Oeno is a low oceanic atoll to the North of Pitcairn. The fringing reef is approximately circular and about 4 km in diameter. It is breached by a single passage, on the northern margin. Through this passage, shallow-draft boats (e.g. the Pitcairners' longboats) may enter the lagoon, which is uniformly shallow, less than 3 m deep, with scattered reefs. There is a single motu, about 80 ha in extent, south-west of the centre of the lagoon. North of the motu and intermittently connected to it, a sandspit runs towards the reef.Oeno is a remarkably undisturbed atoll. The margin comprises either sandy beach or reef flat. Behind this lies shoreline scrub, often dominated by Suriana maritima. At the south end of the motu is closed forest of Argusia argentea and Pisonia grandis, the canopy reaching 7-9 m. Most of the rest of motu is clothed in more or less open Argusia scrub. In total there are only 16 vascular plant species present, but this includes the endemic Bidens hendersonensis var. oenoensis. There is also a stand of coconut trees, introduced by the Pitcairners, at the north of the island.Because the entrance to the lagoon is too shallow to allow the passage of ocean-going yachts and because there is no secure anchorage outside the reef, Oeno is rarely visited. Once or twice a year, the Pitcairners visit the island to camp in the vicinity of the coconut grove. The visits, typically in the southern summer, normally last around a week and are viewed as holidays.
Oeno's chief ornithological importance is the Murphy's Petrel colony, estimated in 1991 at 12,500 pairs and therefore the second largest in the world. It would be worthwhile to assess whether the 1997 eradication of Pacific rats Rattus exulans from Oeno has been followed by an increase in the population of Murphy's Petrels. Similarly, in the light of likely decreases in the breeding populations of Herald and Kermadec Petrels between 1922, the year of the Whitney Expedition visit, and 1991/92, checking for signs of recovery of these populations would be valuable. Unfortunately, monitoring this situation will be difficult because of the intrinsic difficulty of reaching the island and because the Pitcairners' holiday visits to the island are usually in the austral summer rather than during the Murphy's Petrel winter nesting season. Phoenix Petrels, apparently disappeared from Oeno between the Whitney visit in 1922 and the 1991/1992 Expedition, but perhaps 12-20 pairs were present in 1997 and 1998. Oeno supports populations of other seabirds, notably Red-tailed Tropicbird (500-1000 pairs), Masked Booby (250-300), Red-footed Booby (25), Greater Frigatebird (100) and Fairy Tern (< 1000).Oeno is also important as a wintering site for the Bristle-thighed Curlew Numenius tahitiensis. In January 1990 about 100 were recorded.It is possible that the widespread Spotless Crake Porzana tabuensis breeds on Oeno for it is seen regularly. Otherwise there are no landbirds.
Non-bird biodiversity: The flora of Oeno is poor in species. Only the local variety of Bidens hendersonensis is endemic; this was not seen in 1991/92, but was relocated in 1997. On the other hand, the marine mollusc fauna is much richer. Around 240 taxa are known, of which about two percent are endemic to the Pitcairn group. One species, a liotinine gastropod, belongs to a genus (and therefore species) known only from Oeno.
BirdLife International (2019) Important Bird Areas factsheet: Oeno Island. Downloaded from http://www.birdlife.org on 12/12/2019.