Justification of Red List Category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km² combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population size may be moderately small to large, but it is not believed to approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). The population trend appears to be stable, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). For these reasons the species is evaluated as Least Concern.
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 (U.S.A.) 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, Baker et al. 1997, Enticott and Tipling 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 in 2005. The species is not recorded on the Volcano Islands, where it bred before the Second World War (Chiba et al. 2007). The global population is likely no greater than 30,000 birds, including 10,000 pairs (Brooke 2004). The 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 1992, 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 past and ongoing threat to this species is predation by invasive mammals. In Japan, rats and cats limit breeding to small predator-free islets in the Ogasawara Archipelago (McClelland et al. 2008). House rats Rattus rattus have been implicated in dramatic declines in Tristram’s Storm-petrel numbers on multiple islets by predation of eggs and adults, as well as local extirpation on some islands (Kawakami 2008). House rat eradication programs are currently in progress in multiple areas (McClellend et al. 2008, Yabe et al. 2009). Polynesian rats Rattus exulans caused the extirpation of the population on Kure Atoll (Rauzon et al. 1985), to which it is yet to return despite eradication being completed (McClelland et al. 2008). Invasive Pharaoh Ants on Laysan have been shown to be a source of chick mortality; however, it is thought the effect of this is relatively minor (McClelland & Jones 2008).
Due to breeding in burrows on sandy islands, Tristram’s Storm-petrel suffers significant annual nest loss as a result of weather and the fragility of sand burrows (McClelland et al. 2008). Sea level rise and additional wave action associated with climate change are likely to impact the breeding success of this species, though no prediction as to the severity of this threat has been made as of yet.
Tristram’s Storm-petrel experiences significant breeding failure due to interspecific competition. Shearwaters were the largest cause of chick loss on both Laysan and Tern Islands due to their aggressive eviction of Storm-petrels (McClelland et al. 2008). On Laysan, the large Bonin Petrel colony was the greatest cause of hatching failure due to competition for burrows (Fefer et al. 1984). Albatrosses are known to alter, block up and cause the collapse of Storm-petrel burrows (McClelland et al. 2008), possibly aggravating this competition.Laysan and Nihoa Finches predate unattended and abandoned eggs on their respective islands, enhancing the risk associated with the stress-related abandonment behaviour shown by Tristram’s Storm-petrel (Marks and Leasure 1992, McClelland et al. 2008). It is unclear whether, in the absence of finch predation, these eggs would have been further brooded (McClelland et al. 2008) and as such, this may not represent a loss of hatching success.
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 habitation, 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 Mukotori-Jima in 2008, but failed; so Black Rats 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 (M. Sato in litt. 2011).
Text account compilers
Hermes, C., Calvert, R., Martin, R., Taylor, J., Miller, E., Capper, D., Stuart, A., O'Brien, A., Anderson, O., Fjagesund, T., Smith, D.
Harrison, C.S., Nisbet, I. & VanderWerf, E.
BirdLife International (2020) Species factsheet: Hydrobates tristrami. Downloaded from http://www.birdlife.org on 23/10/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 23/10/2020.