The site is a barren, steep-sided trachytic rock, about 340 m by 220 m in size, rising to 104 m, with a relatively flat basaltic top nearly 3 ha in extent, located 305 m north of the eastern part of Ascension Island. The site is heavily overlaid with guano, and there are traces of a guano industry that operated for a short period in the 1920s.
See table for key species. The site supports nine of the 11 breeding seabird species of Ascension, these being Hydrobates castro, Phaethon aethereus, P. lepturus, Sula dactylatra, S.sula, S. leucogaster, Fregata aquila, Anous minutus and Gygis alba. A.stolidus is not currently recorded as being present on Boatswainbird Island (although it does offer suitable habitat) but breeds on other small offshore rock stacks from Comfortless Cove to Porpoise Point, and Onychoprion fuscata are numerous in the area, historically with small numbers breeding at this site. There have been no records of O. fuscata breeding since the 1990s with the exception of one pair being recorded in 2001–2002 (Ratcliffe et al., 2009; Weber pers. comm. 2015). For several of the species listed this is the main breeding colony for Ascension Island. This site notably supports c. 99% of the worlds’ population of F.aquila which, despite data discrepancies, has appeared to have remained stable for the last 70 years supporting c. 6,250 breeding females and c. 9,350 mature females from data collected over 2001 - 2002 (Ratcliffe et al., 2009). Studies are currently on-going to reliably update this estimate. A current Darwin Plus project is underway to assess the taxonomic and conservation status of storm-petrels in the South Atlantic. This will clarify whether the storm-petrels nesting on St Helena and Ascension constitute one or more new species. An updated population estimate will be included in this project as H.castro have not been surveyed since 1958-59, mostly due to accessibility issues. Similarly P. aethereus and P. lepturus have not been surveyed since 1958-59 due to accessibility limitations to the cliff face. S.sula has a small population present on BBI with 30 individuals in 1958 and 27 recorded in 2002 (Pelembe, 2006). S.leucogaster had a population of 666 in 2002 in comparison to 900 individuals in 1958 (Pelembe, 2006). A.minutus had a population size of 9,600 individuals in 2002, significantly less than the population of 1958 with 25,000 birds (Pelembe, 2006). Puffinus lherminieri is present in the area, and although not confirmed, it is possible that this species may be breeding on Boatswainbird Island and is assumed to have bred at this site in the past (Ratcliffe et al., 2009).
Non-bird biodiversity: Chelonia mydas (EN), protected locally since 1926, are present in the waters of Ascension as the beaches are important breeding grounds for green turtles and the three main nesting beaches (Long Beach, Pan Am and North East Bay) are designated as Nature Reserves under the National Protected Areas Order, 2014. The dolphin Tursiops truncatus (LC) is common around the island and there are also regular sightings of Stenella attenuate (LC). There are several invertebrates, including the endemic pseudoscorpions Garypus titanius, Neocheiridium sp. and Stenowithius duffeyi. There are 11 species of coastal fish which are probably endemic to Ascension along with a further 16 fish species which appear to be shared endemics with St Helena (Wirtz et al., 2014).
BirdLife International (2019) Important Bird Areas factsheet: Boatswainbird Island. Downloaded from http://www.birdlife.org on 20/10/2019.