Justification of Red List Category
This species is classified as Vulnerable as it has a very small area of occupancy, with the vast majority of the global population restricted to one tiny island where invasion by feral cats was a concern. Following the cat eradication on the adjacent main island of Ascension, this risk has now been substantially reduced, but censusing the population and ascertaining trends is particularly problematic.
The current population of mature females is estimated at 9,341 (95% CI: 8,587-10,113), based on census data from 2001-2002, suggesting there may be c.18,682 mature individuals, assuming an equal sex ratio. The confidence intervals for the number of mature females are doubled and rounded to provide a range estimate of 17,000-21,000 for the number of mature individuals, roughly equivalent to 25,000-32,000 individuals in total.
It is difficult to determine the current population trend owing to poor baseline information, the difficulties of carrying out census work and the high number of mature non-breeders in the population (Pickup 1998; Ratcliffe 1999). However, the use of a 'virtual ecologist' model on recent census data, alongside historic data, point to a stable population (Ratcliffe et al. 2008).
Fregata aquila breeds predominately on Boatswainbird Islet, a flat-topped, steep-sided rock, 250 m off the north-east coast of Ascension Island (St Helena to UK) in the Atlantic Ocean, but re-colonised the main island of Ascension in 2012 following successful cat eradication in 2006. Since the early 1800s, when it bred on Ascension Island itself, the population has suffered serious declines and, in 1997, was estimated to lie between 5,000-10,000 individuals (Pickup 1998). The most recent estimates for breeding and mature females are 6,250 and 9,341 respectively, based on census data from 2001-2002; suggesting c.18.682 mature individuals, assuming an equal sex ratio (Ratcliffe et al. 2008). Determining population trends for this species is problematic due to difficulties in carrying out census work, poor baseline information and the high number of mature non-breeders in the population (Pickup 1998, Ratcliffe 1999). However, the use of a 'virtual ecologist' model on recent census data, alongside historic data, point to a stable population (Ratcliffe et al. 2008). The small population that re-established on mainland Ascension in 2012 grew rapidly from 2 pairs in 2012 (of which 1 chick successfully fledged) to >100 pairs in 2016, and is expected to grow further as more birds re-distribute from Boatswainbird Island. At sea, the species forages generally within a 400-600 km radius around Ascension, but satellite tracking indicates use of the Atlantic Ocean to the coast of Brazil.
It is a surface-feeder, feeding on fish, particularly Flying-fish Cypsilurus, Hirundichthyes and Exocoetus volitans, and newly hatched Green Turtles Chelonia mydas. Breeding occurs in four loose colonies (Orta 1992a), mainly on the summit plateau of Boatswainbird Island, especially on rougher areas with some groups of birds occupying ledges on the sides of the plateau (Ashmole et al. 1994). Since 2012 a new colony has formed on the south-west facing slope of Letterbox, a volcanic headland < 1 km from Boatswainbird Island. Over the past 4 years this colony has grown in size both in terms of number of breeding individuals (2 to >100) and also the area covered. Breeding is recorded year-round, but there is evidence of some seasonality with laying increasing from May and peaking in October, then declining to a minimum in February-April (Ashmole et al. 1994). Its clutch-size is one and breeding success is low (50:50).
Historically, the species has suffered severe declines due to hunting by humans, and predation by other invasive non-native species; Black Rats Rattus rattus and feral cats (Ashmole et al. 1994) in particular. These factors were likely responsible for the total desertion of Ascension as a breeding site (Orta et al. 2018). A cat-eradication programme was initiated in 2002, and declared complete in 2006 (Orta et al. 2018). Following the successful eradication of cats, the Ascension Frigatebird required considerably more time to recolonise the island compared to other seabird species. (Pelembe unpubl. data in Ratcliffe et al. 2008). In 2012, two pairs were recorded on Ascension as the first returning breeders since their disappearance (Pitches 2013). There is still a small threat of cats reaching Boatswainbird from Ascension (Orta 1992).
Since 1988, a predominantly Taiwanese and Japanese longline fishery has been operating in the area and could be causing significant mortality (Ratcliffe 1999) although there is no direct evidence for this at present (N. Ratcliffe in litt. 2000, 2003). However, the species is known to be caught on baited hooks by the local sport fishery, indicating potential vulnerability to bycatch mortality (Ratcliffe et al. 2008). Over-fishing of tuna could constitute an indirect threat, as predatory fish herd shoals of small fish to the surface where they become available to surface-feeding seabirds (Ratcliffe 1999).
Breeding areas are still greatly restricted, and despite the reduced risk offered by the protection of Boatswainbird as a bird sanctuary (Orta et al. 2018), the species is still highly susceptible to stochastic events.
Conservation Actions Underway
Following the cat eradication programme, Ascension is maintained free of feral cats by enforcing a strict sterilisation regulation for all domestic cats, and by severely limiting the number of domestic cats that can be brought to the island, which are regulated through a cat registration database. Boatswainbird is a Sanctuary under local legislation (National Protected Areas Ordinance 2015), and the adjacent colony on Letterbox (also protected as a Nature Reserve under local legislation) is closely monitored. In 2016 the Ascension Island Government committed to protecting 50% of its Exclusive Fishing Zone as a marine sanctuary, excluding all commercial international fishing.
Conservation Actions Proposed
Use independent observers on longline vessels to investigate the numbers of this species killed (Ratcliffe 1999). Instigate measures to prevent future mortalities by long-lining if this is proven to be a threat (Ratcliffe 1999). Ensure sustainable use of the fisheries around Ascension Island (Ratcliffe 1999). Conduct further research on breeding behaviour of marked birds (Pickup 1998). Monitor changes in distribution, productivity and long-term population trends. Conduct educational awareness with local recreational fishers on how to safely release hooked birds.
89-96 cm. Large seabird with long wings and long, forked tail. Adult male black overall with glossy green on head and long mantle feathers and bright red gular region which inflates to rugby ball size during courtship. Female dark brown overall with rusty collar and breast. Immature similar to female but has white head. Similar spp. Adult male allegedly inseparable from Magnificent Frigatebird F. magnificens, but female is only female frigatebird with no white on head and body.
Text account compilers
Martin, R., Ekstrom, J., Moreno, R., Shutes, S., Anderson, O., Benstead, P., Stattersfield, A., Taylor, J., Fjagesund, T.
Oppel, S., Leat, E., Ratcliffe, N., Hilton, G., Weber, N., Brown, J.
BirdLife International (2020) Species factsheet: Fregata aquila. Downloaded from http://www.birdlife.org on 27/05/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 27/05/2020.