Bristle-thighed Curlew (Numenius tahitiensis): request for information

BirdLife species factsheet for Bristle-thighed Curlew

Bristle-thighed Curlew (Numenius tahitiensis) is a shorebird breeding in the tundra of Alaska. In late summer, it migrates south to Oceania, where it winters mainly on Micronesian and Polynesian islands. The population is estimated to number 10,000 mature individuals (Partners in Flight 2019).

The species is facing a variety of threats, which caused population declines in the past. On its breeding grounds in Alaska, the species suffers from predation by native and introduced predators and habitat loss, and it may be vulnerable to climate change impacts like migratory mismatches or range shifts (Pearce-Higgins et al. 2017, Marks et al. 2020, Smith et al. 2020). Predation and habitat deterioration caused by introduced species is a potential threat on the non-breeding grounds in Oceania as well (Marks et al. 2020).  Direct harvesting by humans used to threatened the population on the non-breeding grounds, but this practice has already been abandoned in the past (Marks et al. 2020, P. Raust in litt. 2020).

The species has been considered Vulnerable under Criterion C2a(ii) (BirdLife International 2020). However, new information regarding the population size and uncertainty surrounding the population trend suggest that the species may warrant a change in Red List status. Therefore, we have fully reviewed the species here against all Red List criteria.

Criterion A – Thepopulation trend is difficult to quantify. On the breeding grounds in the Arctic, trends are inconclusive. There is concern that the species may be undergoing a decline: Wetlands International (2020) list the species as declining between 2002 and 2011. Partners in Flight (2019) describe the trend as unknown or possibly declining (see also Panjabi et al. 2019), while a population survey found no evidence of declines (G. Butcher in litt. 2007). On the non-breeding grounds in Oceania, the species is thought to be stable (P. Raust in litt. 2020) or even increasing (Tibbitts et al. 2019). Overall, there seems to be growing evidence that population declines have stopped or at least slowed down considerably. It is unlikely that the species meets the threshold for listing as threatened under Criterion A (population decline of ≥30% over three generations; 23.1 years; Bird et al. 2020*); nevertheless, in order to fully assess the species against this criterion, we ask for recent information regarding the population trend, particularly on the breeding grounds in Alaska.

Criterion B – The species has a very large range (EOO breeding = 95,000 km2; EOO non-breeding = 345,000,000 km2). This is too large to warrant listing as threatened under Criterion B. The species is thus considered Least Concern under this criterion.

Criterion C – The population is estimated to number 10,000 mature individuals. This does not meet the threshold for listing as threatened under Criterion C, but approaches it. However, in order to be listed as Near Threatened under this criterion, further conditions must be met.

Despite breeding in two different areas in Alaska, the species does not show spatial segregation in the non-breeding grounds (Sonsthagen et al. 2015) and can therefore be considered one subpopulation. What remains unclear is the overall population trend and the level of confidence in the estimate. Is the species undergoing a decline? If yes, has the decline been assessed directly or is it based on indirect evidence? We are seeking for recent information regarding the population trend. Especially in case that the species is assumed to still be in decline, what is the evidence for the decline?

Criterion D – The species’s population size and range are too large to warrant listing as threatened under Criterion D and thus, Bristle-thighed Curlew is classified as Least Concern under this criterion.

Criterion E – To the best of our knowledge, there has been no quantitative analysis of extinction risk conducted for this species. Therefore, it cannot be assessed against this criterion.

Hence, it appears that the only criterion where the species may approach the threshold for listing as threatened is Criterion C2a(ii). Therefore, up-to-date information is urgently sought regarding the population trend of Bristle-thighed Curlew as well as the intensity of threats the species may be facing.

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 relevant to the species’ Red List status and the information requested. By submitting a comment, you confirm that you agree to the Comment Policy.

*Bird generation lengths are estimated using the methodology of Bird et al. (2020), as applied to parameter values updated for use in each IUCN Red List for birds reassessment cycle. Values used for the current assessment are available on request. We encourage people to contact us with additional or improved values for the following parameters; adult survival (true survival accounting for dispersal derived from an apparently stable population); mean age at first breeding; and maximum longevity (i.e. the biological maximum, hence values from captive individuals are acceptable).

An information booklet on the Red List Categories and Criteria can be downloaded here and the Red List Criteria Summary Sheet can be downloaded here. Detailed guidance on IUCN Red List terms and definitions and the application of the Red List Categories and Criteria can be downloaded here.

References

Bird, J. P.; Martin, R.; Akçakaya, H. R.; Gilroy, J.; Burfield, I. J.; Garnett, S.; Symes, A.; Taylor, J.; Šekercioğlu, Ç.; Butchart, S. H. M. (2020). Generation lengths of the world’s birds and their implications for extinction risk. Conservation Biology online first view.

BirdLife International. 2020. Species factsheet: Numenius tahitiensis. http://www.birdlife.org (Accessed 26 March 2020).

Marks, J. S.; Tibbitts, T. L.; Gill, R. E.; McCaffery, B. J. 2020. Bristle-thighed Curlew (Numenius tahitiensis), versión 1.0. In: Poole, A. F.; Gill, F. B. (eds.). Birds of the World. Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.brtcur.01 (Accessed 26 March 2020).

Panjabi, A. O.; Easton, W. E.; Blancher, P. J.; Shaw, A. E.; Andres, B. A.; Beardmore, C. J.; Camfield, A. F.; Demarest, D. W.; Dettmers, R.; Keller, R. H.; Rosenberg, K. V.; Will, T. 2019. Avian Conservation Assessment Database Handbook, Version 2019. Partners in Flight Technical Series No. 8. http://pif.birdconservancy.org/acad_handbook.pdf.

Partners in Flight. 2019. Avian Conservation Assessment Database, version 2019. http://pif.birdconservancy.org/ACAD.

Pearce-Higgins, J. W.; Brown, D. J.; Douglas, D. J. T.; Alves, J. A.; Bellio, M.; Bocher, P.; Buchanan, G. M.; Clay, R. P.; Conklin, J.; Crockford, N.; Dann, P.; Elts, J.; Friis, C.; Fuller, R. A.; Gill, J. A.; Gosbell, K.; Johnson, J. A.; Marquez-Ferrando, R.; Masero, J. A.; Melville, D. S.; Millington, S.; Minton, C.; Mundkur, T.; Nol, E.; Pehlak, H.; Piersma, T.; Robin, F.; Rogers, D. I.; Ruthrauff, D. R.; Senner, N. R.; Shah, J. N.; Sheldon, R. D.; Soloviev, S. A.; Tomkovich, P. S.; Verkuil, Y. I. 2017. A global threats overview for Numeniini populations: synthesising expert knowledge for a group of declining migratory birds. Bird Conservation International 27(1): 6-34.

Smith, P. A.; McKinnon, L.; Meltofte, H.; Lanctot, R. B.; Fox, A. D.; Leafloor, J. O.; Soloviev, M.; Franke, A.; Falk, K.; Golovatin, M.; Sokolov, V.; Sokolov, A.; Smith, A. C. 2020. Status and trends of tundra birds across the circumpolar Arctic. Ambio 49: 732-748.

Sonsthagen, S. A.; Tibbitts, T. L.; Gill Jr., R. E.; Williams, I.; Talbot, S. L. 2015. Spatial genetic structure of bristle-thighed curlews (Numenius tahitiensis): breeding area differentiation not reflected on the non-breeding grounds. Conservation Genetics 16: 223-233.

Tibbitts, T. L.; Ruthrauff, D. R.; Underwood, J. G.; Patil, V. P. 2020. Factors promoting the recolonization of Oahu, Hawaii, by Bristle-thighed Curlews. Global Ecology and Conservation 21: e00785.

Wetlands International. 2020. Waterbird Population Estimates. wpe.wetlands.org (Accessed 26 March 2020).

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4 Responses to Bristle-thighed Curlew (Numenius tahitiensis): request for information

  1. Brad Andres says:

    I am not aware of any new breeding ground information on either population size or trend. Curlews occur on a few Christmas Bird Counts on Hawaii, but counts are highly variable, and no other information exists on population trend (therefore unknown). Note that the information in the Avian Conservation Assessment Database came from Andres et al. 2012.

  2. Eric VanderWerf says:

    In Hawaii the Christmas Bird Count circles do not encompass the areas where the species occurs most regularly, so CBC data are of limited value in assessing the trend. A more comprehensive data set is provided by eBird, and those data could be used to help assess the population trend in Hawaii, and perhaps elsewhere in the Pacific. There are some data quality issues with eBird, but for Bristle-thighed Curlew the data probably are pretty accurate. I am somewhat dubious that harvest for food by humans on the non-breeding grounds has completely stopped, as described above. I don’t doubt that it has decreased, but I suspect harvesting still occurs sporadically in some areas.

  3. Red List Team (BirdLife International) says:

    Many thanks to everyone who has contributed to this discussion. We greatly appreciate the time and effort invested by so many people in commenting. The window for consultation is now closed. We will analyse and interpret the new information and post a preliminary decision on this species’s Red List status on this page in early July.

    Thank you once again,
    BirdLife Red List Team

  4. Red List Team (BirdLife International) says:

    Preliminary proposal

    The overall population trend is difficult to assess, as trend appear inconclusive across the range. While information from the breeding grounds mention declining, possibly declining or stable trends, on the non-breeding grounds the species was found to be stable or increasing. In the absence of data covering the entire range and using a precautionary approach, we can suspect that the species has been undergoing a slow decline over the past three generations, which is unlikely to exceed a rate of 5-10%. The species hence does not meet the threshold for a listing under Criterion A, but approaches it under Criterion C.

    Based on available information, our preliminary proposal for the 2020 Red List would be to list Bristle-thighed Curlew as Near Threatened, approaching the threshold for listing as threatened under Criterion C2a(ii).

    There is now a period for further comments until the final deadline in mid-July, after which the recommended categorisations will be put forward to IUCN.

    Please note that we will then only post final recommended categorisations on forum discussions where these differ from the initial proposal.

    The final 2020 Red List categories will be published on the BirdLife and IUCN websites in December 2020/January 2021, following further checking of information relevant to the assessments by both BirdLife and IUCN.

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