Masked Boobies Sula dactylatra were apparently extremely abundant on St Helena in the early sixteenth century, when the island was discovered. It is probable that the breeding population was severely depleted by man and introduced mammalian predators following human settlement of the island in 1659. The species had certainly become very scarce by the nineteenth and twentieth centuries, restricted to a few small offshore stacks that remained predator-free. The first fully documented breeding, on Shore Island a small predator-free stack, was as recent as 1988. Since that time, the population has increased substantially, and here we report the first instance of breeding on the St Helena mainland, despite the presence of Feral Cats Felis catus, Brown Rats Rattus norvegicus and Black Rats R. rattus. In July 2009, along 4 km of coastline in the southwest of the island, one pair was discovered with a large downy chick, four pairs were confirmed to be incubating eggs, and a further 25 pairs were apparently incubating eggs. Subsequent monitoring of 28 breeding attempts up to July 2010 revealed moderate levels of breeding success (0.461 young fledged per attempt), which were substantially higher than those of Masked Boobies that have re-colonised Ascension Island in recent years following cat eradication. The ability of Masked Boobies to nest successfully, on occasion, on islands where cats and rats are present, highlights their potential resilience to these introduced mammalian predators, and the circumstances and long-term viability of successful co-existence with such non-native predators warrant further investigation.

The role of predation in limiting population size has long been of interest to ecologists (Birkhead & Furness 1985; Furness & Monaghan 1987; Croxall & Rothery 1991; Newton 1998) and is increasingly of relevance to conservation managers wishing to maintain or restore populations of rare or vulnerable species (Gibbons et al. 2007; Hilton & Cuthbert 2010). Understanding the population-level impacts of predation is especially important in cases where the main source of predation is an introduced non-native species. In such cases predator eradication may be technically feasible, but costly, opposed by members of the public, logistically complex, and long-term success may not be guaranteed either due to failure to remove all individuals or by subsequent reinvasion/reintroduction (Oppel et al. 2011). Notwithstanding the challenges, introduced mammalian predators (principally Brown Rats Rattus norvegicus, Black Rats R. rattus, Pacific Rats R. exulans and Feral Cats Felis catus) have been successfully eradicated from several hundred islands worldwide (Nogales et al. 2004; Howald et al. 2007). In many cases the anticipated beneficiaries have been seabirds, which, due to their ground- nesting habits and low reproductive rates, may be especially vulnerable to introduced terrestrial predators, against which they have no, or limited, evolved defence. Larger predators such as Feral Cats are likely to impact a wider range of seabird species than the smaller predators (Black Rats, Brown Rats and House Mice Mus musculus), due to their ability to tackle even large-bodied adult prey (Burger & Gochfeld 1994). Predation on adults also results in faster population decline than that associated with predation directed predominantly or solely on eggs or chicks (Hilton & Cuthbert 2010). However, the outcome of the co-occurrence of any particular seabird-predator species combination can be difficult to predict, as demonstrated by the unexpected predation of albatross chicks by House Mice on Gough Island (Cuthbert & Hilton 2004). Whilst their large size, aggressive nature and formidable bill may confer on boobies a degree of resilience to cat predation, Ashmole et al. (1994) provide detailed field evidence of multiple instances of cat predation of adult Brown Sula leucogaster and Masked Boobies S. dactylatra on Ascension Island, and there is evidence of cat-related declines and extirpation of several booby species at numerous sites worldwide: Ascension Island, South Atlantic (Stonehouse 1962; Williams 1984; Ashmole et al. 1994; Ratcliffe et al. 2009); Baker Island, Hawaii (Forsell 1982); Jarvis Island, 1,300 km south of Hawaii (Rauzon 1985); various Indonesian islands (de Korte 1984; de Korte & Silvius 1994). The restoration of booby breeding populations at some of these islands following cat eradication also provides good evidence of population-level impacts of cat predation (e.g. Ratcliffe et al. 2009).

St Helena was discovered by the Portuguese in 1502, and the first mammalian predator, the Black Rat, is likely to have been introduced at that time, or soon afterwards (Rowlands et al. 1998). The island was not permanently settled until 1659, though cats were introduced sometime earlier, in the sixteenth century, to control the large numbers of rats. Brown Rats did not arrive until around the start of the eighteenth century but were well established throughout the island by 1732, by which time cats were said to have reached plague proportions and were as numerous as rats (Rowlands et al. 1998). The earliest accounts of St Helena during the years immediately following human settlement (Ogilby 1670) suggest the avifauna was initially extremely abundant and heavily exploited for human consumption. Extensive sub-fossil bone remains in deposits on St Helena indicate the former existence of large seabird populations on the island, 16 species in total, including three endemic to St Helena (Olson 1975). Remains of Masked Boobies in both the youngest and older deposits indicate the species’ presence on St Helena for a considerable period prior to human settlement. However, a combination of habitat change, human persecution and predation by mammalian predators led to declines and extinction of almost the entire endemic avifauna, including the St Helena Petrel Pterodroma rupinarum and the St Helena Bulweria Bulweria bifax. The third endemic seabird, the St Helena Shearwater Puffinus pacificoides is believed to have become extinct before 1502 (Olson 1975). Currently only eight breeding seabird species are known to remain on St Helena. The earliest confirmed record in recent times of breeding Masked Boobies (on Shore Island) dates from as recently as 1988 (Rowlands et al. 1998). Unconfirmed reports suggest breeding may also have occurred in the previous few years, but the absence of previous records suggests the species did not breeder earlier in the twentieth century. Indeed, Stonehouse (1960) remarked on the absence of both Brown and Masked Boobies when he visited St Helena in 1958. Since 1988, breeding has been documented for both Shore Island and George Island (Rowlands et al. 1998), but has never been reported on the mainland of St Helena, presumably due to the presence of Feral Cats.

This study resulted from anecdotal reports that Masked Boobies were present on the coast of mainland St Helena, but confirmation of breeding was lacking. Given the current conservation efforts to eradicate introduced mammalian predators from many islands worldwide, and the common belief that such introduced predators prevent successful reproduction of the majority of native seabird species, the aim of this study was to determine whether Masked Boobies were breeding on mainland St Helena, and to document the location, size and success of any breeding colony.

We thank Andrew Darlow and Lourens Malan for bringing to our attention the presence of Masked Boobies on the southern cliffs and the possibility that a breeding colony may have become established. We also thank Graham Sim and Ivan Henry for monthly assistance with fieldwork, and Emma Bennett, who established the seabird monitoring programme on St Helena.

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