Justification of Red List Category
Although this species may have a small range, it is not believed to approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend appears to be stable, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
The species has a large global population with an estimated minimum of 1,200,000 individuals (Delany and Scott 2006). In the Houtman Albrolhos, populations were estimated at 7,665, 6,325 and 34,895 pairs on Morley, Wooded and Pelsart Island respectively in 1993 (Higgins and Davies 1996).
The overall population trend is stable, although some populations have unknown trends (Delany and Scott 2006).
This species breeds in the Seychelles, Mascarene Islands and Agalega Islands (Mauritius), Maldives, the Chagos Archipelago (British Indian Ocean Territory), and Houtman Albrolhos Islands and possibly Ashmore Reef (Australia) (Feare 1984, Higgins and Davies 1996). The Australian subspecies melanops may be resident. The nominate race is a winter visitor to Madagascar and the eastern African coast between southern Somalia and Kenya (Higgins and Davies 1996).
This species is largely sedentary (del Hoyo et al. 1996) and remains at its breeding colonies throughout the year, although it may also forage extensively out to sea (Higgins and Davies 1996) and regularly occurs off the coast of East Africa during the non-breeding season (del Hoyo et al. 1996). It breeds from August to October in large colonies of up to tens of thousands of pairs (Higgins and Davies 1996, del Hoyo et al. 1996) and also forages in vast flocks during this season (Higgins and Davies 1996). When not breeding it remains gregarious and is usually observed in groups of up to 45 individuals (Higgins and Davies 1996), often within larger flocks of Brown Noddy Anous stolidus (Langrand 1990). The species breeds and roosts in mangroves on oceanic coral-limestone islands with shallow lagoons (providing seaweed as nesting material), gullies, sink holes and salt-lakes, and may also occur on shingle or sandy beaches (Higgins and Davies 1996, del Hoyo et al. 1996). It largely forages in inshore seas and reefs surrounding these breeding islands during the non-breeding season although it may also forage extensively out to sea (Langrand 1990, Higgins and Davies 1996). Diet consists of small surface-dwelling fish and invertebrates (e.g. squid) that have been driven to the surface by predatory fish (Feare 1984, Urban et al. 1986, Langrand 1990, Higgins and Davies 1996, del Hoyo et al. 1996). Prior to breeding, adults also consume large quantities of coral fragments from beaches as a source of calcium (needed for egg laying) (Skerrett et al. 2001). The nest is constructed of damp vegetation and seaweed in a low bush or on a horizontal or vertical fork of a tall mangrove tree (Higgins and Davies 1996, del Hoyo et al. 1996, Skerrett et al. 2001). It nests colonially, with neighbouring nests spaced between 0.3 and 5 m apart (del Hoyo et al. 1996).
Habitat loss is a threat to tropical Indian Ocean islands where this species breeds, and introduced browsers and predators (e.g. rats) are present on several islands although their current impact is thought to be minimal (Feare 1984, Skerrett et al. 2001). In Australia, eggs, chicks and adults were taken for food last century by people harvesting guano (Higgins and Davies 1996). There are no threats thought to be causing a significant decline.
Text account compilers
Malpas, L., Ekstrom, J., Harding, M., Butchart, S.
BirdLife International (2019) Species factsheet: Anous tenuirostris. Downloaded from http://www.birdlife.org on 20/11/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 20/11/2019.