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The numbers and distribution of scaup Aythya marila in Britain and Ireland
Biological Conservation 43 (1988) 267-278
The Numbers and Distribution of Scaup Aythya marila in Britain and Ireland D. G. Salmon The Wildfowl Trust, Slimbridge, Gloucester GL2 7BT, Great Britain (Received 4 June 1987; accepted 9 July 1987)
ABSTRACT The numbers and distribution of scaup in Britain and Ireland have been monitored since 1947 through the Wildfowl Trust's National Wildfowl Counts scheme and, since the late 1960s, through the Irish Wildbird Conservancy's wetland surveys. The results since 1960 are presented in this paper. In Britain, the predominant feature has been a huge proliferation, of up to 25 000 birds, in the Firth of Forth off Edinburgh from the mid-1960s to mid1970s. The disappearance of this assemblage occurred in isolation from the trend elsewhere in the country, which has been fairly stable, with small influxes in recent cold winters. In Ireland, there are signs of a decline during the 1970s followed by a slight recovery. The total number in Britain and Ireland in the early 1980s is estimated to have been only 6000-7500. The concentration of the species in relatively few resorts and the lack of current knowledge of trends and population levels in northwest Europe as a whole are sources of concern.
INTRODUCTION The scaup is a diving duck, m o r e marine in winter habitat than the closely related pochards. The nominate race A. m. marila breeds in Iceland and across n o r t h e r n m o s t Scandinavia and the U S S R as far east as the Lena River, wintering mainly in northwest Europe and a r o u n d the Black Sea. The latest estimate o f the northwest European population is 150 000 in the 1970s 267 Biol. Conserv. 0006-3207/88/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1988. Printed in Great Britain
268
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(Atkinson-Willes, 1976; Scott, 1980). The winter distribution is governed by the species' specialised feeding habits, the principal food at that time of year being molluscs, chiefly the blue mussel M y t i l u s edulis, which are taken at depths rarely exceeding 6 m (Madsen, 1954; Nilsson, 1969; Atkinson-Willes, 1976; Cramp & Simmons, 1977). The principal winter habitat is therefore shallow coastal bays. In northwest Europe the species winters mainly in Denmark, the Netherlands, Britain and Ireland, with the largest concentrations in the western Baltic, Wadden Sea and IJsselmeer. Between January 1967 and January 1973 over 80% of the scaup estimated to be wintering in northwest Europe were in only 18 coastal areas (Atkinson-Willes, 1976). In Britain the species has readily adapted to manmade food sources, in particular sewage outfalls (Milne & Campbell, 1973; Pounder, 1976). The present paper examines the numerical distribution and trends in numbers in Britain and Ireland since 1960.
METHODS AND DATA Since 1947 the numbers of ducks, geese and swans have been counted every month from September to March at as many coastal and inland localities as possible in Britain and Northern Ireland, under the Wildfowl Trust's National Wildfowl Counts scheme (Owen et al., 1986). The counts are undertaken entirely by volunteers and reserve wardens on the Sunday nearest the middle of each month. Counts carried out on other dates, because conditions were impossible or unsuitable on the set day, are, however, accepted. The data up to the early 1960s were assessed by Atkinson-Willes (1963). Those from 1960-61 onwards have been computerised, and were reviewed by Owen et al. (1986) and, from 1979-80 onwards, by annual summaries (e.g. Salmon et al., 1987). This paper discusses in greater detail than hitherto the results for scaup over the 25 winters 1960--61 to 1984-85. For certain estuaries inadequately covered by the wildfowl counts, data from the British Trust for Ornithology (BTO)/Nature Conservancy Council (NCC)/Royal Society for the Protection of Birds (RSPB) Birds of Estuaries Enquiry (Prater, 1981; Salmon et al., 1987) have been used. In Britain 4500 localities were recorded at least once during the period in question. Coverage of the coast was virtually complete, while most unrecorded areas inland were in upland regions containing little suitable habitat for wintering wildfowl. For the Irish Republic, a listing of positive scaup records was obtained from the various wetland surveys organised by the Irish Wildbird Conservancy (IWC) since the late 1960s (Hutchinson, 1979; Sheppard, 1984). (Earlier, before the founding of the IWC, the Wildfowl Trust's scheme included the Irish Republic.)
The scaup in Britain and Ireland
269
Scaup are highly gregarious and occur in large, often densely packed flocks. This makes them easy to locate, and it is felt that no major concentrations have been overlooked in Britain and Ireland. Underestimation of big gatherings could occur, however, firstly since the birds dive frequently when feeding, which can be during the day or night (Nilsson, 1969), and secondly if the water is less than calm. Tests with counts of waders Charidrii at roosts, where they are tightly packed, also suggest a tendency to underestimate large concentrations (Prater, 1979; Hale, 1974). This factor is not considered to affect the accuracy of the analyses overall, however; counts at the major sites are in any case generally estimated to the nearest thousand or hundred. To ensure as far as possible the competence and reliability of the observers, they are appointed through a system of regional organisers, who are highly experienced fieldworkers with a personal knowledge of the ornithologists and wildfowl haunts in their areas. Three types of analysis of the count data are presented here. (1) To plot the distribution (Fig. 1), the average number of scaup counted at each site was calculated for each month, September to March, for the last five years of available data. The month with the highest average was chosen and its figure converted to a symbol on a graded scale, which was marked on the map at the precise location or mid-point of the site. Localities which both had a peak monthly average of less than 1.0 and held scaup in only one year of the five were omitted. (2) The annual seasonal maxima, regardless of month, were examined for each site, and places which at any time held over 1000 scaup were included in Table 1. (3) The trend in the British counts in January, when the peak numbers are usually present (Fig. 2), was calculated along the following lines, as used by Owen et al. (1986): (a)
Beginning in 1961 each pair of years was considered together, i.e. 1961 with 1962, 1962 with 1963, etc. Only sites counted in both years were included in the paired samples. (b) The total number of ducks in the paired sites in each year was established. (c) The ratio of the two totals was calculated for each of the 25 pairs of years (up to 1985), working backwards and forwards from an arbitrary 'anchor' year, 1971. (d) 1971 was given an index value of 100, and other years' indices calculated, again working back to 1961 and forwards to 1985, by multiplying the previous index by the current year's ratio from the paired sample and dividing by 100. The resultant indices are presented as five-year running means.
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271
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Fig. 2. Trends in the number ofscaup in Britain in January, 1961 to 1985,expressed as fiveyear running mean indices based on 1971 = 100, (a) including Lothian District, (b) excluding Lothian District. This method allows for fluctuations in the count coverage and gives equal weight to all years. Ireland was not included in this analysis as full J a n u a r y data were available from too few sites.
RESULTS AND DISCUSSION
Distribution Scaup were recorded at 514 sites in Britain and 122 in Ireland between 1960-61 and 1984-85. Figure 1 shows the quantitative distribution, based on the last three years o f available data for each site. The preference for the coast is immediately obvious, the only major inland concentration being in
The scaup in Britain and Ireland
273
Lough Neagh, Northern Ireland (see Table 1). The great majority of inland records represent the irregular occurrence of one to three birds. The coastal sites are nearly all shallow, sheltered bays. Many are sandy or have rocky outcrops and are therefore attractive to molluscs; others are near sewage outfalls. In Orkney, the brackish Loch of Stenness and nearby freshwater lochs, north of Scapa Flow, hold 100-200 between them. In North Uist, Western Isles, over 200 were counted in February 1975, including 170 on the highly eutrophic freshwater Lochs Durasay and Leodasay, but fewer than 10 have been found in recent years (Owen et al., 1986). A relative paucity in northwest Scotland and southern England and Wales is evident. In northwest Scotland, apart from the Uists and Benbecula, most of the coastline is rocky but highly exposed and the inshore water is deep; an intensive survey in December 1984 and January 1985 (Moser et al., 1986) f~iled to find any scaup along the west mainland coast and Inner Hebrides north of 56 °. In the south of England and Wales, however, there is much apparently suitable habitat for scaup. The scaup's tendency to congregate at relatively few localities, often adjacent to areas of heavy human development, makes it highly vulnerable to pollution; 220 scaup were killed after an oil-spill in the Firth of Forth in February 1978 (Campbell et al., 1978).
Major sites Table 1 shows the annual peak numbers at the major sites in Britain and Ireland, expressed as five-year means. Scaup began to appear in large numbers on the south shore of the Firth of Forth after the disappearance of flocks of a thousand or more from the Firth of Tay, 50 km to the north, in the 1930s (Berry, 1939; Atkinson-Willes, 1963). The spectacular increase along the 12 km between Granton and Musselburgh, Edinburgh, during the 1960s and early 1970s has been well documented (Thorn, 1969; Player, 1970; Milne & Campbell, 1973; Pounder, 1976; Campbell, 1978). Other ducks, particularly eider Somateria mollissima and goldeneye Bucephala clangula, showed a similar proliferation in the area at that time. Scaup and goldeneye fed mainly in the vicinity of raw sewage outfalls on waste grain from local breweries and on invertebrates, which were especially abundant due to organic enrichment. In a sample of 21 scaup, the chief invertebrate prey was ragworms Nereidae, round worms Nematoda and blue mussel Mytilis edulis (Player, 1970). Although a slight decline had begun a year or two earlier, all three main duck species showed a marked reduction with the modernisation of the local sewage system, begun in 1975. Campbell (1984) concluded that the decrease in the numbers of scaup and goldeneye was directly connected with the introduction of sewage treatment (although eiders showed no
274
D. G. Salmon
definite link). Furthermore, the scaup showed an initial local shift away from the outfalls which were first closed. There was a distinct December/January peak in the large concentrations (see later). Largo Bay, on the opposite, north side of the Firth of Forth, qualifies as Internationally Important for scaup under the agreed criteria (AtkinsonWilles et aL, 1982), through regularly supporting at least 1% (i.e. 1500) of the latest estimate of the northwest European population. Its attraction to scaup lies in both mussel beds and sewage outfalls. Records are not available before the mid-1960s, but since then, apart from an initial increase, the numbers have been remarkably stable, suggesting that the birds at Largo Bay were mainly discrete from those off Edinburgh, or at least that the scaup displaced from Edinburgh did not move to the northerly site. The scaup in Loch Indaal, on the island of Islay in the Inner Hebrides, are attracted by spent barley from a .local distillery. Some occasionally move onto the freshwater Loch Skerrols, 2 km inland, apparently in response to bad weather (Booth, 1975; Ogilvie, 1983). Their peak numbers are generally present in November, in common with most other west coast and Irish sites. The scaup in the inner Solway Firth, on the Scotland/England border, feed mainly at mussel scars (Moser, 1984). They range widely, but are mainly found in Carse Bay at the mouth of the River Nith on the north shore, and along the south shore. There is evidence of movement between the two sides of the firth (Owen et aL, 1986) and double counting may have taken place. Totals for the whole Solway Firth are not therefore attempted, and the data for the two main areas are presented separately in Table 1, those for the south shore being simultaneous totals for the component sectors plotted in Fig. 1. Of the Irish sites, the 25 000 ha Lough Neagh is remarkable for being one of the few major wintering sites for scaup in northwest Europe which is entirely landlocked. The great majority are found on the west shore. In 1985-86, after the period reviewed in the present paper, 1700 scaup were counted in the lough, a level which if maintained would render it an Internationally Important site. The sharp decline at Wexford in the late 1960s was attributed to heavy pollution destroying the mussel beds (Ruttledge, 1970), while Belfast Lough has been the scene of extensive reclamation (Hutchinson, 1979). Trends
Figure 2 shows the trends in January numbers in Britain from 1961 to 1985 expressed as five-year running mean indices (see above for method). Two graphs are presented, one with and one without Lothian District, in which the Musselburgh to Granton (Edinburgh) stretch lies. There was a big influx
The scaup in Britain and Ireland
275
of scaup to Britain in the severe winter of 1962-63, after which the numbers in the Edinburgh area increased from 6000 to their peak of 25 000 in the early 1970s, then declined dramatically, as described earlier. Elsewhere in Britain the numbers showed no clear trend, the total counted fluctuating around a level of 2500-3500, except for influxes in the cold winters of 1978-79, when the total reached 5000, and 1981-82, with nearly 4000; they did not increase to compensate for the decline at Edinburgh, as might be interpreted from the graphs. The maximum number counted in the whole of Britain was 26 200 in 1973-74, the subsequent reduction clearly being due to the disappearance of the Edinburgh birds. Allowing for occasional missed counts at major resorts, the actual British numbers in the early 1980s are estimated to have been 350(0-4500. The data from Ireland, though not complete enough to allow a similar analysis, suggest that a steady decline occurred there between about 1970 and 1980, followed by a slight recovery. This impression should be treated with caution, however, since coverage of some of the major resorts has been more thorough in recent years. No estimate of the Irish population has been published, but Owen et al. (1986) used the quantitative distribution map in Hutchinson (1979) to conclude that there were no more than 3000 in the 1970s. The data from the IWC suggest a wintering stock of 2500-3000 in the early 1980s. I therefore estimate the total in Britain and Ireland at that time at 6000-7500, at the lower end of ranges of 6000-10 000 estimated by Owen et al. (1986) and 5000-10000 by Lack (1986).
Origins and movements It is apparent that the assemblage which disappeared from Edinburgh did not move to another part of Britain. Unfortunately it has not been possible to obtain reliable recent trends or population estimates for the other principal countries in northwest Europe for this species from the International Waterfowl Research Bureau's January International Waterfowl Censuses, owing to inconsistencies in the count coverage of coastal areas, especially in the western Baltic (Riiger et al., 1986). The evidence from ringing as to the movements of scaup is also unclear. Of 78 foreign-ringed scaup recovered in Britain and Ireland up to 1984, 75 were ringed in Iceland and 3 in Finland, but those figures probably reflect the intensity of ringing rather than any accurate impression of the origins of British and Irish birds. Fifty of the Icelandic-ringed birds were recovered in Ireland, 7 on the east coast of Scotland, 7 elsewhere in Scotland and 11 in England. Only 14 scaup ringed in Britain and Ireland have been recovered overseas: 3 in Iceland, 3 in Germany, one each in Denmark and Finland and 6 in the Soviet Union. The Icelandic recoveries were all of birds ringed in Aberdeenshire, northeast
276
D. G. Salmon
Scotland. Of the remainder, 5 had been ringed in southeast England, away from the main wintering area, one in Ireland and 5 in Aberdeenshire (Spencer & Hudson, 1982; Mead & Hudson, 1984, 1985; BTO, pers. comm.). The rise and fall in the Edinburgh concentration did not bear any relation to the summer numbers in Iceland. In fact, the Icelandic population declined during the 1960s and early 1970s, from a possible 35 000 to 10000-13 000 in 1975, then recovered to 17 000-20 000 by 1982 (A. Gardarsson, pers. comm.). This suggests that the Edinburgh concentration at its peak of 25 000 in the mid 1970s consisted mostly, if not entirely, of birds from Scandinavia and/or the USSR, an assumption supported by their marked December and January peak, as opposed to the bulk autumn arrival in Britain of most wildfowl breeding in Iceland, e.g. whooper swan (Boyd & Eltringham, 1962), greylag goose (Boyd, 1959) and wigeon (Owen & Mitchell, in press). It is tempting to surmise that the autumn arrival of the major concentrations in Ireland and western Scotland suggests Icelandic origin.
CONCLUSIONS While the numbers of scaup in Britain and Ireland have declined during the period under review, this should be put in the context of the sudden increase which occurred at the start of the period and the much larger numbers present on the Continent. Furthermore, the disappearance of the big flocks off Edinburgh is better understood than is often the case when such a drastic change occurs. Concern should perhaps be felt, however, at the apparent decline in Ireland. The lack of directly comparable count data from the species' main areas in the Netherlands and the Baltic is regrettable, both for general conservation reasons, and because it prevents the changes in Britain and Ireland from being put in their full context. It is impossible to say whether the birds from Edinburgh were simply absorbed in the great flocks on the Continent, or whether there has been a general decline in the northwest European population. It is vital that the species' full status on an international scale be ascertained as soon as possible, and constantly monitored thereafter.
ACKNOWLEDGEMENTS The author holds a post financed under contract to the Nature Conservancy Council, whose long-term funding of the National Wildfowl Counts scheme is gratefully acknowledged. Dr M. Owen of the Wildfowl Trust gave extensive advice and assistance at all stages of the writing of this paper. Dr
The scaup in Britain and Ireland
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M. W. Pienkowski of the N C C and Prof. G. V. T. Matthews and Dr A. D. Fox of the Wildfowl Trust made constructive comments on an early draft. Information was kindly provided by Dr M. E. Moser, BTO (data from the B T O / N C C / R S P B Birds of Estuaries Enquiry); C. J. Mead and Mrs J. Marchant, BTO (ringing recovery details); R. Sheppard and Dr R. Nairn, IWC (data from the Irish Republic). My greatest debt is to the thousands of volunteer observers who have carried out the wildfowl counts over the years, often braving very difficult conditions.
REFERENCES Atkinson-Willes, G. L. (ed.) (1963). Wildfowl in Great Britain, 1st edn. Nature Conservancy Monograph, No. 3. London, HMSO. Atkinson-Willes, G. L. (1976). The numerical distribution of ducks, swans and coots as a guide to assessing the importance of wetlands. Proc. int. Conf. Conserv. Wetlands and Waterfowl, Heiligenhafen, 1974, 199-271. Slimbridge, International Waterfowl Research Bureau. Atkinson-Willes, G. L., Scott, D. A. & Prater, A. J. (1982). Criteria for selecting wetlands of international importance. Proc. Conf. Conserv. Wetlands of int. Importance especially as waterfowl habitat, Cagliari, 1980, 1017-42. Bologna, Istituto Nazionale di Biologia della Selvaggina. Berry, J. (1939). The status and distribution of wild geese and wild duck in Scotland. International Wildfowl Inquiry, II. Cambridge, Cambridge University Press. Booth, C. G. (1975). Birds in Islay. Port Charlotte, Argyll Reproductions. Boyd, H. (1959). Greylag geese in Britain in winter. Wildfowl, 10, 43-58. Boyd, H. & Eltringham, S. K. (1962). The whooper swan in Great Britain. Bird Study, 9, 217-41. Campbell, L. H. (1978). Patterns of distribution and behaviour of flocks of seaducks wintering at Leith and Musselburgh, Scotland. Biol. Conserv., 14, 111-23. Campbell, L. H. (1984). The impact of changes in sewage treatment on seaducks wintering in the Firth of Forth, Scotland. Biol. Conserv., 28, 173-80. Campbell, L. H., Standring, K. T. & Cadbury, C. J. (1978). Firth of Forth oil pollution incident, February 1978. Mar. Pollut. Bull., 9, 335-9. Cramp, S. & Simmons, K. E. L. (eds) (1977). The birds of the Western Palearctic, 1. Oxford, Oxford University Press. Hale, W. G. (1974). Aerial counting of waders. Ibis, 116, 412. Hutchinson, C. (1979). Ireland's wetlands and their birds. Dublin, Irish Wildbird Conservancy. Lack, P. (1986). The atlas of wintering birds in Britain and Ireland. Calton, Poyser. Madsen, F. J. (1954). On the food habits of diving ducks in Denmark. Dan. Rev. Game Biol., 2, 160-266. Mead, C. J. & Hudson, R. (1984). Report on bird-ringing for 1983. Ring. & Migr., 5, 153-92. Mead, C. J. & Hudson, R. (1985). Report on bird-ringing for 1984. Ring. & Migr., 6, 125-72.
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Milne, H. & Campbell, L. H. (1973). Wintering sea-ducks off the east coast of Scotland. Bird Study, 20, 153-72. Moser, M. E. (1984). Solway Firth shorebird survey 1982-1984. Peterborough and Sandy, Nature Conservancy Council and the Royal Society for the Protection of Birds (Unpublished report). Moser, M. E., Broad, R. C., Dennis, R. H. & Madders, M. (1986). The distribution and abundance of coastal birds (non-waders) on the west and north-west coasts of Scotland in winter. Scot. Birds, 14, 61-7. Nilsson, L. (1969). Food consumption of diving ducks wintering at the coast of south Sweden. Oikos, 20, 128-35. Ogilvie, M. A. (1983). Wildfowl of Islay. Proc. R. Soc. Edinb. B, 83, 473-89. Owen, M., Atkinson-Willes, G. L. & Salmon, D. G. (1986). Wildfowl in Great Britain, 2nd edn. Cambridge, Cambridge University Press. Owen, M. & Mitchell, C. (in press). Movements and migrations of wigeon Anas penelope wintering in Britain and Ireland. Bird Study. Player, P. V. (1970). Thefood andfeeding habits of diving ducks at Seafield, Edinburgh. BSc thesis, University of Edinburgh. Pounder, B. (1976). Waterfowl at effluent discharges in Scottish coastal waters. Scot. Birds, 9, 5-36. Prater, A. J. (1979). Trends in accuracy of counting birds. Bird Study, 26, 198-200. Prater, A. J. (1981). Estuary birds of Britain and Ireland. Calton, Poyser. Riiger, A., Prentice, C. & Owen, M. (1986). Results of the IWRB International Waterfowl Census 1967-1983. IWRB Spec. Pubis, No. 6. Slimbridge, International Waterfowl Research Bureau. Ruttledge, R. F. (1970). Winter distribution and numbers of scaup, long-tailed duck and common scoter in Ireland. Bird Study, 17, 241-6. Salmon, D. G., Moser, M. E. & Kirby, J. S. (1987). Wildfowl and wader counts 1985-86. Slimbridge, Wildfowl Trust. Scott, D. A. (1980). A preliminary inventory of wetlands of international importance for waterfowl in Western Europe and Northwest Africa. IWRB Spec. Pubis, No. 2. Slimbridge, International Waterfowl Research Bureau. Sheppard, R. (1984). Winter wetland survey. IWC News, 41, 6. Irish Wildbird Conservancy. Spencer, R. & Hudson, R. (1982). Report on bird-ringing for 1981. Ring. & Migr., 4, 65-128. Thom, V. M. (1969). Wintering duck in Scotland 1962-68. Scot. Birds, 5, 417-66.
The Numbers and Distribution of Scaup Aythya marila in Britain and Ireland D. G. Salmon The Wildfowl Trust, Slimbridge, Gloucester GL2 7BT, Great Britain (Received 4 June 1987; accepted 9 July 1987)
ABSTRACT The numbers and distribution of scaup in Britain and Ireland have been monitored since 1947 through the Wildfowl Trust's National Wildfowl Counts scheme and, since the late 1960s, through the Irish Wildbird Conservancy's wetland surveys. The results since 1960 are presented in this paper. In Britain, the predominant feature has been a huge proliferation, of up to 25 000 birds, in the Firth of Forth off Edinburgh from the mid-1960s to mid1970s. The disappearance of this assemblage occurred in isolation from the trend elsewhere in the country, which has been fairly stable, with small influxes in recent cold winters. In Ireland, there are signs of a decline during the 1970s followed by a slight recovery. The total number in Britain and Ireland in the early 1980s is estimated to have been only 6000-7500. The concentration of the species in relatively few resorts and the lack of current knowledge of trends and population levels in northwest Europe as a whole are sources of concern.
INTRODUCTION The scaup is a diving duck, m o r e marine in winter habitat than the closely related pochards. The nominate race A. m. marila breeds in Iceland and across n o r t h e r n m o s t Scandinavia and the U S S R as far east as the Lena River, wintering mainly in northwest Europe and a r o u n d the Black Sea. The latest estimate o f the northwest European population is 150 000 in the 1970s 267 Biol. Conserv. 0006-3207/88/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1988. Printed in Great Britain
268
D. G. Salmon
(Atkinson-Willes, 1976; Scott, 1980). The winter distribution is governed by the species' specialised feeding habits, the principal food at that time of year being molluscs, chiefly the blue mussel M y t i l u s edulis, which are taken at depths rarely exceeding 6 m (Madsen, 1954; Nilsson, 1969; Atkinson-Willes, 1976; Cramp & Simmons, 1977). The principal winter habitat is therefore shallow coastal bays. In northwest Europe the species winters mainly in Denmark, the Netherlands, Britain and Ireland, with the largest concentrations in the western Baltic, Wadden Sea and IJsselmeer. Between January 1967 and January 1973 over 80% of the scaup estimated to be wintering in northwest Europe were in only 18 coastal areas (Atkinson-Willes, 1976). In Britain the species has readily adapted to manmade food sources, in particular sewage outfalls (Milne & Campbell, 1973; Pounder, 1976). The present paper examines the numerical distribution and trends in numbers in Britain and Ireland since 1960.
METHODS AND DATA Since 1947 the numbers of ducks, geese and swans have been counted every month from September to March at as many coastal and inland localities as possible in Britain and Northern Ireland, under the Wildfowl Trust's National Wildfowl Counts scheme (Owen et al., 1986). The counts are undertaken entirely by volunteers and reserve wardens on the Sunday nearest the middle of each month. Counts carried out on other dates, because conditions were impossible or unsuitable on the set day, are, however, accepted. The data up to the early 1960s were assessed by Atkinson-Willes (1963). Those from 1960-61 onwards have been computerised, and were reviewed by Owen et al. (1986) and, from 1979-80 onwards, by annual summaries (e.g. Salmon et al., 1987). This paper discusses in greater detail than hitherto the results for scaup over the 25 winters 1960--61 to 1984-85. For certain estuaries inadequately covered by the wildfowl counts, data from the British Trust for Ornithology (BTO)/Nature Conservancy Council (NCC)/Royal Society for the Protection of Birds (RSPB) Birds of Estuaries Enquiry (Prater, 1981; Salmon et al., 1987) have been used. In Britain 4500 localities were recorded at least once during the period in question. Coverage of the coast was virtually complete, while most unrecorded areas inland were in upland regions containing little suitable habitat for wintering wildfowl. For the Irish Republic, a listing of positive scaup records was obtained from the various wetland surveys organised by the Irish Wildbird Conservancy (IWC) since the late 1960s (Hutchinson, 1979; Sheppard, 1984). (Earlier, before the founding of the IWC, the Wildfowl Trust's scheme included the Irish Republic.)
The scaup in Britain and Ireland
269
Scaup are highly gregarious and occur in large, often densely packed flocks. This makes them easy to locate, and it is felt that no major concentrations have been overlooked in Britain and Ireland. Underestimation of big gatherings could occur, however, firstly since the birds dive frequently when feeding, which can be during the day or night (Nilsson, 1969), and secondly if the water is less than calm. Tests with counts of waders Charidrii at roosts, where they are tightly packed, also suggest a tendency to underestimate large concentrations (Prater, 1979; Hale, 1974). This factor is not considered to affect the accuracy of the analyses overall, however; counts at the major sites are in any case generally estimated to the nearest thousand or hundred. To ensure as far as possible the competence and reliability of the observers, they are appointed through a system of regional organisers, who are highly experienced fieldworkers with a personal knowledge of the ornithologists and wildfowl haunts in their areas. Three types of analysis of the count data are presented here. (1) To plot the distribution (Fig. 1), the average number of scaup counted at each site was calculated for each month, September to March, for the last five years of available data. The month with the highest average was chosen and its figure converted to a symbol on a graded scale, which was marked on the map at the precise location or mid-point of the site. Localities which both had a peak monthly average of less than 1.0 and held scaup in only one year of the five were omitted. (2) The annual seasonal maxima, regardless of month, were examined for each site, and places which at any time held over 1000 scaup were included in Table 1. (3) The trend in the British counts in January, when the peak numbers are usually present (Fig. 2), was calculated along the following lines, as used by Owen et al. (1986): (a)
Beginning in 1961 each pair of years was considered together, i.e. 1961 with 1962, 1962 with 1963, etc. Only sites counted in both years were included in the paired samples. (b) The total number of ducks in the paired sites in each year was established. (c) The ratio of the two totals was calculated for each of the 25 pairs of years (up to 1985), working backwards and forwards from an arbitrary 'anchor' year, 1971. (d) 1971 was given an index value of 100, and other years' indices calculated, again working back to 1961 and forwards to 1985, by multiplying the previous index by the current year's ratio from the paired sample and dividing by 100. The resultant indices are presented as five-year running means.
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50-
O- 1~133
6'6
71
YEAR
7'6
Fig. 2. Trends in the number ofscaup in Britain in January, 1961 to 1985,expressed as fiveyear running mean indices based on 1971 = 100, (a) including Lothian District, (b) excluding Lothian District. This method allows for fluctuations in the count coverage and gives equal weight to all years. Ireland was not included in this analysis as full J a n u a r y data were available from too few sites.
RESULTS AND DISCUSSION
Distribution Scaup were recorded at 514 sites in Britain and 122 in Ireland between 1960-61 and 1984-85. Figure 1 shows the quantitative distribution, based on the last three years o f available data for each site. The preference for the coast is immediately obvious, the only major inland concentration being in
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Lough Neagh, Northern Ireland (see Table 1). The great majority of inland records represent the irregular occurrence of one to three birds. The coastal sites are nearly all shallow, sheltered bays. Many are sandy or have rocky outcrops and are therefore attractive to molluscs; others are near sewage outfalls. In Orkney, the brackish Loch of Stenness and nearby freshwater lochs, north of Scapa Flow, hold 100-200 between them. In North Uist, Western Isles, over 200 were counted in February 1975, including 170 on the highly eutrophic freshwater Lochs Durasay and Leodasay, but fewer than 10 have been found in recent years (Owen et al., 1986). A relative paucity in northwest Scotland and southern England and Wales is evident. In northwest Scotland, apart from the Uists and Benbecula, most of the coastline is rocky but highly exposed and the inshore water is deep; an intensive survey in December 1984 and January 1985 (Moser et al., 1986) f~iled to find any scaup along the west mainland coast and Inner Hebrides north of 56 °. In the south of England and Wales, however, there is much apparently suitable habitat for scaup. The scaup's tendency to congregate at relatively few localities, often adjacent to areas of heavy human development, makes it highly vulnerable to pollution; 220 scaup were killed after an oil-spill in the Firth of Forth in February 1978 (Campbell et al., 1978).
Major sites Table 1 shows the annual peak numbers at the major sites in Britain and Ireland, expressed as five-year means. Scaup began to appear in large numbers on the south shore of the Firth of Forth after the disappearance of flocks of a thousand or more from the Firth of Tay, 50 km to the north, in the 1930s (Berry, 1939; Atkinson-Willes, 1963). The spectacular increase along the 12 km between Granton and Musselburgh, Edinburgh, during the 1960s and early 1970s has been well documented (Thorn, 1969; Player, 1970; Milne & Campbell, 1973; Pounder, 1976; Campbell, 1978). Other ducks, particularly eider Somateria mollissima and goldeneye Bucephala clangula, showed a similar proliferation in the area at that time. Scaup and goldeneye fed mainly in the vicinity of raw sewage outfalls on waste grain from local breweries and on invertebrates, which were especially abundant due to organic enrichment. In a sample of 21 scaup, the chief invertebrate prey was ragworms Nereidae, round worms Nematoda and blue mussel Mytilis edulis (Player, 1970). Although a slight decline had begun a year or two earlier, all three main duck species showed a marked reduction with the modernisation of the local sewage system, begun in 1975. Campbell (1984) concluded that the decrease in the numbers of scaup and goldeneye was directly connected with the introduction of sewage treatment (although eiders showed no
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definite link). Furthermore, the scaup showed an initial local shift away from the outfalls which were first closed. There was a distinct December/January peak in the large concentrations (see later). Largo Bay, on the opposite, north side of the Firth of Forth, qualifies as Internationally Important for scaup under the agreed criteria (AtkinsonWilles et aL, 1982), through regularly supporting at least 1% (i.e. 1500) of the latest estimate of the northwest European population. Its attraction to scaup lies in both mussel beds and sewage outfalls. Records are not available before the mid-1960s, but since then, apart from an initial increase, the numbers have been remarkably stable, suggesting that the birds at Largo Bay were mainly discrete from those off Edinburgh, or at least that the scaup displaced from Edinburgh did not move to the northerly site. The scaup in Loch Indaal, on the island of Islay in the Inner Hebrides, are attracted by spent barley from a .local distillery. Some occasionally move onto the freshwater Loch Skerrols, 2 km inland, apparently in response to bad weather (Booth, 1975; Ogilvie, 1983). Their peak numbers are generally present in November, in common with most other west coast and Irish sites. The scaup in the inner Solway Firth, on the Scotland/England border, feed mainly at mussel scars (Moser, 1984). They range widely, but are mainly found in Carse Bay at the mouth of the River Nith on the north shore, and along the south shore. There is evidence of movement between the two sides of the firth (Owen et aL, 1986) and double counting may have taken place. Totals for the whole Solway Firth are not therefore attempted, and the data for the two main areas are presented separately in Table 1, those for the south shore being simultaneous totals for the component sectors plotted in Fig. 1. Of the Irish sites, the 25 000 ha Lough Neagh is remarkable for being one of the few major wintering sites for scaup in northwest Europe which is entirely landlocked. The great majority are found on the west shore. In 1985-86, after the period reviewed in the present paper, 1700 scaup were counted in the lough, a level which if maintained would render it an Internationally Important site. The sharp decline at Wexford in the late 1960s was attributed to heavy pollution destroying the mussel beds (Ruttledge, 1970), while Belfast Lough has been the scene of extensive reclamation (Hutchinson, 1979). Trends
Figure 2 shows the trends in January numbers in Britain from 1961 to 1985 expressed as five-year running mean indices (see above for method). Two graphs are presented, one with and one without Lothian District, in which the Musselburgh to Granton (Edinburgh) stretch lies. There was a big influx
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of scaup to Britain in the severe winter of 1962-63, after which the numbers in the Edinburgh area increased from 6000 to their peak of 25 000 in the early 1970s, then declined dramatically, as described earlier. Elsewhere in Britain the numbers showed no clear trend, the total counted fluctuating around a level of 2500-3500, except for influxes in the cold winters of 1978-79, when the total reached 5000, and 1981-82, with nearly 4000; they did not increase to compensate for the decline at Edinburgh, as might be interpreted from the graphs. The maximum number counted in the whole of Britain was 26 200 in 1973-74, the subsequent reduction clearly being due to the disappearance of the Edinburgh birds. Allowing for occasional missed counts at major resorts, the actual British numbers in the early 1980s are estimated to have been 350(0-4500. The data from Ireland, though not complete enough to allow a similar analysis, suggest that a steady decline occurred there between about 1970 and 1980, followed by a slight recovery. This impression should be treated with caution, however, since coverage of some of the major resorts has been more thorough in recent years. No estimate of the Irish population has been published, but Owen et al. (1986) used the quantitative distribution map in Hutchinson (1979) to conclude that there were no more than 3000 in the 1970s. The data from the IWC suggest a wintering stock of 2500-3000 in the early 1980s. I therefore estimate the total in Britain and Ireland at that time at 6000-7500, at the lower end of ranges of 6000-10 000 estimated by Owen et al. (1986) and 5000-10000 by Lack (1986).
Origins and movements It is apparent that the assemblage which disappeared from Edinburgh did not move to another part of Britain. Unfortunately it has not been possible to obtain reliable recent trends or population estimates for the other principal countries in northwest Europe for this species from the International Waterfowl Research Bureau's January International Waterfowl Censuses, owing to inconsistencies in the count coverage of coastal areas, especially in the western Baltic (Riiger et al., 1986). The evidence from ringing as to the movements of scaup is also unclear. Of 78 foreign-ringed scaup recovered in Britain and Ireland up to 1984, 75 were ringed in Iceland and 3 in Finland, but those figures probably reflect the intensity of ringing rather than any accurate impression of the origins of British and Irish birds. Fifty of the Icelandic-ringed birds were recovered in Ireland, 7 on the east coast of Scotland, 7 elsewhere in Scotland and 11 in England. Only 14 scaup ringed in Britain and Ireland have been recovered overseas: 3 in Iceland, 3 in Germany, one each in Denmark and Finland and 6 in the Soviet Union. The Icelandic recoveries were all of birds ringed in Aberdeenshire, northeast
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Scotland. Of the remainder, 5 had been ringed in southeast England, away from the main wintering area, one in Ireland and 5 in Aberdeenshire (Spencer & Hudson, 1982; Mead & Hudson, 1984, 1985; BTO, pers. comm.). The rise and fall in the Edinburgh concentration did not bear any relation to the summer numbers in Iceland. In fact, the Icelandic population declined during the 1960s and early 1970s, from a possible 35 000 to 10000-13 000 in 1975, then recovered to 17 000-20 000 by 1982 (A. Gardarsson, pers. comm.). This suggests that the Edinburgh concentration at its peak of 25 000 in the mid 1970s consisted mostly, if not entirely, of birds from Scandinavia and/or the USSR, an assumption supported by their marked December and January peak, as opposed to the bulk autumn arrival in Britain of most wildfowl breeding in Iceland, e.g. whooper swan (Boyd & Eltringham, 1962), greylag goose (Boyd, 1959) and wigeon (Owen & Mitchell, in press). It is tempting to surmise that the autumn arrival of the major concentrations in Ireland and western Scotland suggests Icelandic origin.
CONCLUSIONS While the numbers of scaup in Britain and Ireland have declined during the period under review, this should be put in the context of the sudden increase which occurred at the start of the period and the much larger numbers present on the Continent. Furthermore, the disappearance of the big flocks off Edinburgh is better understood than is often the case when such a drastic change occurs. Concern should perhaps be felt, however, at the apparent decline in Ireland. The lack of directly comparable count data from the species' main areas in the Netherlands and the Baltic is regrettable, both for general conservation reasons, and because it prevents the changes in Britain and Ireland from being put in their full context. It is impossible to say whether the birds from Edinburgh were simply absorbed in the great flocks on the Continent, or whether there has been a general decline in the northwest European population. It is vital that the species' full status on an international scale be ascertained as soon as possible, and constantly monitored thereafter.
ACKNOWLEDGEMENTS The author holds a post financed under contract to the Nature Conservancy Council, whose long-term funding of the National Wildfowl Counts scheme is gratefully acknowledged. Dr M. Owen of the Wildfowl Trust gave extensive advice and assistance at all stages of the writing of this paper. Dr
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M. W. Pienkowski of the N C C and Prof. G. V. T. Matthews and Dr A. D. Fox of the Wildfowl Trust made constructive comments on an early draft. Information was kindly provided by Dr M. E. Moser, BTO (data from the B T O / N C C / R S P B Birds of Estuaries Enquiry); C. J. Mead and Mrs J. Marchant, BTO (ringing recovery details); R. Sheppard and Dr R. Nairn, IWC (data from the Irish Republic). My greatest debt is to the thousands of volunteer observers who have carried out the wildfowl counts over the years, often braving very difficult conditions.
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Milne, H. & Campbell, L. H. (1973). Wintering sea-ducks off the east coast of Scotland. Bird Study, 20, 153-72. Moser, M. E. (1984). Solway Firth shorebird survey 1982-1984. Peterborough and Sandy, Nature Conservancy Council and the Royal Society for the Protection of Birds (Unpublished report). Moser, M. E., Broad, R. C., Dennis, R. H. & Madders, M. (1986). The distribution and abundance of coastal birds (non-waders) on the west and north-west coasts of Scotland in winter. Scot. Birds, 14, 61-7. Nilsson, L. (1969). Food consumption of diving ducks wintering at the coast of south Sweden. Oikos, 20, 128-35. Ogilvie, M. A. (1983). Wildfowl of Islay. Proc. R. Soc. Edinb. B, 83, 473-89. Owen, M., Atkinson-Willes, G. L. & Salmon, D. G. (1986). Wildfowl in Great Britain, 2nd edn. Cambridge, Cambridge University Press. Owen, M. & Mitchell, C. (in press). Movements and migrations of wigeon Anas penelope wintering in Britain and Ireland. Bird Study. Player, P. V. (1970). Thefood andfeeding habits of diving ducks at Seafield, Edinburgh. BSc thesis, University of Edinburgh. Pounder, B. (1976). Waterfowl at effluent discharges in Scottish coastal waters. Scot. Birds, 9, 5-36. Prater, A. J. (1979). Trends in accuracy of counting birds. Bird Study, 26, 198-200. Prater, A. J. (1981). Estuary birds of Britain and Ireland. Calton, Poyser. Riiger, A., Prentice, C. & Owen, M. (1986). Results of the IWRB International Waterfowl Census 1967-1983. IWRB Spec. Pubis, No. 6. Slimbridge, International Waterfowl Research Bureau. Ruttledge, R. F. (1970). Winter distribution and numbers of scaup, long-tailed duck and common scoter in Ireland. Bird Study, 17, 241-6. Salmon, D. G., Moser, M. E. & Kirby, J. S. (1987). Wildfowl and wader counts 1985-86. Slimbridge, Wildfowl Trust. Scott, D. A. (1980). A preliminary inventory of wetlands of international importance for waterfowl in Western Europe and Northwest Africa. IWRB Spec. Pubis, No. 2. Slimbridge, International Waterfowl Research Bureau. Sheppard, R. (1984). Winter wetland survey. IWC News, 41, 6. Irish Wildbird Conservancy. Spencer, R. & Hudson, R. (1982). Report on bird-ringing for 1981. Ring. & Migr., 4, 65-128. Thom, V. M. (1969). Wintering duck in Scotland 1962-68. Scot. Birds, 5, 417-66.