Open image gallery
Within the region, the Lesser Sheathbill Chionis minor occurs only at two localities, namely Marion Island and Prince Edward Island, which qualifies the species as regionally Vulnerable under the restricted population criterion D2. However, the regional population is increasing, and there are no known threats that have had a severe impact on the species. Consequently its regional conservation status is reduced to Near Threatened.
The Lesser Sheathbill occurs at Subantarctic islands in the south-west Indian Ocean. The subspecies C. m. marionensis occurs only at the Prince Edward Islands, a South African territory in the south-west Indian Ocean (del Hoyo et al. 1996). At Marion Island, the species occurs almost exclusively along the coastline, but at Prince Edward Island it also forages throughout the coastal plain (Huyser et al. 2000). However, as Lesser Sheathbills occur mostly within 1 km of the coast and the coastline of Marion Island is c. 72 km while that of Prince Edward Island is only c. 28 km (Ryan et al. 2003), the minimum AoO of Lesser Sheathbills at the Prince Edward Islands is c. 100 km2. Lesser Sheathbills banded at Marion Island have been sighted at Prince Edward Island, suggesting some movement between the two localities (BM Dyer pers. obs.), although this was not observed by Burger (1979).
The global population has been estimated at 8 700-13 000 mature individuals (BirdLife International 2014). Estimates of numbers of Lesser Sheathbills at the Prince Edward Islands are gauged from counts of birds undertaken in winter, when birds are not breeding and are most conspicuous. No complete count has ever been undertaken at Prince Edward Island. From 1977-1997, the mean count (±SD) on the east coast of this island was 1 408 ± 153 mature individuals (Huyser et al. 2000). In March 2011, 311 mature individuals were counted on the west coast of the island (Department of Environmental Affairs unpubl. data). Therefore there are probably at least 1 700 mature individuals at Prince Edward Island. At Marion Island, there were 3 600-3 800 mature individuals in 1976 (Burger 1979, Huyser et al. 2000). It is estimated that there were 4 300 mature individuals in 2012 (Department of Environmental Affairs unpubl. data). The combined population of the Prince Edward Islands is c. 6 000 mature individuals. Confidence in this population estimate is high.
From 1994-2012, winter counts of Lesser Sheathbills at Marion Island increased significantly (Department of Environmental Affairs unpubl. data). Confidence in this trend estimate is high.
Starvation and inclement weather can kill fully-grown birds (Burger 1979). Lesser Sheathbills are vulnerable to predation by Subantarctic Skuas Stercorarius antarcticus, especially when foraging inland (Burger 1979, Huyser et al. 2000), but do not form a major portion of skuas' diets (Burger 1979). A milder climate may lead more Subantarctic Skuas to overwinter at the Prince Edward islands, thereby increasing predation on Lesser Sheathbills (Huyser et al. 2000). Until they were removed from Marion Island, Feral Cats Felis catus also killed Lesser Sheathbills (Burger 1979). Competition for food with the introduced House Mouse Mus musculus, which feeds on terrestrial macro-invertebrates at Marion Island (Huyser et al. 2000), has not yet led to a decrease of Lesser Sheathbill at Marion Island. A proposed bait drop to eradicate mice may negatively affect the population, with Lesser Sheathbills being susceptible to both primary and secondary poisoning (Wanless et al. 2010). Decreasing numbers of winter-breeding Gentoo Penguins Pygoscelis papua at Marion Island may have an impact on winter survival, as they provide a potentially important food resource in this period, but numbers of Subantarctic Fur Seals Arctocephalus tropicalis have increased (Huyser et al. 2000). Plastics in marine debris at Marion Island have recently increased (Department of Environmental Affairs unpubl. data). As Lesser Sheathbills are very inquisitive birds, it is not unlikely that they may ingest such plastic particles. Other threats include outbreaks of disease and oil spills.
It is not permitted to feed sheathbills. At field huts on Marion Island, which are usually attended by sheathbills, grey water and waste food are put into outdoor toilets to discourage mice entering huts; other refuse is placed in waste bins that are emptied annually. No access to Prince Edward Island is allowed except for bona fide research purposes about every five years. Strict measures are in place to prevent accidental introduction of mice to this locality. The Prince Edward Islands are listed as a Ramsar Wetland of International Importance, as a Special Nature Reserve under the Environment Conservation Act (No 73 of 1989) and managed under the Prince Edward Islands Management Plan 1996. The Prince Edward Islands Marine Protected Area was proclaimed in 2013.
Annual monitoring of numbers of sheathbills at Marion Island should be continued. The precautionary approach should be applied regarding possible eradication of mice at Marion Island (Wanless et al. 2010)
* Safe methods of eradicating mice on Marion Island should be sought. Means to mitigate sheathbill mortality, if bait is dropped to eradicate mice, e.g. by holding a sheathbill founder population in captivity until danger of poisoning has passed, should be investigated further. Four of six sheathbills held in captivity at Marion Island for four days lost from 5-13% of their body mass despite being fed at libitum (Wanless et al. 2010).
* On account of potential incidental eradication of sheathbills at Marion after a bait drop, research on taxonomic separation between sheathbills at Marion and Prince Edward islands should be undertaken.
* A full count of numbers of birds at Prince Edward Island in winter is needed.
* Further studies should be conducted on demographic parameters (breeding success, age at breeding, sex ratios, survival and longevity) of Lesser Sheathbills at Marion Island, to compare with earlier estimates and facilitate population modelling.
* Further insight on potential competition between sheathbills and mice for food in winter will be useful.