THE SEPTEMBER DRIFT-MOVEMENTS OF 1956 AND 1958 By KENNETH WILLIAMSON (Migration Research Officer, B.T.O.) Part 1—General Summary and Weather Conditions INTRODUCTION DURING THE FIRST WEEK of September in 1956 and 1958 there were extensive drift-arrivals of migratory birds on the east coast of Britain. These were made up almost entirely of species which are summer-visitors to Europe and widely distributed in Scandinavia and the countries surrounding: the Baltic Sea. As might be expected from the coincidence in the time of these falls, the chief participants were much the same in both years. A number of wader and other non-Passerine birds were concerned, but by far the most important numerically were the mainly night- migrating- Passerines, among" which Wheatear, Whinchat, Red­ start, Garden and Willow Warblers and Pied Flycatcher* were dominant. The discussion which follows largely concerns these Passerine species. In this part of the paper the migration is summarized and its general character is considered in relation to the meteorological environment. Part II is concerned with a more detailed examina- *Scientific names of all species and races mentioned are given in an Appendix on pages 376-377. continued ... 334 VOL. LII] SEPTEMBER DRIFT-MOVEMENTS 335 tion of various aspects, especially the occurrence of extra-limital vagrants, and with a theoretical discussion of this material, particularly on the 1958 season. These drift-movements are worth a detailed analysis because, although much the same species were concerned in the two years, there were some rather striking (and unexpected) differences in composition and differential abundance; and a comparison clearly demonstrates the vitally important effects of meteorological phenomena on bird migration, and throws interesting light (some of it new) on the function of drift in the evolutionary sphere. Migrational drift, which in brief is concerned with the directional influence of wind on migratory movements in circumstances when accurate navigation is not possible, has been discussed in a number of recent papers (particularly Williamson, 1952, 1955). Perhaps the three most striking differences between the early September falls in the two years were: (a) that certain species which reached "avalanche" strength at Fair Isle in 1956 were more numerous in the south-east than the north-east in 1958; (b) that the movements were almost confined to the east coast in 1956, but were more general and noticeable as far west as the Irish Sea in 1958; and (c) that while certain of the rarer migrants, such as the Wryneck, Bluethroat, Red-backed Shrike and Ortolan Bunting, were well represented in both years, it was only in 1958 that eastern and north-eastern vagrants were concerned, a feature being the remarkable influx of Red-breasted Flycatchers. One aim of this paper is to compare the meteorological developments over the Continent and the North Sea in the two seasons, and enquire what part these developments played in determining these differences. The full data of the 1956 migration have not previously been presented, except in species lists in the separate annual reports of the bird observatories. The general pattern of the 1958 migration has, however, already received brief mention, with detailed accounts of the birds involved, in Bird Migration (vol. 1, no. 1) and the "Recent reports and news" feature of British Birds, (vol. li, pp. 361, 405, 432-445). For this reason the 1956 move­ ments are given a fuller treatment in the present paper, those of 1958 being drawn upon mainly for comparison and for the discussion of instructive and important points which emerge. The data are taken mainly from the "Daily Census" record books of the bird observatories, from their annual reports and (in a few cases) their ringing records, and from the two publications cited above. ACKNOWLEDGEMENTS My thanks are due to the bird observatories for so readily placing their records for these periods at my disposal, and to many migration students who were "in the field" at these times and have kindly made their observations available. I am grateful to Sir Landsborough Thomson, R. C. Homes, FIG. I—THE LOCATIONS OF THE MORE IMPORTANT PLACES MENTIONED IN THE TEXT (I) Foula, Shetland (16) Cley and Blakeney Point, Norfolk (2) Dunrossness and Sumburgh, (17) Walberswick, Suffolk Shetland (18) Bradwell, Essex (3) Fair Isle, Shetland (19) Dungeness, Kent (4) Dornoch, Sutherland (20) St. Catherine's Point, Isle of (5) Isle of May, Fife Wight (6) Aberlady, East Lothian (21) Portland Bill, Dorset (7) Holy Island, Northumberland (22) Jersey and Guernsey, Channel (8) Monks' House, Northumberland Islands (9) Fame Islands, Northumberland (23) St. Agnes, Isles of Scilly (10) Whitley Bay, Northumberland (24) Lundy, North Devon (11) Teesmouth, Co. Durham/York- (25) Skokholm, Pembrokeshire shire (26) Great Saltee and Tuskar Rock, (12) Flamborough Head, Yorkshire Co. Wexford (13) Spurn Point and Kilnsea, York- (27) Bardsey, Caernarvonshire shire (28) Copeland Islands, Co. Down (14) Gibraltar Point, Lincolnshire (29) Tory Island, Co. Donegal (15) Holme, Norfolk VOL. LII] SEPTEMBER DRIFT-MOVEMENTS 337 R. K. Cornwallis, R. Spencer, P. A. D. Hollom and J. H. B. Munro for kindly reading through preliminary drafts of the manuscript and offering valuable comments and suggestions, and I am especially indebted to I. J. Ferguson-Lees for much advice and help, including the extraction of data from county reports. Thanks are also due to the Controller of H.M. Stationery Office for permission to reproduce drawings based on the weather-maps published in the Daily Weather Report of the Meteorological Office (Air Ministry). MIGRATION AND THE WEATHER IN I956 During the last week of August 1956, European weather was characterized by low pressure disturbances affecting a very extensive area between the Baltic countries and the British Isles. There was a complicated depression with centres over the Baltic Sea, the north German coast and southern England on the 25th, and this system moved northwards to envelop the whole of Scandinavia next day. The British Isles were then in a trough which brought moderate-to-fresh W.S.W. winds to England and the whole of west and middle Europe. A col developed briefly over south Norway and Denmark on the 27th, but there was much rain in the area, with continuing S.W. wind, and on the 28th another cold front, stemming from a low which developed over Jutland, swept eastwards across the Continent. A low centre moved into the Skagerrak from the North Sea on the 29th, remaining there throughout the 30th; but with anti­ cyclones building up in mid-Atlantic and over north-eastern Europe the weather showed much promise of improvement. Spreading eastwards, the Atlantic high enveloped Britain on the last day of the month, and a considerable ridge probed far into Europe. A cyclonic disturbance, however, covered Finland and the east Baltic countries, so that winds were westerly except in southern Europe on the south side of the ridge. This ridge extended to southern Scandinavia and most of middle Europe on 1st September, giving easterly winds as the low retreated into northern Russia. During 1st September the two highs joined up to create an extensive anticyclonic ridge penetrating southern Scandinavia, the Baltic region, Poland and western Russia, again establishing an easterly airflow across middle Europe. On the assumption that fine, clear weather is needed as an inducement to begin migration, considerable movement was probably in progress between latitudes 50° and 60°N. 2nd September ig$6. Late on the 1st a somewhat restricted warm front, producing much rain, was associated with a low moving northwards along the Biscay coast of France. The front crossed the English Channel before nightfall and pursued its way through East Anglia, reaching the Humber Estuary at dawn. From Dungeness north to Spurn it was responsible for bringing down a large number 338 BRITISH BIRDS [VOL. LII of drift-migrants caught whilst journeying southwards through the Continental maritime region. These seem likely to have had their origin in southern Norway or Denmark where calm air and clear sky had prevailed the previous evening. Dungeness, Kent. Many birds were already at the lighthouse at 02.30 hours, these including Wheatears and Pied Flycatchers. Assuming they had travelled in a S.W. "standard direction" for some 74 hours in the N.E. airstream at 20 knots plus their own flight-speed, an origin in Denmark seems likely for the vanguard. When the traps were driven at 06.15 hours it was obvious that there was a great increase in birds from the previous day, and at that time most of these were still in the neighbourhood of the lighthouse. The Garden Warbler was the commonest Passerine, FIG. 2—POSITION OF THE WARM FRONT AT 06.00 HOURS ON 2ND SEPTEMBER 1956 closely followed by Redstart, Pied Flycatcher, Whitethroat, Wheatear, Whinchat and Willow Warbler in that order. How­ ever, although Whitethroats showed a tenfold increase from 1st September it is more than likely that a proportion were of British origin, since 5 of the 30 trapped had been ringed previously at Dungeness. The same probably applies to the Sedge Warblers, since of the three captured one had been ringed before. Cley, Norfolk. There was rain during the night and early morning on the north Norfolk coast. Redstarts were dominant, Garden Warblers less so—a reversal of the situation at VOL. LII] SEPTEMBER DRIFT-MOVEMENTS 339 Dungeness, and this may indicate a different area of origin, perhaps the Skagerrak crossing. An increase in Wheatears was noted, and Pied Flycatchers and Whinchats were common, but Willow Warblers were few and there is no suggestion that either Sedge Warblers or Whitethroats were involved. Holme, Norfolk. At the north-eastern corner of the Wash the numbers of most species doubled between 14.00 and 18.00 hours, perhaps the result of coasting in the Wash area leading to concentration at this point, although another explanation might be that the birds had passed the front in clear weather to east­ ward and had been brought to the coast by the veering wind.