Following a taxonomic reassessment, Leach’s Storm-petrel (Hydrobates leucorhous) has been split on the HBW/BirdLife International checklist into H. leucorhous, H. socorroensis and H. cheimomnestes. When treated as subspecies, H. l. socorroensis was referred to as Townsend’s Storm-petrel, and H. l. cheimomnestes was referred to as Ainley’s Storm-petrel (Carboneras et al. 2018), but the use of these common names for the newly recognised species is still to be confirmed.
The newly recognised H. socorroensis and H. cheimomnestes both are only known to breed on islets by Guadalupe Island, Mexico, with potentially a combined population of only 5,000 pairs (Brooke 2004). H. socorroensis breeds on the islets of Islote Negro and Islote Afuera from May/June to October/November, while H. cheimomnestes breeds at the opposite time of the year (November/December to April/May) on the same islets as well as also on Gargoyle Rock (Howell 2012). H. socorroensis may also breed on the mainland of Guadalupe, but feral cats have had a major impact on storm-petrels on the island (e.g. Guadalupe Storm-petrel, Hydrobates macrodactylus [see BirdLife International 2018b]), and its persistence there is uncertain (Howell 2012).
The pre-split species was widespread across the North Atlantic and North Pacific (and a limited number of breeding records from the southern hemisphere) (see Carboneras et al. 2018), with an overall population size estimate of 6.7 to 8.3 million breeding pairs. Therefore, the removal of the two newly recognised species from an assessment of the newly defined H. leucorhous will have a negligible impact on the species’s status. Therefore, it is proposed that the newly defined H. leucorhous continue to be listed as Vulnerable under criteria A2bce+3bce+4bce as per the pre-split species (see BirdLife International 2018a).
As mentioned above, the major threat to species on Guadalupe comes from invasives. Feral cats will prey upon individuals, and goats may compound this threat by trampling breeding burrows. It is not certain whether such invasives have made it to the offshore islets though, and so their direct impact on populations there are uncertain. The Guadalupe Murrelet (Synthliboramphus hypoleucus), which has a slightly larger breeding range than both of these storm-petrels (see BirdLife International 2018c), is also thought to be threatened by drowning in gill-nets, disturbance, light pollution and organochloride pollution (Drost and Lewis 1995), though these have not been identified as important threats to the pre-split H. leucorhous. Therefore, it is not certain whether these threats are having any impact at all on the newly recognised taxa in this region. However, because of their highly restricted ranges and the key threat from invasives, both species could warrant listing as threatened. Thus, both H. socorroensis and H. cheimomnestes have been assessed here against all criteria.
It is uncertain to what extent the threats faced by these species are impacting their populations. Therefore, we cannot accurately assess either species against the thresholds for this criterion.
While the Extent of Occurrence (EOO) for both species has not been calculated per IUCN guidelines (IUCN 2001, 2012), based on the maps presented in Howell (2012), their EOOs are unlikely to approach the threshold for Vulnerable. As the species have highly restricted breeding ranges though, occurring on only tiny islets (with H. socorroensis potentially also having a very localised distribution on Guadalupe), the Area of Occupancy (AOO) of both species is likely to be tiny, at least falling below the threshold for Endangered under criterion B2 (500km2).
Hydrobates cheimomnestes (Ainley’s Storm-petrel) – With the main threat being the spread of invasive predators onto the islets where the species breeds, the species can be said to occur at three locations, meeting the threshold for Endangered under subcriterion a. The species is not known to undergo extreme fluctuations, and it is uncertain whether it is currently in decline. Therefore, the species may be considered to be approaching the threshold for listing as threatened under this criterion and so would warrant listing as Near Threatened under criterion B2ab(v). If there were clear evidence to show the species is in decline though, then the species would warrant listing as Endangered under the same criterion.
Hydrobates socorroensis (Townsend’s Storm-petrel) – The main threat to the two islets off of Guadalupe, where it is known to breed, is from the spread of invasives onto these islands; and any remaining population on Guadalupe itself will likely be being directly impacted by the invasives on the island. Therefore, the species may be said to occur at <5 locations (likely only 3), and it may be precautionarily assumed to be in decline, as any population that still breeds on Guadalupe is likely declining. Therefore, the species likely warrants listing as Endangered under criterion B2ab(v).
Hydrobates cheimomnestes (Ainley’s Storm-petrel) – In Brooke (2004), the population size for both H. cheimomnestes and H. socorroensis combined was placed at 5,000 pairs (i.e. 10,000 mature individuals). Howell (2012) suggests that the population for this taxon alone is likely no more than a few thousand individuals. Therefore, the population size may be best placed in the range 2,500-9,999 mature individuals.
There is uncertainty over whether the species is declining though and, although the nature of seabird movements suggest that it should be considered to be all in one subpopulation, given current information it would likely only warrant listing as Near Threatened under criterion C2a(ii). If there were any further evidence to show a decline in the population then instead it would warrant listing as Vulnerable under the same criterion.
Hydrobates socorroensis (Townsend’s Storm-petrel) – In Brooke (2004), the population size for both H. cheimomnestes and H. socorroensis combined was placed at 5,000 pairs (i.e. 10,000 mature individuals). Howell (2012) suggests that for this taxon alone there may be 4,000 birds on Islote Negro and 3,000 on Islote Asfuera. Therefore, the overall population size may be best placed in the range 2,500-9,999 mature individuals.
As the species potentially still breeds on Guadalupe, it may be precautionary to assess the species as undergoing a continuing decline. Thus, given the nature of seabird movements suggest the species should be considered to be all in one subpopulation, the species would warrant listing as Vulnerable under criterion C2a(ii).
Hydrobates cheimomnestes (Ainley’s Storm-petrel) – Based on current information, it is unlikely that this species would approach the threshold for Vulnerable under criterion D1. However, as it breeds on only 3 islets with a high level of threat from invasive predators, such that they could drive the species to Critically Endangered or Extinct in only a short period of time, then the species warrants listing as Vulnerable under criterion D2.
Hydrobates socorroensis (Townsend’s Storm-petrel) – Based on current information, it is unlikely that this species would approach the threshold for Vulnerable under criterion D1. However, as it breeds on only 2 islets (and potentially on mainland Guadalupe) with a high level of threat from invasive predators, such that they could drive the species to Critically Endangered or Extinct in only a short period of time, then the species warrants listing as Vulnerable under criterion D2.
To the best of our knowledge no quantitative analyses of extinction risk have been conducted for either of these species. Therefore, we cannot assess them against this criterion.
Thus, overall it is proposed that;
– Hydrobates leucorhous be listed as Vulnerable under criteria A2bce+3bce+4bce
– H. cheimomnestes be listed at least as Vulnerable under criterion D2, and potentially as Endangered under criterion B2ab(v).
– H. socorroensis be listed precautionarily as Endangered under criterion B2ab(v).
Comments are welcome on these proposed listings, but please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’ Red List status. Therefore, please make sure your comments are about the proposed listings.
BirdLife International. 2018a. Species factsheet: Hydrobates leucorhous. Downloaded from http://www.birdlife.org on 10/04/2018
BirdLife International. 2018b. Species factsheet: Hydrobates macrodactylus. Downloaded from http://www.birdlife.org on 10/04/2018.
BirdLife International. 2018c. Species factsheet: Synthliboramphus hypoleucus. Downloaded from http://www.birdlife.org on 11/04/2018.
Brooke, M. 2004. Albatrosses and Petrels of the World. Oxford University Press, Oxford, UK.
Carboneras, C.; Christie, D. A.; Jutglar, F.; Kirwan, G. M.; Sharpe, C. J. 2018. Leach’s Storm-petrel (Hydrobates leucorhous). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/52595 on 10 April 2018).
Drost, C. A.; Lewis, D. B. 1995. Xantus’ Murrelet (Synthliboramphus hypoleucas). In: Poole, A.; Gill, F. (ed.), The birds of North America, No. 164, pp. 1-24. The Academy of Natural Sciences, and The American Ornithologists’ Union, Philadelphia, and Washington, DC.
Howell, S. N. G. 2012. Petrels, albatrosses & storm-petrels of North America: a photographic guide. Princeton University Press, Princeton.
IUCN. 2001. IUCN Red List Categories and Criteria: Version 3.1. IUCN Species Survival Commission. IUCN, Gland, Switzerland and Cambridge, U.K.
IUCN. 2012. IUCN Red List Categories and Criteria: Version 3.1. Second edition. IUCN Species Survival Commission. IUCN, Gland, Switzerland and Cambridge, U.K. Available at: www.iucnredlist.org/technical-documents/categories-and-criteria.