In the days of big data sets and complex analyses, it is pleasing that local, focused studies can still answer specific gaps in our understanding of migratory behaviour. Ron Summers and his colleagues have responded to a comment by Patrick Thompson, at the end of his contribution to The Migration Atlas, published by BTO in 2002: “… it is of major concern that we know so little about where Scottish birds [Greenshank] go once they leave the breeding grounds.” With a small number of geolocators and a few dozen plastic colour-rings, they are now able to answer this implied question, in a 2020 paper in Bird Study.
On the edge of range
Greenshank (or Common Greenshank) breed across the boreal zone of the Palearctic, from eastern Siberia in the east to Scotland in the west. The map below, produced by BirdLife International clearly shows that Scottish Greenshank lie at the southern and western extremes of this distribution. The species is even more widely distributed outside the breeding season; you can hear their three-note tew-tew-tew calls from New Zealand to the west of Ireland.
Scotland holds a small population of Greenshank, estimated at 1440 pairs in 1995 (Hancock et al. 1997) but revised to be 1100+ in the latest population estimate in British Birds. This is a tiny part of a global population estimated as between 440,000 and 1.5 million individuals (Wetlands International). The lack of precision implicit in this large range is unsurprising; Greenshank tend to occur in small numbers in most of their wintering locations, not conveniently collecting in large flocks, such as Redshank or Knot.
The northwest corner of mainland Scotland, together with the northern isles of the Outer Hebrides, are the main focus of the UK distribution, as is clear in the map showing relative abundance, taken from the BTO’s MapStore, based on Bird Atlas 2007-11 (BTO, BirdWatch Ireland & SOC).
The Greenshank is far more widely spread across the British Isles in the winter. The three maps below, also based on data from Bird Atlas 2007-11 show a growth in the number of inland and east-coast sightings between 1981-84 (left) and 2007-11 (middle), with the highest densities in west coast sites in Ireland and Scotland.
The Greenshank of Tongue
The sound of a calling Greenshank takes me back to an Easter holiday that involved cycling into the wind (and odd snow flurry) along the A roads of the north coast of Scotland. These elegant waders are very much at home in this terrain of bogs, moorland and rocky outcrops. The study area for the project at the heart of this blog was near Tongue, two-thirds of the way from John o’ Groats, in the east, to Cape Wrath, in the west.
Between 2010 and 2015, twenty-four breeding Greenshank were trapped at the nest, by laying a mist-net over them. All were colour-ringed (most uniquely) and geolocators were attached to twenty of them. In most cases, feather samples were taken to determine gender. As many as possible of the geolocators were retrieved when the birds returned to breed, by again catching birds on their nests but one was recovered by a French hunter and another when a bird was recaptured in North Wales.
Stories from colour-rings
Prior to this study, the only ringing recoveries of Scottish-breeding birds refer to a chick ringed at Forsinard (Sutherland) on 6 June 1926 and shot in Ireland four months later, and a chick ringed in Perthshire in June 1974 and shot in northwest France that September. As both of these were autumn records, the young birds could still have been on migration when shot. The first definite winter location was established when an adult colour-ringed in Sutherland in 2010 was found to winter in Essex (eastern England).
Ten of the colour-ringed birds from Tongue have been seen away from the breeding grounds: two in Scotland, five in Ireland and one each in Wales, northwest England, southeast England and southwest France. These movements represent migrations of between 530 km and 1560 km, which must seem unimpressive to a Greenshank that flies 10,000 km to reach southern Australia. Scottish colour-ringed birds have been seen on staging areas but once they arrive in their wintering areas in late June and July, they don’t move again until the following March or April.
In the case of one pair of Greenshank, both male and female were ringed and resighted. The male was in Wales and the female in France. If you have read Black-tailed Godwits: the importance of synchrony, this will come as no surprise!
Additional data from geolocators
Colour-rings cannot really establish whether migration is non-stop or staged, just because sighting probability is low. By using geolocators, which collect data on the timing of dusk and dawn, it is possible to work out where individuals go between one year and the next – as long as you can re-catch individuals wearing geolocators! Seven geolocators were recovered (five by recapture of breeding birds, one shot in France and one recaptured at its Welsh wintering site – see image alongside). One geolocator did not work but there was sufficient information from the other geolocators to show that:
- Geolocations for four birds indicated that they spent the winter in Ireland (but geolocators do not give precise information and one of these birds was actually seen (and subsequently caught) in North Wales)
- One bird spent the winter in France (proved by being resighted and later shot there)
- One tagged bird was recorded as being in Ireland in September, after which data collection ceased
- Median departure date from northern Scotland was 16 July
- Median arrival date in final wintering locations was 17 July
- One Irish-wintering bird staged for two days in southwest Scotland
- The French wintering bird appeared to spend a day in southern Ireland on its way south and to stop off in northern France on its way north.
Context and conclusions
Although the Summers et al Greenshank study only involves a small number of birds, it seems that Scottish birds do not migrate far, with none seen outwith Ireland, Britain or France. Further, the data from the geolocators, which do not rely on birdwatchers spotting the birds, provided no evidence of birds going further south than France.
Although the migration distances were relatively short, there was some staging during southward and northward migrations. For most birds, there was no staging and they could have accomplished the migration distance to Ireland or Wales (500-700 km) in about 10 hours, if flying at 60 km per hour (Alerstam et al. 2007).
The size of the wintering populations of Greenshank in Ireland, Great Britain and France are estimated as 1,265, 810 and 200, giving a total population of about 2,300 in these countries. The breeding population in Scotland in 1995 was estimated as being between 1,100 and 1,790 pairs. Adding in chicks might mean that 4,000 birds are on the move each autumn. Although this value is higher than the numbers counted in Ireland, Britain and France in winter, the authors suggest that the two values are not so far apart as to discount the conclusion that the majority of birds from these wintering populations are derived from the Scottish-breeding population.
Winter visitors and passage migrants
The map alongside shows the movements of ringed Greenshank to and from Britain & Ireland, as extracted from the BTO’s online ringing report on 20 March 2020. There is evidence of the classic leap-frog migration, with individuals that breed further north spending the winter months further south. British breeding Greenshank may not move very far but many birds from Norway, Sweden and Russia spend time in Britain on their way to western African countries such as Ghana and Ivory Coast. Spring records in Italy suggest some birds might take a more easterly route on their way north.
Data from a UK-based colour-ring and tagging programme, coordinated by Pete Potts of Farlington Ringing Group, are being analysed. Initial findings suggest that a single estuary can host birds from a range of breeding locations. Some passage birds stay to complete moult but others just fatten and move on. We should soon know more about the relative importance of these British stop-over sites to western European Greenshanks.
To read the paper
The full story is available in Bird Study:
Scottish-breeding Greenshanks Tringa nebularia do not migrate far. Ron Summers, Nick Christian, Brian Etheridge, Stuart Rae, Ian Cleasby and Snæbjörn Pálsson. Bird Study. March 2020.
The study was a Highland Ringing Group project, supported by the Scottish Ornithologists’ Club Endowment Fund.
More about migration
If you found this blog interesting, here’s a selection from the WaderTales catalogue that may also appeal. The first two articles reveal the results of a local study of Green Sandpipers that spend the winter in Hertfordshire.
In the next two blogs, you can read how migration patterns vary in Oystercatchers, depending upon how far north birds breed, the locality in which they nest and the obstacles that lie in the way if they undertake migratory journeys.
- Mission impossible: counting Iceland’s wintering Oystercatchers
- Which Oystercatchers cross the Atlantic?
And this blog is all about ‘happenstance’, the seemingly random processes that presumably determine why Sanderling from the same small part of Greenland may fly as far as South Africa or just to Scotland:
WaderTales blogs are written by Graham Appleton, to celebrate waders and wader research. Many of the articles are based on previously published papers, with the aim of making wader science available to a broader audience.