Chasing ghosts: how technology is helping track the bird that mysteriously disappeared
The Slender-billed Curlew hasn’t been seen since 1995, and could very well be extinct. But before we write it off for sure, we need to scour its vast, inhospitable breeding range for straggling populations. A groundbreaking new technique, which studies tiny atoms left in the feathers of long-dead specimens, is telling us where we should look first.
How do you look for a Critically Endangered species’ final few nesting sites, when you were never really sure where they bred in the first place?
That's the magnitude of the task facing conservationists who are attempting to chase the tail feathers of the world's final remaining Slender-billed Curlew Numenius tenuirostris population. That is of course, if any such population even still exists at all.
In an attempt to narrow the search for this lost species, a new paper published by BirdLife’s journal, Bird Conservation International, involving staff from, or linked to, the RSPB (BirdLife in the UK), has used data gleaned from tiny atoms, harvested from the feathers of deceased specimens, to pinpoint where in the vast Siberian wilderness we should begin our search.
How do you look for a possibly extinct species, when you didn't even know where it bred when it was plentiful?
We didn’t always need to resort to such elaborate measures to catch a glimpse of this medium-sized wader. At the beginning of the 19th Century, it was a somewhat common bird that wintered all across the Middle East, North Africa and central and eastern Europe.
But even in these bountiful times, the species’ breeding habits were poorly understood. We knew they retreated to remote Central Asia in Spring, but not much more beyond that. To date, the only fully-documented Slender-billed Curlew nests are a handful that were discovered in the 1910s and 1920s, near the town of Tara in Omsk, Siberia.
Also poorly understood are the exact reasons for its rapid decline, although we can make a few educated guesses. Widespread hunting across its wintering grounds in the late 19th and early 20th Century had a noticable impact, and the extensive drainage of wetlands across the Mediterranean and North Africa only served to put further pressure on this migratory species. However, the threats the species faces across its breeding grounds, wherever they may be, are largely unknown.
Either way, eventually things got so dire that the Slender-billed Curlew stopped appearing at its wintering grounds altogether. The last fully-verified sighting was in Morocco in February 1995, and although there have since been claimed sightings in places as far apart as France and Ukraine, the species’ visual similarity to more common birds such as Eurasian Curlew Numenius arquata and Whimbrel Numenius phaeopus means they are difficult to verify. See the video below for a comparison, and to watch the only known footage of a Slender-billed Curlew:
It's possible that as numbers began to thin, Slender-billed Curlews, being gregarious birds, joined flocks of these similar species, making them almost impossible to spot. Equally as likely, the Slender-billed Curlew has died out altogether, in the process becoming the first bird extinction in Europe since the Great Auk Pinguinus impennis was hunted into the history books in 1852.
But to know for sure, we need to scour its vast potential breeding ranges for straggling subpopulations. In any case, even if we were unsuccessful in finding any Slender-billed Curlews, any knowledge we could gain about its breeding habits, and the factors that contributed to its extinction, could be helpful in our efforts to protect other species of waders from suffering the same fate.
The study involved the analysis of stable isotopes harvested from feathers of 35 Slender-billed Curlew specimens kindly donated from all around the world. These atoms are tiny traces of the environment (such as food or water) that are transferred into body tissues when the bird consumes them, and subsequently becomes a part of the bird's feathers as they are formed. By studying the isotopes from juvenile specimens – that is to say, young birds at the breeding site preparing to embark on their first migration – staff at Iso-Analytical, a laboratory specialising in isotope analysis, were able to calibrate their findings with existing large-scale isotope ratio maps from across the world, allowing the team to discover where these young birds first grew their feathers.
The team was able to discern the birds' origins by studying hydrogen atoms within the isotopes. "Heavy water differs from normal water in that the hydrogen element has an extra neutron" explains Dr Graeme Buchanan, Principal Conservation Scientist, RSPB, and one of the authors of the paper. "As you move further away from the equator, the ratio of heavy water to normal water decreases; it’s this established ratio that we used to place the species into a specific band of latitude”.
The potential breeding areas of Slender-billed Curlew, This is based on the number of juvenile birds assigned to cells based on δ2H values from feather samples in comparison to an isoscape (Buchanan et al 2017). Stars represent locations of sightings of birds in the study region between May and July, while crosses indicate birds shot in these months. Broken lines indicate boundaries of ecoregions from Olsen et al. (2001). A filled circle indicates the only known nesting site.
The results of the study proved what had long been suspected: that we were looking in the wrong place all this time. It appears that the Slender-billed Curlew’s traditional breeding stronghold is (or indeed, was) not in Omsk, where the only known nests were found, but further south, in the steppes and forest-steppes of Kazakhstan and southern Russia. Another area, in the north of the Ural Mountains, was identified as a likely source for some of the sampled birds. "This suggests there is some degree of specialisation in the Slender-billed Curlew's breeding habits", says Buchanan. "Previously it was thought of as a forest-steppe species, but the findings suggest that it is more of a steppe environment species".
The study confirmed what many conservationists had long believed: that we had been looking in the wrong place for the bird all along
While the findings came as a surprise, they do back up long-held beliefs in some conservation circles. “It has been suggested in the past that the breeding range was further south. In the late nineties, a group from Glasgow University went on an expedition to this area to look for the Slender-billed Curlew – but didn’t find it" says Buchanan. "But while people have speculated it for a long time, but this is the first evidence that this is where we should look.”
By no means should we be discouraged by the failed expedition - while we have managed to pin down a latitude, the potential breeding area still covers a huge swathe of land, and the prospect of locating the breeding site of a bird that probably numbers less than 50 seems, ahem, slender at best. But the Eurasian steppes have previous form for hiding birds from view. In 2000, a field survey in the area uncovered a far larger population of Sociable Lapwing Vanellus gregarius than the previously-estimated worldwide figure of 200-600.
But is it realistic to expect to find any Slender-billed Curlews? After all, the conditions are too inhospitable for the birds to stay there all year round. They would still need to migrate, and as we know, there have been no fully-documented sightings since 1995. But Buchanan suggests that the species could very well still be making the trips south. “The species does have an extensive wintering range which includes parts of north Africa, which almost certainly haven’t been surveyed in great detail. So it remains a possibility that it is still migrating to the Mediterranean Basin, but the population is so small, and going to such a little-studied area, that we don’t know about it.”
If we're planning a repeat trip to find the Slender-billed Curlew, however, time is of the essence. It was previously thought that habitat loss across its core breeding area was not a major factor in the Slender-billed Curlew's slide towards extinction, but the paper's findings mean we have to rethink that assumption. A considerable amount of steppe areas in northern Kazakhstan have been ploughed for use in agriculture, leaving only the areas with poorer soil quality. This change in land use has been linked with the decline of numerous steppe species in the region, and it seems we can add the Slender-billed Curlew to that list.
Nonetheless, we can't give up hope that the Slender-billed Curlew persists. And if the 22 years that have passed since the last confirmed sighting feels like too long, then take heart from the story of the Steppe Whimbrel Numenius phaepous alboaxillaris, a sub-species of Whimbrel that was thought extinct in 1994, until sightings in Russia and, more recently, Mozambique. In the search for extinct species, hope springs eternal - no matter how slender.
You can help in the search. Anyone birdwatching across its range who suspects they are looking at a potential Slender-billed Curlew should take detailed notes and photographs. These will be essential to assess the veracity of the record. They should contact the RSPB's Slender-billed Curlew working group without delay (email firstname.lastname@example.org). Further information on Slender-billed Curlews, including a downloadable identification leaflet can be found at www.slenderbilledcurlew.net.