Winter is a very different prospect for a Green Sandpiper that spends the non-breeding season in the UK, rather than Spain, Portugal or countries in northern Africa. Are British winterers living life ‘on the edge’ and how do they cope? One way is to defend resources within clearly-defined territories.
Life in Hertfordshire
Ken Smith, Mike Reed and Barry Trevis have been studying a colour-ringed population of Green Sandpipers in Hertfordshire (an English county just north of London) since 1983. The focal point of their activities is Lemsford Spring, a Hertfordshire & Middlesex Wildlife Trust nature reserve. The site is a set of disused watercress beds that provide plenty of freshwater shrimp (Gammarus pulex) in shallow lagoons fed by freshwater springs. Over the years they have been able to use their colour-ringed birds to study return rates from year to year, behaviour and local habitat use. Using radio-tags they have looked at nocturnal activity patterns and more recently they have been using GPS geolocators to find where their birds go to breed.
The highest numbers of Green Sandpipers in southern England occur in late summer, when many passage birds pass through the country, but a few birds stay for the whole winter. The Lemsford studies show that colour-ringed birds have a high return rate from one winter to the next. These birds are actually in the general area of Lemsford for most of the year, only leaving to breed for two or three months. Upon return, in late summer, they use other local sites such as gravel pits, only switching to the watercress beds in the autumn.
Individuals birds visit a suite of sites; by tracking birds wearing radio tags, the team were able to establish that even birds that fed at Lemsford Springs (below) during the daytime would roost communally at the local gravel pits overnight.
Effects of cold weather
The research team found that the patterns of behaviour of the Hertfordshire Green Sandpipers varied with local weather conditions. During extremely cold weather in January and February the birds switched to roosting overnight at the watercress beds, where they could also feed. In these conditions, with the water frozen at gravel pits, roost sites were accessible to predators. Automated monitoring of tag locations showed that birds were active for around 80% of each daytime period at all times of year. Nocturnal activity varied from around 16% in autumn to over 40% in cold conditions in midwinter. This suggests that a low level of night time activity is normal in Green Sandpipers and that higher levels, found during extremely cold winter conditions, are the result of birds attempting to increase their daily food intake.
The return-rate of colour-ringed birds can be used as a surrogate for survival rates, bearing in mind that any bird that changes its wintering area will reduce the apparent survival rate, although still alive. (See WaderTales blog on measuring survival rates based on review paper in IBIS)
Once established in the Hertfordshire area, individual Green Sandpipers were likely to turn up in subsequent winters, with little difference in return-rates between young birds, ringed in their first winter, and adults. There was considerable variation from year to year, with the return rates varying from 68.7 to 100%; the lowest being between 1986/87 and 1987/88, when another local winter site (Cole Green) became unsuitable. Birds using that site presumably went elsewhere or died. The average apparent annual survival rate was 83.5% (or 84.7% if account is taken of the disappearance of Cole Green). In the only other similar study, in northern Germany, the mean return rate was only 56%, with few birds returning in the year after a severe winter. In the Hertfordshire study, there is a correlation between the overall return rate of colour ringed birds from one winter to the next and the number of nights with air frost in the first winter (r = -0.86, n = 7, P < 0.05). These results suggest that birds suffer increased mortality in severe winters.
One of the interesting things that became apparent, once individuals could be identified using colour-rings, was that some of the Hertfordshire Green Sandpiper were highly territorial, with similar patterns of behaviour each year.
When Green Sandpipers appear in late summer they are usually found in small flocks, which include the colour-ringed full-grown adults and young birds of the year. At some point in the autumn, possibly triggered by low temperature or shortening day-length, the behaviour suddenly changes, with some of the birds establishing defended territories on the lagoons, from which they chase all interlopers. This initial burst of territoriality has been captured in the excellent images above and below this paragraph. Once the territories are established they are maintained by subtle calling displays, with only occasional full-on disputes.
During periods of very cold weather, additional birds arrived from other local sites and, for a short period in January 1984, the number of birds present on the reserve more than doubled (to a total of 16). The newly-arrived birds attempted to establish their own territories and devoted up to half of their time to boundary disputes, far in excess of that of the residents. When the weather conditions improved the additional birds left the reserve.
During the autumn and early winter of 1985, the research team attempted to investigate territoriality through activity watches of birds present on the lagoon at Lemsford. Every minute, each bird was scored as feeding, resting or disputing. Most watches were over at least one hour and all were completed during the period October to December. On the graph, the solid line and points are the counts of the numbers of birds, and the open symbols are the percentage of time that birds spent in dispute.
From early October, which corresponds to the end of the autumn moult, between 6 and 11 birds were present at Lemsford, feeding together as a group with little or no interaction. Around mid-October, the number of disputes started to increase. It varied in intensity from day to day but, over a few weeks, the numbers of birds present fell to four or five. These birds took ownership of exclusive territories that were largely maintained for the whole of the winter. Ken Smith was able to find all of the missing Green Sandpipers within 15 km of Lemsford. Some of these individuals would occasionally pop back to Lemsford, get involved in a fight and leave again. In extreme weather interlopers can spend the whole day trying to feed on the lagoons and being chased off.
From observations at Lemsford, it appears that, once territories have been established, local birds are well aware of their boundaries and neighbours. Should a bird stray into a neighbouring bird’s patch, a low-key call is sufficient warning to avoid a disagreement. Ken reports that the big disputes, with chasing and out-and-out aggression, only occur when the territories are being established or when a new bird tries to feed in an established territory. During extremely cold weather, in the 1980s, there would be lots of extra birds attempting to feed in the spring-fed watercress beds, by trying to squeeze into perceived gaps between territories. Such cold conditions are very rare nowadays.
A changing climate
Over the three decades of the project, there has been a tendency for cold winter temperatures to become less frequent across Britain & Ireland. During this time, the distribution of Green Sandpipers has expanded northwards, as you can see in the figure below.
The left-hand and central maps show the winter distributions of Green Sandpiper in the periods of the two winter atlases of 1981-84 and 2007-11, respectively. The surveys underpinning these maps were organised by the British Trust for Ornithology with BirdWatch Ireland and the Scottish Ornithologists’ Club. The expansion of the range represents a 56% increase in occupied 10-km squares, with an obvious extension into Scotland and across northern England, where winter temperatures are colder. Data collected for Bird Atlas 2007-11 included measures of effort, enabling relative abundance to be quantified. In the right-hand map, darker squares represent areas with higher numbers of Green Sandpipers. The Lemsford site is marked with a star.
As, mentioned earlier, there is an indication that annual return rates to Lemsford (and in the German study) relate to the conditions birds experience in the winter period prior to departure for their breeding grounds. In Scotland and Northern England, where freezing conditions occur more frequently, it might be expected that survival rates will be lower than in southern England. For winter populations to establish themselves in new (colder) sites, young birds will need to spend their first winters in these areas and then return in subsequent years. Local population can potentially grow unless or until knocked back by a cold winter.
Where to in spring?
In recent years, the research team have deployed harness-mounted geolocators to ascertain the movements of a small number of individuals during the course of a full annual cycle. Another WaderTales blog discusses migration and the effects of harnesses on the behaviour of individuals. See Green Sandpipers & Geolocators.
Summary and papers
This thirty-year study is an excellent example of what can be discovered through long-term observations of a small population of waders. You can read more about the Hertfordshire Green Sandpiper research in these papers (links in bold):
- K W Smith, J M Reed & B E Trevis (1984) Studies of green sandpipers wintering in southern England. Wader Study Group Bull, 42.
- K W Smith (1986) Green Sandpiper. In The Atlas of Wintering Birds in Britain and Ireland, pp 222-223, Poyser, Carlton.
- K W Smith, J M Reed & B E Trevis (1992) Habitat use and site fidelity of green sandpipers Tringa ochropus wintering in southern England. Bird Study, 39, 155-164.
- Smith, K W, Reed, J M & Trevis, B E (1999) Nocturnal and diurnal activity patterns and roosting sites of Green Sandpipers Tringa ochropus wintering in southern England. Ringing & Migration, 19, 315-322.
- Smith K W (2002) Green sandpiper in The Migration Atlas: movements of the birds of Britain and Ireland. Wernham C V, Toms M P, Marchant J H, Clark J A, Siriwardena G M & Baillie S R. (eds). T & A D Poyser, London.
- Smith, K.W., Trevis, B.E. & Reed, M. (2018). The effects of leg-loop harnesses and geolocators on the diurnal activity patterns of Green Sandpipers Tringa ochropus in winter. Ringing & Migration 32: 104-109.
Graham (@grahamfappleton) has studied waders for over 40 years and is currently involved in wader research in the UK and in Iceland. He was Director of Communications at The British Trust for Ornithology until 2013 and is now a freelance writer and broadcaster.