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WINTERING SNOW BUNTINGS IN CAITHNESS
Several bird species come to spend the winter in Britain; and Caithness gets its share of these winter visitors. Some of the species involved are Redwings from Fennoscandia and Iceland, Fieldfares from Scandinavia, Whooper Swans from Iceland, and Waxwings from Fennoscandia and Western Siberia. They invariably come in search of food which, because of a milder climate, is more plentiful in Britain than in their summer breeding areas.
One species, not mentioned already, which visits Caithness in winter is the Snow Bunting Plectrophenax nivalis. This species has been of particular interest to me over the last few years, and has been the subject of many hours of study, all aimed principally at discovering where the birds cam from. It is well known that Snow Buntings breed in arctic Greenland, Iceland and Scandinavia but from which area do our winter visitors come?
The Snow Bunting is a small sparrow-sized bunting with a short stubby bill. Basically sandy brown in colour, the males are spectacular when they fly, as they show huge patches of white on their wings; hence some writers refer to them as 'Snowflakes". The female Snow Buntings are much less colourful, being largely sandy brown with only small patches of white in their wings.
In winter, the Snow Buntings rely very much for their food on the seeds of weeds, heathers and grasses or cereal grains from stubble fields, but in their summer breeding areas they prefer to eat insects and other invertebrates, and feed these to their young. There is a small breeding population of Snow Buntings based on the Cairngorms of Scotland but the number of pairs involved seldom enters double figures (Thom, 1986); thus the majority of Snow Buntings occurring in Britain are winter visitors.
Occurrence and Abundance
The most important winter areas in Britain for Snow Buntings are Shetland, Orkney, the east coast from Caithness to Kent and the Cairngorms. (Lack, 1986)
Snow Buntings generally spend around five months each winter in Caithness, from November to March, although a few early birds arrive in September and a few stragglers stay on into April. Local birdwatchers, members of the Caithness branch of the Scottish Ornithologists' Club, have been recording the numbers and locations of flocks of Snow Buntings in Caithness for many years, and their records for the years 1976-1988 allow some very interesting estimates of Snow Bunting numbers to be made. Estimates made by Banks et al. (Scottish Birds, 1991) show that Snow Bunting numbers in Caithness have varied greatly from year to year; numbers peaked in the winter of '78/'79 with just under 2,000 Snow Buntings present, and an even greater peak occurred in the winter of '87/'88 when approximately 2,500 birds were present. The other winters between 1976 and 1988 had much fewer birds with perhaps as few as 100 birds around in some winters. An average winter population of 1,100 birds was calculated; this value demonstrates the importance of Caithness as a wintering area for Snow Buntings since this figure represents 7-11% of the British wintering population, given by Lack (1976) as 10,000 - 15,000 birds.
Snow Buntings are social birds and tend to flock together throughout the winter as they forage for food. In Caithness, flock sizes usually range from 10 to 300 birds, and depend to some extent on whether the winter is one of abundance or shortage. Caithness birdwatchers have established that there are certain general areas of the county which are favourite Snow Bunting haunts; these favoured areas are centered on Reay, Thurso, Loch Calder, Dunnet Bay and Keiss links and have an abundance of either stubble fields or grassy sand dunes (Banks et al, Scottish Birds, 1991).
The outstanding question in relation to these wintering Snow Buntings is of course, "Where do they come from?". This question has been largely answered as the result of work which I and a few colleagues carried out in Caithness in the winters between January 1986 and March 1990 (Banks et al, Bird Study, 1991). During this study, a site in the sand dunes at Keiss links (58 31'N; 3 3'W) was baited with grain every winter and another site at Dunnet bay (58 35'N; 3 21'W) was baited in winters '86/'87 and '87/'88. Once Snow Buntings had become accustomed to feeding at these sites, the birds were trapped periodically by having small elastic-propelled nets fired over them. Thus, 2,227 Snow Buntings were caught and ringed with rings supplied by the British Trust for Ornithology; in addition 14 birds which had been ringed outside Caithness were also trapped. When some of these Caithness-ringed birds were captured by foreign ringers in other countries then the origins of our winter visitors began to be revealed. While handling all these Snow Buntings caught in Caithness we were able to make a detailed examination of their plumage, and this too helped to shed some light on the origins of the birds.
A previous writer, Williamson in 1966, in the well known book "The Snow Bunting" by D. Nethersole-Thompson, had proposed SW Greenland and Scandinavia as the most likely origins of Snow Buntings wintering in Britain; but Williamson had very little evidence at his disposal in those days. He had also argued that Snow Buntings breeding in Iceland were largely sedentary and seldom visited Britain.
Another ornithologist, Salomonson, as far back as 1931 had described slight variations in plumage among Snow Buntings related to their different breeding areas e.g. the males of Snow Buntings breeding in SW Greenland and Scandinavia have whitish rumps whereas the males of Snow Buntings breeding in Iceland have black rumps. While these differences are very obvious in summer when the birds are in breeding plumage, they are far more difficult to see in winter when the tips of other feathers tend to conceal the "tell-tale" white or black rump feathers; indeed these, and other plumage differences, especially in females, are only observable in winter when the birds are captured and examined in the hand. Plumage differences, such as those very briefly described, cause ornithologists to split Snow Buntings into different sub species or races; birds from SW Greenland and Scandinavia are the sub species Plectrophenax nivalis nivalis and Snow Buntings from Iceland are the sub species Plectrophenax nivalis insulae.
So, are the Snow Buntings which winter in Caithness of the race Plectrophenax nivalis nivalis from SV Greenland and/or Scandinavia, or are they of the race Plectrophenax nivalis insulae from Iceland?
The re-capture in Iceland in spring of 8 Snow Buntings ringed earlier at Keiss links in winter, and the capture at Keiss links in winter of 3 Snow Buntings ringed as freshly hatched birds in Iceland, established beyond doubt that Iceland was the main source of wintering Snow Buntings in Caithness; and, since several Snow Buntings caught in Caithness had also been caught further south in Britain (in Grampian region and in Cleveland in England), it is probable that birds from Iceland moving through Caithness account for a large proportion of Snow Buntings wintering throughout Britain.
Our detailed examination of the plumages of Snow Buntings in the hand showed, in fact, that around 80% of the birds were of the race Plectrophenax nivalis insulae from Iceland, and the other 20% were of the race Plectrophenax nivalis nivalis from either SW Greenland or Scandinavia.
Other evidence accumulated in the course of our work in relation to the amount of extra weight or fat put an by the birds in preparation for their long return flights to their breeding grounds at the end of March, suggested that Snow Buntings leaving Caithness for their northerly migration were not putting on enough fat reserves to take then to SW Greenland. The Snow Buntings of the sub species Plectrophenax nivalis insulae heading for Iceland were putting on more weight and fat than the birds of the Plectraphenax nivalis nivalis race. So the latter must have been preparing for a journey shorter than that to Iceland; SW Greenland is thus ruled out and Scandinavia becomes the most likely destination (Banks et al, 1989).
It has thus been established that around 80% of the Snow Buntings over- wintering in Caithness come from Iceland and the remainder from Scandinavia. The variation in numbers coming to Caithness each year seems to depend on events in Iceland. Most Snow Buntings breeding in Iceland remain there for the winter (Breuil 1989), and the numbers which migrate south represent only a small proportion of Iceland's breeding population. However, in some years the numbers moving south are much greater than others, and so we get our peak years in Caithness; but what determines the numbers moving out is unknown. It may be that after exceptionally good breeding seasons the number of birds is so great that the food supply in Iceland is insufficient for their needs, and so more birds move south in search of food, coming down through Shetland and Orkney into Caithness and beyond, as our ringing studies have shown. That some individuals migrate in some years and not others is demonstrated by an individual Snow Bunting which was caught at Keiss links in January '86 and again the following winter in December '86, but then in a later winter was caught in Iceland in January 1990 years - some years in Caithness, others in Iceland.
Banks,K.W., Clark,H., Mackay,I.R.K., Mackay,S.G. and Sellars,R.M. 1989. Biometrics and premigratory fattening in the Snow Bunting Plectrophenax nivalis. Ringing and Migration, l9: 141-157.
Banks,K.W., Clark,H., Mackay,I.R.K., Mackay,S.G. and Sellars,R.M. 1991. Origins, population structure and movements of Snow Buntings Plectrophenax nivalis. wintering in Highland Region, Scotland. Bird study, 38: 10-19.
Banks,K.W., Clark,H., Mackay,I.R.K., Mackay,S.G. and Sellars,R.M. 1991. Snow Buntings in Caithness. Scottish Birds, 16: 57-65.
Breuil,M. 1989. Les Oiseaux d'lslande - Ecologie et Biogeographie. R.Chabaud -Lechevalier, Paris.
Lack,P. 1986. The Atlas of Wintering Birds in Britain and Ireland. Poyser, Calton.
Nethersole-Thompson,D. 1966. The Snow Bunting. Oliver and Boyd, Edinburgh.
Salomonsen,F. 1931. On the geographical variation of the Snow Bunting (Plectrophenax nivalis). Ibis, 57-70.
Thom,V.X. 1986. Birds in Scotland. Poyser, Calton.