atra), or a small plump sora (one of the species remaining in Porzana today). Probably weighing 65-70 g, it was less than 20 cm long altogether, including a short tail of about 35 mm and a bill perhaps 20-25 mm long. By Zapornia crake standards, the midfoot bone (the "lower leg" of birds) was long, the femur fairly short but notably stout for a bird of its size and bulk. Humerus and ulna were shortened (and the ulna was also more delicate), even a bit shorter than in the smaller but volant Spotless crake; assuming its remiges were short as in the Henderson crake, the wing length of "P." rua was probably about 6 cm or even less.
Its plumage colour is unknown. If it does indeed belong to a group including some or all of as Henderson's crake, the Spotless crake, the Kosrae crake and the Tahiti crake, it was almost certainly mostly blackish, with perhaps some barring on the hindflanks, and/or some contrast between a slightly paler underside and darker and olive-tinged back and wings. Its bill would in this case most likely have been red or blackish, the eyes chestnut brown, and the legs almost certainly bright red.
Ecology and extinction edit
The Mangaia crake was of the most abundant species on Mangaia before human settlement, and for a considerable time even afterwards. The type locality Te Rua Rere[2] in Tava'enga District (northwest) has yielded bones of more than a dozen individuals alone. By 2006, the species' remains had been found also in ana (Mangaian for cave/den/rockshelter) Toruapuru[3] in Ivirua District (east), ana Tapukeu in Tamarua District (southeast), ana Tuatini[4] and Tangatatau in Veitatei District (southwest), ana Manuku in Keia district (west), as well as ana Tuara. It is thus documented essentially all around the island, though the localities where it has been found generally lie at the edge of the makatea karst escarpment (an uplifted fossil coral reef) towards the ring of heavily-cultivated taro fields which surround the central hill in the island's interior. While most sites have yielded only one or two bones, more than a hundred bones had been recovered from both Tangatatau and Manuku. Apparently, the Mangaia crake formerly was a plentiful species – together with the still-present little white tern, and the now-extinct local populations of Black-winged petrel and Polynesian ground dove, it must have made up the bulk of the birdlife of pre-settlement Mangaia.
The ecology of this rail is almost entirely conjectural, but probably did not differ radically from that of the Henderson crake. As such, it was probably omnivorous and made use of anything edible it found, eating arthropods, seeds and perhaps occasionally fruit, as well as carrion and bird eggs and chicks. Considering that it was the second-largest and by far the most numerous of up to four rail species previously sharing the fairly small (some 50 square kilometres/20 sq. mi.) island, some competitive exclusion mechanism must have been present; most likely, the Mangaia crake was more inclined to predation than its smaller relatives, so eggs and nestlings of the Black-winged petrel and other ground-nesting seabirds might have been a significant food item. Like on Henderson Island, the Polynesian rats introduced by early settlers may occasionally have been successfully attacked by the rails, too.
The cause of its extinction is ascribed to a combination of predation and habitat alteration following human settlement of the island and the introduction of exotic mammals.[1]
See also edit
References edit
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