Double-banded Plover (Charadrius bicinctus): request for information.

BirdLife species factsheet for Double-banded Plover

The Double-banded Plover (Charadrius bicinctus) breeds in New Zealand, the Chatham Islands and the Auckland Islands (Wiersma et al. 2019). It usually breeds inland on sand or gravel banks alongside rivers, or in dry montane habitats in New Zealand, although it also breeds in a range of other habitats including coasts, agricultural land and moors (Wiersma et al. 2019). It is partially migratory, with birds that breed at high altitude mainly migrating to winter in northern New Zealand, eastern and southern Australia, Tasmania, Norfolk Island and Lord Howe Island (Wiersma et al. 2019). Outside the breeding season, it occurs in a range of coastal and freshwater habitats (Wiersma et al. 2019).

The species is currently listed as Least Concern. In the Conservation status of New Zealand birds 2016 (which uses different criteria from the IUCN Red List), the subspecies C. b. bicinctus, which breeds in New Zealand and the Chatham Islands and winters in New Zealand, Australia, Tasmania, Norfolk Island and Lord Howe Island, is listed as Nationally Vulnerable (Robertson et al. 2017). This is because it is judged to have 5,000–20,000 mature individuals with a predicted decline of 30–70% over three generations (Robertson et al. 2017). Introduced mammals are considered the main cause of the decline, with feral cats, ferrets, stoats, and hedgehogs predating eggs and chicks (Rerbergen et al. 1998), along with native Kelp Gulls (Larus dominicanus [Moon 1992]), Swamp Harriers (Circus approximans [Keedwell and Sanders 2002]) and introduced Australian Magpies (Gymnorhina tibicen [Keedwell and Sanders 1999]). Rats and weasels are also reported to have known impacts on the subspecies, while habitat loss and disturbance is considered to have displaced birds from some breeding sites (Pierce 2013). The subspecies C. b. exilis, found on the Auckland Islands, is listed as Naturally Uncommon (Robertson et al. 2017). As a result of this, we are reviewing the species’s Red List Category.

Our current information on the species’s conservation status will now be compared to all Red List Criteria.

Criterion A – This species’s generation length is currently estimated as 5 years, meaning that population size reductions should be measured over a period of 15 years for the application of Criterion A. According to Robertson et al. (2017), the subspecies C. b. bicinctus has a predicted or ongoing decline of 30-70% over three generations, even though the time span was not stated. The total population of C. b. bicinctus has been estimated at 5,000–20,000 mature individuals (Robertson et al. 2017). No further information is provided on the derivation of these figures.

We have no recent population estimate for C. b. exilis, but it was estimated to number 730 birds in 1989 (Walker et al. 1991), which is roughly equivalent to 487 mature individuals. Given that it is not judged to be threatened by Robertson et al. (2017) and is unlikely to have a population size exceeding 5,000 mature individuals, we can deduct from the New Zealand Threat Classification System (Townsend 2008) that the population is considered to be stable or increasing.

Assuming that Robertson et al. (2017) used a generation length of 5 years, an exponential rate of decline in the population of C. b. bicinctus, and that the population size of C. b. exilis has remained stable, then the population sizes and reductions described above would equate to a reduction over three generation lengths of 28 – 69%. This range of estimates could qualify the species for listing as Near Threatened, Vulnerable or Endangered under Criterion A4, and possibly A3 too.

Criterion B – The species’s Extent of Occurrence (EOO) is estimated at 1,650,000 km2. This does not approach the threshold for listing the species as threatened under Criterion B1. The species’s Area of Occupancy (AOO) has not been quantified, but is not likely to approach the threshold for listing the species as threatened under Criterion B2. The species is therefore assessed as Least Concern under Criterion B.

Criterion C – The total population of C. b. bicinctus has been placed in the band of 5,000–20,000 mature individuals for the New Zealand conservation status assessment (Robertson et al. 2017), which is the same band as the species has been placed in since Miskelly et al. (2008). No further information is provided on the evidence behind this figure, but Lane (1987) estimated the minimum number of individuals to be 12,450. The estimate of 50,000 individuals in Heather and Robertson (2005) does not indicate how the number was derived (Dowdling and Moore 2006), but it is also clear that the estimate by Lane (1987) is an absolute minimum, based on the maximum observed counts in New Zealand and Australia. Pierce (1999) considered that the population probably exceeded 50,000 individuals, from several separate lines of evidence including an assessment of the proportion of colour-ringed birds.  We have no recent population estimate for C. b. exilis, but it was estimated to number 730 birds in 1989 (Walker et al. 1991), which is roughly equivalent to 487 mature individuals. Taking the New Zealand conservation status assessment estimate for the nominate subspecies and adding that of C. b. exilis, we obtain a precautionary population estimate of 5,478-20,478 mature individuals, but an improved estimate is needed.This range of population size estimates could meet the population size thresholds for listing the species as Vulnerable or Least Concern under Criterion C. However, to list the species as threatened on the Red List under Criterion C further conditions must also be met.

We do not have population data from which to estimate the rate of decline, so the species cannot be currently assessed as threatened under criterion C1. Further information is required, but given the potentially high rate of decline and the small population size it may be precautionary to assess the species as Near Threatened under criterion C1 at present.

As described above, the species’s population has been predicted to undergo a reduction over three generations, although we do not have further information on the evidence base for the assessed reduction. The species can be inferred to be undergoing a continuing decline. The species has at least two subpopulations and potentially more. Subpopulations are defined as “geographically or otherwise distinct groups in the population between which there is little demographic or genetic exchange (typically one successful migrant individual or gamete per year or less)” (IUCN 2001, 2012). However, the largest breeding stronghold in Canterbury was estimated to hold 20,000 mature individuals (Ornithological Society of New Zealand 2010). Therefore, even taking into account declines, and the fact that the overall population is now estimated here to be smaller than this number, it is unlikely that the largest subpopulation has fewer than 1,000 mature individuals. Therefore, the species would not meet condition 2a(i). If the species is considered to have just two subpopulations representing the two subspecies, then between 91 and 98% of the species’ population may be found in a single subpopulation. This would not meet condition 2a(ii) at the level of Vulnerable. Considering that different parts of the species’s population have different migratory strategies, it is likely that there are in fact more than two subpopulations, which would mean that the percentage of mature individuals in the largest subpopulation would be lower than 91%. There is no evidence that the species’s population size is undergoing extreme fluctuations so the species does not meet condition 2b.

Although the species’s population size falls beneath the threshold for listing the species as threatened under Criterion C, and the species’s population is declining, none of conditions 2a(i), 2a(ii) or 2b are met. The species, therefore, qualifies as Near Threatened under Criterion C1.

Criterion D – Based on the population estimates described above, the species’s population size does not meet or approach the threshold for Vulnerable under Criterion D1. The species does not have a restricted Area of Occupancy of number of locations such that deforestation could drive the species to Critically Endangered or Extinct within a very short time. The species does not meet the criteria for listing as Vulnerable under Criterion D2. The species is therefore assessed as Least Concern under Criterion D.

Criterion E – To the best of our knowledge no quantitative assessment of the probability of extinction has been conducted for this species, and so it cannot be assessed against this criterion.

To allow us to achieve a clearer assessment of the species’s status, information is requested on population size and trends. In particular, we request information on the species’s population size and rate of decline since the 2000s, and into the future over a 15-year time scale.

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.

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

Dowding, J. E., & Moore, S. J. (2006). Habitat networks of indigenous shorebirds in New Zealand. Science & Technical Pub., Department of Conservation.

Heather, B.D.; Robertson, H.A. 2005. The field guide to the birds of New Zealand. 2nd edition. Penguin, Rosedale, Auckland.

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. Gland, Switzerland and Cambridge, UK: IUCN. Available at www.iucnredlist.org/technical-documents/categoriesand-criteria.

Keedwell, R. and Sanders, M.D. 1999. Australian Magpie preys on Banded Dotterel chicks. Notornis 46(4): 499–501.

Keedwell, R.J. & Sanders, M.D. (2002) Nest monitoring and predator visitation at nests of Banded Dotterels. Condor 104(4): 899–902.

Moon, G.J.H. (1992) Egg predation by Black-backed Gull. Notornis 39(2): 93.

Ornithological Society of New Zealand (2010). Checklist of the birds of New Zealand, Norfolk and Macquarie islands, and the Ross Dependency, Antarctica. Te Papa Press.

Pierce, R.J. 2013. Banded dotterel. In Miskelly, C.M. (ed.) New Zealand Birds Onlinewww.nzbirdsonline.org.nz

Rebergen, A., Keedwell, R., Moller, H., & Maloney, R. (1998). Breeding success and predation at nests of banded dotterel (Charadrius bicinctus) on braided riverbeds in the central South Island, New Zealand. New Zealand Journal of Ecology, 33-41.

Robertson, H. A., Baird, K., Dowding, J. E., Elliott, G. P., Hitchmough, R. A., Miskelly, C. M., McArthur, N., O’Donnell, C. F. J., Sagar, P. M., Scofield, R. P. and Taylor, G. A. (2017) Conservation status of New Zealand birds, 2016. New Zealand Threat Classification Series 19. Department of Conservation, Wellington.

Townsend, A. J., de Lange, P. J., Duffy, C. A. J., Miskelly, C. M., Molloy, J., Norton, D. A. (2007) New Zealand Threat Classification System manual. Department of Conservation, Wellington.

Walker, K., Moore, P. and Elliott, G. (1991) The Auckland Island banded dotterel has apparently increased. Notornis 38(4): 257-265.

Wiersma, P., Kirwan, G. M. and Boesman, P. (2019) Double-banded Plover (Charadrius bicinctus). 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/53844 on 9 April 2019).

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2 Responses to Double-banded Plover (Charadrius bicinctus): request for information.

  1. Hannah Wheatley (BirdLife) says:

    As part of next year’s comprehensive Red List update, we will be undertaking a revision of species’s generation lengths. Given that a change in generation length could affect this species’s Red List assessment, our proposal for the 2019 Red List is to pend the decision on this species and keep the discussion open until 2020, while leaving the current Red List category unchanged in the 2019 update.

    Final 2019 Red List categories will be published on the BirdLife and IUCN websites in December, following further checking of information relevant to the assessments by both BirdLife and IUCN.

  2. John Dowding says:

    The population range of 5000-20000 is from Hansen et al (2016), who estimated population size at 19000. This is probably total population size, so number of mature individuals will be somewhat lower.
    I don’t think there is any useful evidence for or against the existence of sub-populations within C. b. bicinctus. Different parts of the population do have different migration strategies (Pierce 1999), but these are not absolute (i.e. most but not ALL birds from an area migrate to the same wintering grounds). Gene-flow between regions may also be increased by movement of juveniles.
    There are no recent demographic data to allow calculation of generation time, but based on similar small-medium plovers in NZ, a range of 5-7 years seems likely.
    There is little doubt the population is in decline, but the rate of that decline is the big unknown. I understand work is currently under way to estimate this, but it may be biased towards the parts of the population breeding in braided river systems, where we have the most useful count series. This analysis will be available by 2020, so deferring a decision until next year makes sense.

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