Justification of Red List Category
This species is classified as Near Threatened because it is thought to have a small breeding range. A higher level of threat classification might be justified if the range size is confirmed and found to be declining.
Population estimates for breeding colonies suggest that the world population is no greater than c.10,000 pairs (i.e. c.20,000 mature individuals) and c.30,000 birds (Brooke 2004), while the population in Japan has been estimated at c.100-10,000 breeding pairs and c.50-1,000 individuals on migration in Japan (Brazil 2009).
The population is suspected to be stable although predation by invasive species, human disturbance, marine pollution and stochastic events may cause future declines.
Hydrobates tristrami breeds in the Hawaiian archipelago (USA), on Nihoa (2,000-3,000 pairs), Necker, French Frigate Shoals (max 280 pairs), Laysan (500-2,500 pairs), Pearl and Hermes Reef (1,000-2,000 pairs), and may also breed on Midway, Lisianski and Kure (Rauzon et al. 1985, Harrison 1990, Enticott and Tipling 1997, Baker et al. 1997, McClelland et al. 2008). It also breeds on a few small predator-free islets of the Bonin and Izu Islands, Japan (McClelland et al. 2008), including six islands in the Bonin group, three of which were newly identified as part of a survey that concluded in 2005. They were not detected on the Volcano Islands, where the species bred before WWII (Chiba et al. 2007). These estimates suggest that the world population is no greater than 30,000 birds, including 10,000 pairs (Brooke 2004). This species is difficult to monitor due to its high sensitivity to human disturbance during the incubation phase (Marks and Leasure 1992) and due to its habit of breeding in the winter months when access to its remote breeding colonies is more difficult. Little is known about its post-breeding dispersal, but some move north to the seas east of Japan (Carboneras 1992a, Enticott and Tipling 1997).
The species feeds while pattering on the sea surface, principally consuming squid and fish (Brooke 2004). It nests in burrows in sand or guano, under clumps of vegetation or in recesses in scree (Brooke 2004). In Hawaii, first egg laying occurs in December with the last chicks fledging in June (McClelland et al. 2008). The species has been known to live for at least 14 years (Marks and Leasure 1992).
The most serious threat to the species is the potential introduction of rats or other predators to any of its primary breeding sites (Harrison 1990): a large population of H. tristrami which bred on Torishima Island until the early 1960s was apparently exterminated by introduced cats and rats (Carboneras 1992a, Enticott and Tipling 1997), and potentially large colonies could have been extirpated from the Northwestern Hawaiian Islands for the same reason (C. S. Harrison in litt. 1999). It is known to breed on only a very few rat-free islets in Japan, indicating a high degree of susceptibility to introduced predators on its natal colonies. It is thought that rats may predate on both adults and chicks in burrows (Chiba et al. 2007). Predation and irritation by introduced tramp ants Monomorium pharaonis could pose a problem for the species (E. van der Werf in litt. 2007), although studies on Laysan Island indicate that impacts on breeding success are minimal, with nest failures attributable to ants recorded as 1% (n=116) (McClelland and Jones 2008). Nest failures were due to aggravation rather than direct predation, with chicks abandoning nest sites (McClelland and Jones 2008). Nests most likely to be affected were situated under vegetation (McClelland and Jones 2008). On Laysan, the species's eggs are predated by Laysan Finches Telespiza cantans and on Nihoa, possibly by Nihoa Finches T. ultima (Marks and Leasure 1992). Telespiza cantans is known to routinely check H. tristrami burrows. A study on Laysan confirmed T. cantans as an effective egg predator, depredating 90% of unattended eggs, and despite this vulnerability egg neglect was observed in 10% of nests (McClelland et al. 2008). The same process is expected on Pearl and Hermes Reef where T. cantans has been introduced, possibly resulting in a reduction in nesting success (Marks and Leasure 1992). Other native species, including the Bonin Petrel Pterdroma hypoleuca, Laysan Albatross Phoebastria immutabilis, and particularly the Wedge-tailed Shearwater Ardenna pacifica may hamper reproductive success on Laysan and Tern Island (McClelland et al. 2008). Extreme weather events can affect breeding; in 1913, birds on Laysan suffered heavy losses owing to the flooding of burrows, followed by sandstorms (Warham 1990). Light pollution and the contamination of the sea with plastics and other litter are no longer considered likely threats to this species (I. C. T. Nisbet in litt. 2010).
Conservation Actions Underway
Limited research has been carried out into the species's life history and ecology. Most of the known breeding population is protected by the Hawaiian Islands National Wildlife Refuge, which is devoid of human inhabitation, except when researchers are present (Marks and Leasure 1992). A recent research project has investigated the potential threat from introduced ants (E. van der Werf in litt. 2007). Black rat Rattus rattus eradications were attempted on Muko-jima and (Muko)tori-jima in 2008 but failed, and so must be considered as candidates for any future eradication plans for the islands (M. Sato in litt. 2011). Plans to eradicate rats from these islands, among others, are underway with results expected Feb-Mar 2012 (M. Sato in litt. 2011).
Text account compilers
Anderson, O., Calvert, R., Capper, D., O'Brien, A. & Taylor, J.
Harrison, C., Nisbet, I. & VanderWerf, E.
BirdLife International (2018) Species factsheet: Hydrobates tristrami. Downloaded from http://www.birdlife.org on 21/02/2018. Recommended citation for factsheets for more than one species: BirdLife International (2018) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 21/02/2018.