The level of threat facing a shorebird species is assessed using a set of criteria that tries to answer three key questions: How many breeding pairs are there? How quickly are numbers falling? and How restricted is the distribution? In a paper published in Ibis at the end of 2021, Birgita Hansen and colleagues produced population estimates for 37 species of shorebird on the East Asian-Australasian Flyway, for which the non-breeding distribution includes Australia and New Zealand. This new study shows that there are at least nine million shorebirds on this flyway. How many more would there have been ten, twenty or fifty years ago?
This is a good time to attempt to assess population sizes of shorebird species on the East Asian-Australasian Flyway because the quality of the data available is so much better than it was even ten years ago. Increased cooperation across borders, fostered through individual efforts by concerned birdwatchers, by conservation NGOs and through bodies such as the East Asian-Australasian Flyway Partnership, has delivered reliable counts of shorebirds in previously inaccessible areas, especially around the Yellow Sea.
The East Asian-Australasian Flyway covers an area of 85 million square kilometres, from Russia and Alaska, in the north, through to Australia and New Zealand in the south. Birgita Hansen and colleagues have revised the population estimates for 37 migratory shorebird species protected under Australian national environmental legislation. Population estimates were generated by:
- Summarising existing count data in the non-breeding range
- Extrapolating the data to try to include the numbers of birds using uncounted areas and
- Modelling abundance on the basis of estimates of breeding range and density.
This was not just a number-crunching exercise; expert opinion was sought, especially to refine estimates for species with low data quality.
The main count data available to the authors came from the Australian National Shorebird Monitoring program (formerly Shorebirds 2020), the Queensland Wader Study Group, the Ornithological Society of New Zealand, and the Asian Waterbird Census. Two major strengths of the paper are a clear explanation of the methods used to create the population estimates and open consideration of the quality of the estimates. The authors explain how it is possible to produce population estimates in areas where there are significant gaps in the data. These features will be important to teams making future assessments of numbers.
The Hansen paper builds on previous estimates by Bamford et al, in a paper published in 2008, that used data collected between 1986 and 2000, and through an expert-assessment process (Wetlands International 2012). However, the various estimates have been derived in different ways and cannot directly be compared.
How many shorebirds?
Species that form coastal flocks in a relatively well-established set of sites are easier to census than others that are spread thinly or across a range of habitats. Flyway counts of Curlew Sandpiper are thought to include almost all of the sites used by the population, so a total of 85,086 is adjusted to an estimate of 90,000 to account for gaps. For Latham’s Snipe, the flyway estimate is 35,000, based on the breeding range and density estimates, but actual flyway counts only total 1124. For Curlew Sandpipers, non-breeding season numbers appear to be reliable indicators of population size but for Latham’s Snipe breeding season estimates are more realistic. Variability in data quality is not unusual; the British & Irish estimates of winter numbers of Common Snipe, Jack Snipe, Eurasian Woodcock, Golden Plover and Northern Lapwing are similarly imprecise, as discussed in Do population estimates matter? and Ireland’s wintering waders.
Estimates of population size obtained through analyses of breeding ranges and density indicated that non-breeding counts for 18 species in the Hansen study were relatively uncertain. Breeding ground estimates were considered the best available data for ten species that mostly use poorly-surveyed freshwater and marine habitats, especially in south-east Asia, or are thinly distributed on the open coastline in the non-breeding season. The WaderTales blog Waders on the coast reflects on the latter issue for UK waders.
Birds at risk
Prioritisation of conservation action for shorebirds is underpinned by the threat levels assigned to them. This means that it is probably easier to access funding for Critically Endangered Spoon-billed Sandpipers than for Near Threatened Curlew Sandpipers. There are three categories indicating imminent risk of extinction in the IUCN Red List – Critically Endangered, Endangered and Vulnerable. The definition of each level is set out on this page from BirdLife.
Several of the wader species on the East Asian-Australasian Flyway that are most at risk, such as Nordmann’s (or Spotted) Greenshank and Spoon-billed Sandpiper, are not included in the review by Hansen et al. because they do not generally migrate as far south as Australia (but there is a WaderTales blog called Spoon-billed Sandpiper: track and trace).
Two species that winter in Australia in significant numbers are categorized as Endangered by IUCN – Far Eastern Curlew and Great Knot. When comparing the Hansen estimates (mainly based on data from 2011 to 2016) with Bamford et al (data from 1986 to 2000), the population of Far Eastern Curlew is now assessed to be lower (35,000, as opposed to 38,000), while the population of Great Knot is now thought to be bigger than previously realised (425,000, as opposed to 375,000). This does not mean that numbers have gone up – we know that annual counts at key sites have fallen significantly – it indicates a more comprehensive estimate of numbers, particularly in uncounted areas.
Indeed, as shown by Colin Studds et al, summarised in Wader declines in the Yellow Sea, both Far Eastern Curlew and Great Knot numbers have been declining at an alarming rate for two decades. The fact that the drop in Far Eastern Curlew numbers is less striking than might have been expected and that there is an apparent increase in Great Knot numbers reflects more comprehensive counts from what are now known to be important sites along the flyway. The increased figures do not mean that the two species are any less threatened!
Seven species considered in the Hansen paper are listed as Near Threatened. Estimates of Grey-tailed Tattler and Red-necked Stint have been raised, from 50,000 to 70,000 for the tattler and from 325,000 to 475,000 for the stint. There is no change in the estimate of Bar-tailed Godwits (325,000), despite significant declines in numbers in Australia, again reflecting the discovery of significant numbers in sites where they were previously unrecorded or under-recorded. The Black-tailed Godwit estimate remains at 160,000.
The estimates of two Near Threatened species have dropped alarmingly, despite more comprehensive information on the sites that they use. Red Knot and Curlew Sandpiper numbers have halved (220,000 down to 110,000 and 180,000 down to 90,000, respectively).
The figures in the Hansen paper are based on the best available information for the period up until 2016. The welcome, increased focus upon shorebird conservation issues throughout the flyway is uncovering new and better count data. Birgita shared the following cautionary tale about the Near Threatened Asian Dowitcher, the population estimate of which is given as 14,000 in the paper, somewhat lower than the estimate of 24,000 that was published by Bamford et al in 2008.
“In 2019 there was a count of 22,432 Asian Dowitchers at Jiangsu Lianyungang, on the Chinese coast, which was interpreted as being 97.5% of the global population at the time (based on Bamford). This figure well exceeded our 2016 estimate of 14,000, clearly indicating that, despite access to greater volumes of data through the Asian Waterbird Census, there was a substantial number of dowitchers that had been missed in standard monitoring. So we can see already how, even with the extrapolations we used, the numbers can be highly uncertain.”
Population estimates are only estimates – the clue is in the name – but it is important to work out, as best as possible, how many birds there are, in order to assess the vulnerability of individual species and the importance of the sites that are used by assemblages of shorebirds. When 1110 Curlew Sandpipers were counted at Broome (Western Australia) in 2021, that was over 1% of the reduced flyway total of just 90,000 – and that’s important. Similarly, 4674 Bar-tailed Godwits in Tasman Bay (New Zealand) in 2021 exceeds the 1% threshold for the species and an amazing count of 9810 Sharp-tailed Sandpipers at Lake Martin (Victoria) in 2019 represents 11.5% of the 85,000 estimate in the Hansen paper. There are many more examples.
This population information is complementary to regular counts at key sites, that track population trends, and colour-ring observations that monitor annual survival rates of adults (there is a WaderTales blog about this – Measuring Shorebird Survival).
In the Ibis paper, Birgita Hansen and her colleagues discuss the rationale and limitations of the approaches they have used to obtain population estimates and how their methods could be used in other situations. Data available for population estimates will always vary in quality and extent among species, regions and migration stage, and approaches need to be flexible enough to provide relevant information for conservation policy and planning. Anyone considering this sort of exercise would be advised to read the whole paper:
Generating population estimates for migratory shorebird species in the world’s largest flyway. Hansen, B.D., Rogers, D.I., Watkins, D., Weller, D.R., Clemens, R.S., Newman, M., Woehler, E.J., Mundkur, T. and Fuller, R.A. Ibis. DOI/10.1111/ibi.13042
WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.