Hawaiian Petrel (Pterodroma sandwichensis) is known only to breed on the Hawaiian islands of Hawai`i, Lana`i, Moloka`i, Kaua`i and Maui, and it ranges through the central Pacific. Analysis by Spear et al. (1995) gave a best estimate for the population size of 3,750-4,500 pairs (7,500-9,000 mature individuals) based on at-sea surveys. However, thanks to the discovery of potentially thousands of breeding birds on Lana`i and two small colonies of tens or hundreds of individuals on West Maui (D. Ainley in litt. 2007, J. Penniman in litt. 2007), the population size may in fact have been closer to the upper estimate of Spear et al. (1995) – 6,500-8,300 pairs (11,000-16,600 mature individuals).
The subpopulation size on Kaua`i was estimated to be c.1,600 pairs (Ainley et al. 1997), and so could contain 19-43% of the overall breeding population, depending on which population size estimate is used. Radar surveys have shown that since Hurricane Iniki, the Kaua`i subpopulation may have declined by as much as 78% between 1993 and 2013, driven by key threats such as introduced predators, light attraction, collisions with powerlines and habitat alteration (Raine et al. 2017). At the moment this data is only from Kaua`i, although the subpopulations of West Maui and at Mauna Loa do also appear to be in trouble (BirdLife International 2017, del Hoyo et al. 2017). However, the subpopulation trends of different subpopulations may vary quite a lot, and it had been suggested that the overall population could be stable or increasing (D. Ainley in litt. 2007). That said, the rate of decline on Kaua`i is worryingly rapid, given the proportion of the global population that is found there. Therefore, we compare the species here against all criteria to reassess its Red List status.
Criterion A – If we were to extrapolate the rate of decline on Kaua`i to the entire population then the decline is at such a rate that if it continued into the future, the decline over 3 generations (59.4 years) would be c.99%. This would then qualify the species for listing as Critically Endangered.
However, it is thought that the different subpopulations may be undergoing different population trends (see BirdLife International 2017). Even though the subpopulations on Mauna Loa and West Maui are considered to be under threat, we have no quantitative idea of population trends at these sites, or for any others. In the absence of any quantified trends, if we make an assumption that, Kaua`i aside, all other subpopulations are overall stable, then the global rate of decline would be c.19-42.5% over 3 generations, depending on total population size.
Therefore, with the available information, we cannot accurately assess the species against this criterion, as it could fall anywhere between Least Concern and Critically Endangered; although we may be justified in suggesting the current population trend is an ongoing decline. Thus, further information about trends on other islands would be extremely useful in helping to decide whether the overall population decline is sufficient for the species to warrant listing under this criterion.
Criterion B – The Extent of Occurrence for this species is 22,000,000km2; and the Area of Occupancy has been listed as 370km2. Occurring on five islands, the species is currently considered to occur at 6-10 locations*, with a continuing decline in population size. Two sites are potentially under threat, but there is not sufficient evidence to show a continuing decline in the number of locations. Thus the species would likely warrant listing as Vulnerable under criterion B2ab(v).
Criterion C – Despite the information generated from surveys on Lana`i and West Maui suggesting that the population size could be towards the upper estimate of Spear et al. (1995), the rapid decline in at least the population on Kaua`i would suggest that the population size may now in fact be smaller than this. However, the uncertainty over population trends on other islands means that it would not be possible to accurately estimate the current population size by just extrapolating forwards from the 1995 estimates of Spear et al. It may then be appropriate to use the lower boundary of the best estimate of population size from Spear et al. (1995) and the upper boundary of the upper population estimate as the limits of the population size. Therefore, the population size may be assessed as falling in the range 3,750-8,300 pairs. This may then warrant the species listing as Vulnerable under criterion C, given the species is considered to be declining, as long as other conditions are met.
It is uncertain whether there is any genetic mixing between colonies, so we cannot fully accurately know whether the species may be considered to occur in one sub-population or not. However, if each island where the species breeds on is treated as one sub-population (as has been the case is past assessments), then the species would not warrant listing under criterion C2a(ii). The estimate of several thousand breeding birds on Lana`i and the fact that the species is not known to undergo extreme fluctuations would also mean that it does not warrant listing under criteria C2a(i) or C2b either.
To be listed under criterion C1 would require Hawaiian Petrel to be undergoing at least an estimated 10% continuing decline over 3 generations. It appears that even if all other subpopulations were considered stable, the population trend may be estimated to be declining by >10%. Therefore, given the current information, the species may precautionarily warrant listing as Vulnerable under criterion C1. However, any further information regarding the trends of other subpopulations outside of Kaua`i would be very useful, as if they were in fact increasing then the overall trend may instead be <10% decline over 3 generations. Conversely, if the ranging behaviour of the species was considered sufficiently high that the species should be assessed as occurring in only one subpopulation, then it would warrant listing as Vulnerable under criterion C2a(ii).
Criterion D – The population size and range are too large to warrant listing as threatened under this criterion.
Criterion E – To the best of our knowledge no quantitative analysis of extinction risk has been conducted for this species. Therefore, it cannot be assessed against this criterion.
Overall, without further information regarding population trends on other islands, the species would likely warrant remaining as Vulnerable, but under criteria B2ab(v); C1, and potentially under C2a(ii) too. The hesitation to extrapolate the Kaua`i trends across all subpopulations is based mainly on the prior statements that trends of different subpopulations may vary substantially in this species, and it has even been suggested that the overall population trend could be stable or an increase (D. Ainley in litt. 2007). However, if this is no longer considered to be the case, and in fact the trends from Kaua`i may be representative of the species as a whole, then Hawaiian Petrel may warrant uplisting.
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’s Red List status. Therefore, please make sure your comments are about the proposed listing.
*The term ‘location’ refers to a distinct area in which a single threatening event can rapidly affect all individuals of the taxon present, with the size of the location depending on the area covered by the threatening event. Where a taxon is affected by more than one threatening event, location should be defined by considering the most serious plausible threat (IUCN 2001, 2012).
Ainley, D. G.; Podolsky, R.; de Forest, L.; Spencer, G. 1997. New insights into the status of the Hawaiian Petrel on Kauai. Colonial Waterbirds 20: 24-30.
BirdLife International. 2017. Species factsheet: Pterodroma sandwichensis. Downloaded from http://www.birdlife.org on 05/12/2017.
del Hoyo, J.; Collar, N.; Kirwan, G. M. 2017. Hawaiian Petrel (Pterodroma sandwichensis). 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/467271on 5 December 2017).
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.
Raine, A. F.; Holmes, N. D.; Travers, M.; Cooper, B. A.; Day, R. H. 2017. Declining population trends of Hawaiian Petrel and Newell’s Shearwater on the island on Kaua’i, Hawaii, USA. Condor 119: 405-415.
Spear, L. B.; Ainley, D. G.; Nur, N.; Howell, S. N. G. 1995. Population size and factors affecting at-sea distributions of four endangered Procellariids in the tropical Pacific. Condor 97: 613-638.