BRITISH BIRDS NUMBER 3, VOL. XLVI, MARCH, 1953.
MIGRATION IN LATE SEPTEMBER AND EARLY OCTOBER, 1951.
BY DAVID JENKINS.
INTRODUCTION. DURING the autumn of 1951 a party from Cambridge University visited south Norway in order to study migration. Towards the end of their visit a large scale passage of many species was observed (Mylne and Hyatt, 1952). On their return it became apparent that migration had been observed at many places throughout western Europe at the same time. In this paper an attempt has been made to collate observations of this movement and to establish some correlation with the prevailing weather conditions. No detailed conclusions can be drawn from fragmentary observations obtained from widely separated stations for a single migratory passage, but it is hoped that the paper may show that this approach can be developed with profit in the future. As it is thought that it would be unwise to draw any conclusions from a consideration of the meteorological conditions associated with one peak of passage alone, and since observations on the Continent showed that others occurred during September 1951, the weather during the month as a whole is considered and a comparison is made of the climatic features at the time of each peak.
General pattern of migration during September-October, 1951. Although there were few days in September when no migration occurred, observations on the Continent indicate that there were definite peaks of movement. These peaks were spread over several days. Their existence may be seen by an examination of Tables I, II, III and IV which show the migration of certain Passerine species at observatories in Finland, Sweden, Norway and N. Germany. At the different observatories different species were principally involved. This makes the coincidence of dates (Septem ber 6th-9th, I9th-22nd and 29th—October 2nd and slightly earlier in Finland) remarkable and encourages the belief that the initiation of migration in many species may have been due to a common factor or factors. 78 BRITISH BIRDS. [VOL. XLVI.
TABLE I : SIZNILISCAR BIRD OBSERVATORY, FINLAND. MIGRATION SEPTEMBER I2TH-27IH, 1951.
(At this station some birds were migrating to the east, some to the west. In order to establish peaks of passage the totals have been summated).
Species 12th 13th 14th 15th i6th 17th 18th 19th 20th 2ist 22nd 23rd 26th 27th
Woodlark ... 1 3 - - 3 I 1 2 1 —
Skylark 2 — 5 6 - 6 14 3 3 6 7
Nutcracker ... 39 3 809 1 — 16162 433 nII 33 44 2S2 S0o 4 2 36 6 2 Robin 10 2 —- —- —- —- -— 3 -— -— 67 135 5 3 5 2
Meadow Pipit ... 8877 81 94 139 — 35 133 6 118 229 194 92 27 95 17 28
White Wagtail... 48 2 '3*3 23 — ix11 7 1 17 rI — 6 626 1 - Starling 44 2 — 5 — 286 39 - 32 - 2 - 10 - Siskin 227 100 185 912 6 — 348 48 46 345 301 36 533 169
Chaffinch 252 130 86 152 - 30 2,054 40 52 243 436 57 4.374 3.S94 MINOR MAJOR PEAK PEAK Explanation of the Tables. For diurnal migrants the figures represent counts of all birds seen flying past the observa tion posts from shortly after dawn until migration ceased. For nocturnal migrants the figures represent daily counts, made in a specified area, of birds which had arrived during the night. A dash (—) indicates that a watch for migrating birds was kept, but none was seen. Where migrating birds were recorded, but actual counts not made, the relative numbers seen are indicated by the following signs : X shows that the species was observed. » shows that less birds were present than on the previous day. + shows that more birds were present than on the previous day, while (++) or three (+ + +) plus signs indicate heavier or very heavy passage.
TABLE II : SELECT ED DIURNAL MIGRANTS SHOWING PEAKS OF PASSAGE AS RECORDED IN SWEDEN-. September OTTENBV. 1st 2nd 3rd 4th 5th 6th 7th 8th gth 10th nth 12th 13th 14th 15th 16th Species Sparrow Hawk 1 3 3 7 _. 8 32 6 7 I - I - - - Kestrel 2 1 1 4 Crane _ 9 275 zgq 422 11 — 21 __ _ Dunlin — 15 — 10 — 626 472 204 189 174 15 15 169 75 — 5o Black-headed Gul1l 3 - — 12 9 75 213 240 85 17 2 — 2 — — — House Martin 178 460 190 575 20 6,771 1,982 200 — — — 35 3 — Tree Pipit ... 25 81 — 40 96 18 30 19 24 — 5 2 4 — 2,172 — 310 5,960 2,210 257 — White Wagtail 323 730 — 935 5-572 2.665 3 ,587 1,383 1,277 — 18 — 14 17 15 — Starling — 75 Linnet
September. October. OTTENBV 17th 18th 19th 20th 21st 22nd 23rd) 24th 25th 26th 27th 28th 29th 30th 1 1st 2nd 3rd Species Sparrow Hawk — — 6 19 27 Kestrel ... — — 12 6 5 5 Crane ... — — 117 372 62 25 — 42 98 Dunlin ... 214 ^Q 411 129 4 97 50, 15 443 4 — 19 — 16 Black-headedGull— 109 6 573 555 378 6 6 — House Martin — — 166 Tree Pipit ... 32 12 37 10 1 13 31 24 — 83 32 2 35 20 — White Wagtail 254 23 1,870 1,323 i,i39 8,381 253 1,090 196 798 :,98o 368 77 no 90 Starling ... — 15 13 62 — 104 141 ,677 149 161 102 — — — —217 Linnet ... — — n11 385 744 561 132 900 i,03o 1,017 475 281 40 VOL. XLVI.] MIGRATION IN AUTUMN, 1951. 79
TABLE III : SELECTED MIGRANTS (DIURNAL UNLESS OTHERWISE INDICATED) SHOWING PEAKS OF PASSAGE AS RECORDED IN NORTH GERMANY.
NORTH GERMANY. September. Species ist 2nd 3rd 4th 5th 6th 7th 8th 9th 10th nth 12th 13th 14th 15th 16th Skylark Song Thrush* + — Redwing* Wheatear* Redstart* + + +++++++++ Robin* — + + + — Meadow Pipit Tree Pipit Chaffinch — — + +
NORTH GERMANY. September October. Species 17th 18th 19th 20th 21st 22nd 23rd 24th 25th 26th 27th 28th 29th 30th 1 st 2nd 3rd Skylark + — — - + x x + x x % + + + ++ — Song Thrush* +++++++++ Redwing* ... + + + + +++++++++ Wheatear* ... Redstart* ... + + — — + — — Robin* + — — + — Meadow Pipit — ++ -+ ++ ++ + + X X Tree Pipit ... — — + + + Chaffinch ... ++++++
* Night migrant.
TABLE IV : SELECTED DAY MIGRANTS SHOWING PEAKS OF PASSAGE AS RECORDED IN SOUTH NORWAY.
ElGVAAG September. O ctober. Species 15th 16th 17th 18th 19th 20th 21s2ISt 22nd 23rd 24th 25th 26th 27th 28th 29th 30th ist
Skylark 21 1 — 17 77 2 Swallow 22 135 3 IS — 69 7 Meadow Pipit 13 0 II 5 7 2 — 6 4 White Wagtail 147 bi 3 55 21 ib S 479 595 Starling 2 37 9 2 — 11 — 119 93 Siskin... 120 45 47 2 108 176 Linnet 13 4 — 67 41 28 17 no Chaffinch 123 773 582 7« 68 — 44 — 13,555 11,807 (observations discontinued on October ist) PEAK
LISTA FYR. September. Species 5th 16th 17th 18th I9th 20th 2ISt 22nd 23rd Z4th 25th 26th 27th 28th 29th 30th ist Sparrow Hawk — — — 2 2 20 4- _ _ 1 I — — Skylark 50 so 70 300 IOO 100 500 50 15 40 30 X X X X X X Meadow Pipit X X X X X + + X X + + + X X X X X + ++ X + + + Siskin... 6 Linnet — — 30 80 40 60 400 400 100 20 300 IOO 120 300 100 20 t Twite — 3 3 12 30 40 50 »» «« X 10 15 30 15 40 X X Chaffinch 3 3 50 6 — — I 3 10 40 6 + + + Yellowhammer 6 25 30 45 100 20 3 10 2 10 30 40 20 6 — — 2 10 I I Reed Bunting — — 15 4 10 20 4 — I 2
There was a marked peak of migration on September gth, loth, nth : most noticeable were birds of prey of which at least eight species were on passage. The movement also involved many species of Passerines. Compared with the intervening period there was another marked wave beginning on September igth and lasting until September 23rd. 80 BRITISH BIRDS. [VOL. XLVI.
THE WEATHER. Analysis of weather conditions at the place of origin of these birds may give some idea of the extrinsic factors which initiated the migration. It is reasonable to assume that many of the birds which migrate through western Europe breed in Norway, Sweden and Finland. Weather conditions in these countries will, therefore, be considered in relation to migration in September, 1951. It is not to be supposed that a precise correlation between migratory behaviour and any single weather factor may be found, since the latter are possibly additive in effect, and since the internal state of the bird must be taken into account. Hinde (1951) has considered some of the complications of this approach to the study of bird migration. General pattern of weather. At the beginning of September pressure over much of Europe was low, but an anticyclone built up over France and moved to the east. It was followed by another high pressure area coming from the Atlantic which moved across Britain and on September 9th was centred off the west coast of Norway. This " high " then proceeded south-east across Europe and was succeeded by low pressure weather which lasted till September 17th. On this date a ridge of high pressure extended across France and Germany from an anticyclone centred west of Ireland. Pressure over northern Europe was still low, but began to rise on September 19th and 20th as this " high " moved east. How ever, it did not proceed as far north as Norway but changed to an easterly direction. By September 23rd it was centred near the Black Sea. Meanwhile the barometer had dropped again in the north. The " high " centred near the Black Sea intensified and began to move to the north on September 26th. On September 27th pressure was high over a large area of eastern and north eastern Europe extending from Bear Island to the Black Sea. The movement to the north continued and took on a westerly component so that on September 29th the anticyclone was centred over Finland. It then moved to the south-west and at the end of the month its centre was over southern Norway. Detailed consideration of weather. In general the weather conditions associated with high barometric pressure (anticyclones) in summer include light winds and clear skies. Winds blow outwards in a clockwise direction from areas of high pressure and inwards in an anticlockwise direction towards depressions. Thus, in uncomplicated circumstances, winds to the south of an anticyclone will be easterly and to the north, westerly. In late September, 1951, there was a gradual build-up of anticyclonic weather conditions over practically the whole of northern Europe VOL. XLVI] MIGRATION IN AUTUMN, 1951. 81 as a high pressure centre was transferred from the Black Sea to southern Norway. As the anticyclone moved westwards so were the regions to the south of it exposed to easterly winds.
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GRAPH II : BAROMETRIC PRESSURE IN NORTH-CENTRAL NORWAY AND SWEDEN (68°N.). 82 BRITISH BIRDS. [VOL. XLVI.
Barometric pressure. As is shown in Graphs I and II, pressure was high over Scandinavia at the times of peak migration in Sweden, Norway and in North Germany. Finland was, however, under the influence of low pressure centres on September 15th and 18th when minor peaks occurred. The major peak at the end of the month took place when the barometer was rising sharply. However, it is difficult to believe that birds can be sensitive to pressure changes as such and for this reason further consideration of isobaric patterns has not been attempted.
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I 1 I I I I I I I I I 1 1 1 I I 1 1 1 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 2 4 6 SEPTEMBER OCTOBER GRAPH III : WIND STRENGTH—SUMMATION FOR SCANDINAVIAN STATIONS.
Wind. In Graph III units of wind measured as steps on the Beaufort Scale have been totalled for 20 stations in Scandinavia. There is a marked drop in wind from September 5th-ioth, from September i6th-24th and from September 28th-October 2nd. These periods of reduced wind strength correspond fairly accurately with the periods of peak migration. A more detailed analysis of wind conditions at stations in Sweden and Finland has also been undertaken (Graphs IV and V). At no time was there much wind. There were no periods of absolute calm over the whole of either country, but in Sweden there were VOL. XLVI] MIGRATION IN AUTUMN, 1951. 83
KARESUANDO 68° 27' N. 6 22° 30' E. U 4 HI
J I jI LrVN/ i i i i i\ i I\ i iW i
ASELE 6 64° 10' N. 17° 22' E. 4
2- J^ IX. I P WVWWWWW FALUN <>\- 60° 37' N 15° 38' E. = Migration Peak
10 12 14 16 18 20 22 24 26 28 30 2 4 SEPTEMBER OCTOBER GRAPH IV : WIND AT FOUR REPRESENTATIVE INLAND SWEDISH STATIONS.
some which approached this. These were September cjth-nth, i6th-i7th, 2oth-23rd, 2C;th-30th. Peak migration at Ottenby (the Swedish Bird Observatory at the southern tip of Oland in the Baltic) occurred on September 7th-8th, ic;th-2ist and 29th. Records from the Finnish Bird Observatory at Sizniliscar show that peaks of migration occurred there on September 15th, 18th, 23rd (Robins only) and 26th-27th (See Table I). Graphs V and VI show that there was as much wind on September 15th as on any other day in the period for which migration records are available, but that calm conditions prevailed on September 18th, 23rd and 26th-27th. It is suggested by Williamson (1952) that calm weather may provide an active stimulus for the initiation of migration. The 84 BRITISH BIRDS. [VOL. XLVI.
weather conditions associated with most of the waves of migration in September, 1951, may perhaps be regarded as supporting this view, although those at the time of the earliest peak in Finland cannot. It is, however, unknown exactly where the birds came from and the evidence is suggestive that lack of wind may be one of a series of heterogeneous external stimuli which mount up, especially as there was a fall in temperature of 29 degrees Fahrenheit between September 7th and 10th and of 14 degrees overnight on September I4th-I5th in central Sweden. It has long been known that the direction of the wind is all important in determining whether birds are drifted across the North Sea to British shores (Rintoul and Baxter, 1918). This was well shown in September, 1951. During the earlier peaks of passage
14 LUONETJARVI 62°50'N. 25°50'E 12 z 10 o
H LLI Z z >- u o 14
I I- e> z 10, LU I- Q 5z 6
9 10 I I 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 I SEPTEMBER OCTOBER DATE GRAPH V : WIND STRENGTH AT TWO FINNISH STATIONS AT DIFFERENT LATITUDES. VOL. XLVI] MIGRATION IN AUTUMN, 1951. 85 on the Continent when winds across the North Sea were in the main westerly few birds came to Britain. When the winds were easterly, as at the end of the month and in early October, many vagrants were recorded at British observatories. (The mechanics of the process of migrational drift by wind are considered in Williamson, 1952).
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GRAPH VI : WIND FORCE FINLAND—SUMMATION 32 STATIONS. Temperature. Eagle Clarke (1912) found that " movements did not begin in earnest until a decided fall in temperature took place." and he decided that this factor was the precursor of each of the pronounced movements he studied. Ritchie (1939), in his detailed study of a single Waxwing (Bombycilla garrulus) invasion, on the other hand, concluded that " the most favourable condition for departure seems to be a temperature which has remained steady for two or three days." 86 BRITISH BIRDS. [VOL. XLVI.
Temperature may, then, be considered under two headings :— (a) Daily variation (maximum minus minimum). Graphs VII and VIII show the daily variation in temperature at Finnish and Swedish Stations. In each case the variation was reduced towards the end of the month when the major peak occurred. However, the variation was great in the middle of the month when the other waves were observed.
10 I KAJAAN 64°20'N. 28°50' E.
SODANKYLA 67°30'N. 26°20'E.
a. D UTTI 60°90' N. 28°S0'E. ESio
P P P LUONETJARVI 62°50'N. 'MM vVVVV 28°S0'E. -mm— Migration Peak
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 2 4 SEPTEMBER OCTOBER DATE GRAPH VII : DAILY VARIATION IN TEMPERATURE (MAXIMUM MINUS MINIMUM)—FINLAND. VOL. XLVI] MIGRATION IN AUTUMN, 1951. 87
15 10 5L STENSELS 65°04'N. I7°I0'E. ASELE 64°I0'N. I7°22'E. KARESUANDO 68°27 N. 22°30 E. - JOKKMOKK. 66°36N, I9°S0 E.
5- ^•^ ^=~K^y-^ D HARNOSAND 62°3l'N. I7°26'E. <20 -— BLAHAMMAREN 63°H'N. I2°II'E.
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--v./ \—~^< - FALUN 60° 37V I5°38'E. \y/ V — TUPPfALA 59°53'Nf I7°36'E. ( y , 6 7 8 9 10 II 12 13 14 IS 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 DATE SEPTEMBER GRAPH VIII : DAILY VARIATION IN TEMPERATURE (MAXIMUM MINUS MINIMUM) AT SWEDISH STATIONS. (b) Mean change from one day to the next. Mean temperature changes at four stations at different lati tudes in both countries are represented in Graphs IX and X. These give an overall picture of the changes in Sweden and Finland as a whole. They show that the mean temperature dropped gradually from September 6th to 10th and from 17th or 18th (earlier in Finland) to 20th. If anything there was an overall rise in mean temperature in these countries at the end of the month, but analysis of the figures given in the Daily Weather Reports of the Meteorological Office for all Scandinavian stations (including Norway, but not Denmark) shows that during the period September 7th-3oth a fall in temperature of 10 or more degrees Fahrenheit occurred as follows :—• north of 65 degrees north latitude, on September i8th-i9th, igth-20th, 2oth-2ist, 26th-27th, 2gth-30th ; between 60 and 65 degrees north, on September I4th-i5th, I7th-i9th, 25th-26th ; south of 65 degrees north, on September 7th-8th, 8th-ioth, I9th-20th, 28th-29th, at different stations. Thus before all the migration peaks in September, 1951, there was a notable fall in temperature, of 10 degrees Fahrenheit or more, somewhere in Scandinavia. Before the first wave there was a drop BRITISH BIRDS. [VOL. XLVI. 6 7 8 9 10 II 12 13 14 IS 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 DATE SEPTEMBER GRAPH IX : MEAN TEMPERATURE AT REPRESENTATIVE SWEDISH STATIONS. of II degrees between September 7th and 8th and of 12 degrees between September 8th and 10th (making in all a fall of 23 degrees between 7th and 10th) in south-west Sweden, before the second a drop of 11 degrees at Haparanda (extreme north of Baltic), of 17 degrees at Hammerfest (north Norway), of 11 degrees in western Finland, of 13 degrees in central Finland and of 12 degrees in south- central Sweden between September 17th and 20th (the drop occur ring within 48 hours at most of these places), and before the last peak a drop of 10 degrees on 25th-26th in central Finland, of 14 degrees on 26th-27th at Narvik (north Norway), of 13 degrees on September 28th-2o,th in south-central Sweden, and of 11 degrees on 29th-3oth in western Finland. It seems significant that all the periods during which temperature fell markedly were succeeded by movements of birds. The places where birds were seen migrating were, however, generally different from the stations where the temperature drop was recorded. This is why it has been thought worth while to plot the recordings for the whole period at representative stations at different latitudes throughout Sweden and Finland. Perhaps in this way an overall picture of the temperature changes can be obtained. The first two migration peaks were associated with a marked drop in temperature at certain places and a gradual fall in temperature throughout VOL. XLVI.] MIGRATION IN AUTUMN, 1951. 89 25 H 20- KAJAANI 64°20'N. 28°50'E. 15- 10- SODANKYLA 67°30'N. 26°20'E 10 o° 5 IXJ a: e ° UTTI 60 90'N. 26°80'E. < i20 a: LUONETJARVI 62°50'N. 25°50'E. MWMi = Migration Peak 12 14 16 18 20 30 DATE SEPTEMBER GRAPH X : MEAN TEMPERATURE CHANGES AT REPRESENTATIVE FINNISH STATIONS. Scandinavia as a whole. The last, and major, peak in which it is known that birds from Norway and Sweden and/or Finland were involved, was associated with a marked drop in temperature in some places in all three countries but with a steady temperature, with minimum daily variation, over the remainder of the area. Cloud. The cloud situation in Finland in September, 1951, is represented in Graph XL Cloud density was high when the peaks of diurnal migration occurred, September 15th, 18th and 26th-27th, but it was at its lowest prior to the maximum nocturnal (Robin) migration on September 23rd. The importance of cloud density in relation to migration is considered below. Figures are not available for cloud density over Sweden. 90 BRITISH BIRDS. [VOL. XLVI. - D - 2000 hours j^fo = Migration Peak d= diurnal migrants Q 0800 hours n= nocturnal migrants Z> O u - c-*-.^'\ A « 200 - z \.\-ycLt\. \ i .^ / !/' y\ . > /^ \ \ / / Q v O 100 \ ' / u Pd Pd Pn , Pd MAAA, hhtM ' JtMdUdLUUUL. »VVVYI TTYVII TIII IF TimnmrVTVr 1 I I I I I 1 1 1 I 1 1 ! 1 1 1 1 1 1 1 I 10 II 12 13 H 16 17 IS 19 20 21 22 23 24 25 26 27 28 29 30 DATE SEPTEMBER GRAPH XI : TOTAL CLOUD FINLAND—SUMMATION 32 STATIONS. Fog. There was little fog over Scandinavia prior to September 23rd, but after that it was more or less foggy or misty over southern Sweden until October 5th and over southern Finland from Septem ber 25th to 29th. There was slight fog in the Kattegat on Sep tember 25th and 28th, heavy mist over the entrance to the Gulf of Finland on September 26th and 28th, with fog over Denmark and extreme south Norway on the morning of September 29th, and widespread fog over the Baltic on October 1st. On this day there was mist over the sea between the Gulf of Finland and Oland and also off the south-west coast of Norway. On October 2nd there was mist over south-west Sweden and the Kattegat extending over the land to the sea west of Denmark. There was still mist over the Kattegat and Denmark on October 3rd and 4th, but thereafter it decreased. Summary. First peak. Second peak Third and main peak. High barometer High barometer High barometer (not Finland) Overall fall in wind Ditto, except in Finland Undoubted fall in wind strength during peak of 15th strength everywhere Very marked tempera Marked temperature Marked temperature ture fall in central fall in all three fall in all three Sweden countries countries succeeded by minimum tem perature variation Temperature variation Temperature variation great great VOL. XLVI] MIGRATION IN AUTUMN, 1951. 91 Summary (continued). First peak. Second Peak. Third and main peak. 5. — High cloud density High cloud density diurnal migrants. diurnal migrants. Low cloud density nocturnal migrants 6. — — Fog over southern Scandinavia MAP I : WEATHER CONDITIONS ON OCTOBER IST, 1951, 0600 HRS. 92 BRITISH BIRDS. [VOL. XLVI. Climatic factors acting on birds at the end of September, 1951, included anticyclonic weather conditions, extending over practically the whole of northern Europe, minimum wind strength and a marked fall in temperature over a wide area succeeded by minimum tem perature variation. From a consideration of the opinions of other workers and of weather conditions at the times of earlier migration MAP II: BIRD OBSERVATORIES MENTIONED IN THE TEXT. VOL. XLVI] MIGRATION IN AUTUMN, 1951. 93 waves during the month, it seems that these conditions are very favourable for migration. Thus it is not surprising that very large numbers of migrating birds were seen at many places in northern and western Europe at this time. THE COURSE OF THE MIGRATION PEAK AT THE END OF SEPTEMBER. The gradual build up of anticyclonic weather conditions over the greater part of Europe at the end of September, 1951, suggests that birds may have been on the move over a very large area towards the end of the month. This is confirmed by the records from west European observatories about this time of such more eastern species as Brown Flycatcher (Muscicafia latirostris), Yellow- breasted Bunting (Emberiza aureola), Petchora Pipit (Anthus gustavi) and Yellow-browed Warbler (Phylloscofus inornatus). These should not be regarded as lost stragglers, but as interesting clues to the state of migration in other parts of Europe. In the account of migration which follows, the period under review is from September 25th to October 10th or for as much of it as there is information available. Only the movements of certain representative species will be discussed. These will be considered in two groups, as diurnal and nocturnal migrants. The distinction between these two groups is not always clear cut since some species may start to migrate in the day time on one occasion and at night on another, but it is believed that without it the pattern of the late September/early October migration of 1951 cannot be understood. In this period there is little evidence for the arrival of birds in Great Britain before the beginning of October. However, marked passage was seen in Scandinavia on September 26th at the Finnish observatory at Sizniliscar, including Robins (probably the birds which were later recorded further west) on September 22nd and 23rd. At Ottenby the situation was slightly complicated as there was fog on September 26th and 28th, but there was a major move ment on the 27th, and it was found at Lista, S. Norway, that small Passerines suddenly appeared in the garden of the ringing station on the same day. These were nocturnal migrants. At Eigvaag, the principal watching post on the Lista peninsula, diurnal migration recommenced equally suddenly. There had been little to see since September 23rd and then on September 29th nearly 15,000 birds were counted flying past within a few hours. It is not, of course, possible to assume that these peak dates represent the start of migration in Scandinavia since the movements at Sizniliscar, Ottenby and Eigvaag were definitely onrushing waves (as contrasted with those arrested by bad weather—see Bagg et al, 1950). However, it is unlikely that the migration started much before September 26th, in the far north. The dates of peak migration at the various observatories are best summarised in tabular form, as on the next page. 94 BRITISH BIRDS. [VOL. XLVI. Passage Country. Station. beginning. Peak. I. Finland Sizniliskar Sept. 26th Sept. 26th-27th 2. Sweden Ottenby Sept. 27th Sept. 29th-3oth 3- Norway Revtangen, Stavanger Sept. 26th Sept. 2gth-3oth Lista (night migrants) Sept. 27th Sept. 30th (day migrants) Sept. 29th Sept. 29th-3oth 4- N.E. Germany Rugen Sept. 28th early October 5- N.W. Germany Wilhelmshaven no well high migration defined peak early October Wangerooge) Mellum I Sept. 26th October ist-2nd Amrum ) 6. Holland The Hague Sept. 28th October 3rd 7- Great Britain Cley Sept. 29th October 1st Gibraltar Point Sept. 30th Oct. ist-2nd Spurn Head Sept. 30th Oct. ist-3rd Isle of May Oct. 1st Oct. ist-2nd Fair Isle Sept. 30th Oct. rst-3rd Little Ross Oct. 3rd Oct. 4th Lundy Oct. 2nd Oct. 2nd Skokholm Oct. 2nd Oct. 2nd (?) Great Saltee Oct. 1 st Oct. 2nd (no real peak) Channel Islands—no comparable movement of Passerines. Tables showing the details of migration at these observatories can be seen here and elsewhere in this paper (pages 78, 79, 96 and part 2 (April). The locality of each Observatory is shown on Map II. TABLE V : OTTENBY. NUMBERS OF SOME DAY MIGRANTS SEEN DEPARTING. September. October. Species. 25th 26th 27th 28th 29th 30th 1st Woodlark — — 5° — 2 Skylark — — 12 -— 165 142 83 Meadow Pipit ... 2 — 27 — 138 6 1 White Wagtail ... 196 — 798 1,980 368 74 Starling ... 104 — 141 1,677 149 161 Greenfinch — — 17 125 6 Siskin 22 — 73 300 11 2 Linnet ... 132 8 900 5-030 1,017 475 Chaffinch — 588 318 22 169 Brambling — — 15 4 213 Yellowhammer ... 15 — 224 252 90 TABLE VI : EIGVAA G, S. NORWAY. NUNUMBERM S OP SOME DAY MIGRANTS SEEN DEPARTir September. Species. 25th 26th 27th 28th 29th 30th Woodlark — — 56 80 Skylark — — 77 2 White Wagtail 7 2 6 4 Starling 11 93 Greenfinch 119 — 11 52 Siskin — — 108 176 Linnet 41 28 17 no Chaffinch 44 13,55° Brambling 2 11,800 129 1,369 Yellowhammer 10 37 29 — 28 44 (Records from Lista Fyr, the ringing station, show that the finch migra- tion continued on a large scale on October 1st). VOL. XLVI.] MIGRATION IN AUTUMN, 1951. 95 TABLE VII: REVTANGEN, SOUTH NORWAY. NUMBERS OF SOME DAY MIGRANTS SEEN DEPARTING ON SEPTEMBER 29TH, 195I. Skylark 400 plus Great Tit ... 15 Blue Tit ... 195 Fieldfare ... 13 Meadow Pipit 45° Greenfinch... 12 Siskin 4 Linnet 36 60-70 Twite 12 Crossbill (Loxia curvirostra) 28 Parrot Crossbill (Loxia pytyopsittacus) 600 Chaffinch ... 85 Bramblin(Counts nog t usually made—peak passage on October ist). TABLE VIII : LISTA FYR. NUMBER OF NIGHT MIGRANTS SEEN. September. October. 25th 26th 27th 28th 29th 30th ist Wren 4 1 3 3 2 6 + Song Thrush — 2 — 2 1 Redwing ... — — - + + + + + + Redstart ... 2 2 7 - 6 15+ 4+ Robin — — 1 2 6 20 + Blackcap ... — — 2 I 2 Garden Warbler . — — 2 3 5 Willow Warbler . — — 3 2 2 5 3 Goldcrest ... — — 1 10 15 Pied Flycatcher . — — 5 5 1 4 Dunnock ... — — 1 1 3 (Passage beginn ing on 27 ix. Redwing peak on 28 .ix. Main passage on 3o.ix). TABLE IX : OTTENBY. NUMBER OF NIGHT MIGRANTS RINGED. September. October. 25th 26th 27th 28th 29th 30th 1 st 2nd 3rd Song Thrush ... 8 — 1 11 21 12 7 11 2 Redwing 1 Redstart Robin ... 1 3 72 3° 416 59 7° 73 13 Blackcap 1 — — 1 8 — — 12 2 Willow Warbler 1 Goldcrest Pied Flycatcher 1 (Passage beginni ng on 27.ix Main passage on 29.ix). Main passage Song Thrush and Redwing on i.x. (beginning 2g.ix.) Main passage Robins 25.ix and 3-4.x. (some i.x). REVTANGEN, STAVANGER, S. NORWAY. Main passage i.x. (150 Robins seen). Beginning (Goldcrest only) 29.ix passage over 3.x. With the exception of a small easterly passage at the Finnish Observatory all the diurnal migrants at the Scandinavian stations were flying in a southerly direction. Thus in the early October 96 BRITISH BIRDS. [VOL. XLVI. rush there were at least two streams of birds moving south out of Norway and Sweden towards Denmark and Germany. There is no bird observatory in Denmark at present but records have been received from several in north Germany. At Lake Hidden (Rugen) in north-east Germany birds have been found in the past to come from the north and north east. In September/ October, 1951, there was little diurnal migration, but what there was corresponded with the Ottenby findings. Thus there was little movement of Chaffinches and Bramblings, but larger passages of Linnets, Siskins and Yellowhammers. Although detailed notes are not available the picture in north west Germany was a different one. Here, at Wangerooge-Ost, Mellum and Amrum, the most prominent feature was the finch migration. Chaffinches were on the move from September 26th onwards reaching a peak on October 1st and 2nd at Wangerooge and Amrum where exceptional migration was noted, " many thousands " being reported at the latter observatory. There can be little doubt that these birds are the ones that were seen in Norway, since, in spite of a relatively large finch passage in Finland, few Fringillidce were seen at Ottenby or Rugen. At Amrum, one of the North Frisian Islands, the birds were flying in a north-easterly direction against the wind on October 2nd. However, the whole problem of " riickzug " in Chaffinches is too complicated to discuss here. The finches continued their passage to the south and at The Hague, Holland, where there had been no passage since September 28th, they were recorded on October 3rd flying high and accompanied by Siskins and crows. No information is available from farther south, but it seems that in the five days September 29th to October 3rd the finches migrated south from Lista in Norway to The Hague in Holland. The North Sea crossing. Although a large number of Chaffinches were moving on the Continent, scarcely any were reported in Britain during the same period, but as shown in Table X another Passerine species, the Robin, arrived in great numbers during the first week of October. TABLE X -. NUMBERS OF TYPICAL NOCTURNAL MIGRANT—BRITISH EAST COAST OBSERVATORIES. September. October. ROBIN. 25th 26th 27th 28th 29th 30th 1st 2nd 3rd 4th 5th 6th 7th 8th 9th Cley ______105 37 39 1 — 1 2 — Gibraltar Point ...... — — — — — 300 300 150 80 50 40 30 30 Spurn Head ...... — — — 1 — 300 500 500 300 300 X X IOO Monks' House ...... 1 — — — 1 3 2 - - 500 — — - Isle of May — — — — — 600 600 400 200 150 IOO 70 40 Fair Jsle 3 1 — — 2 30 50 75 75 50 25 4 10 VOL. XLVI.] MIGRATION IN AUTUMN, 1951. 97 Should such a North Sea crossing be regarded as a normal pro cedure in the autumn migration of small Passerines ? Eagle Clarke (1912) envisaged birds flying across the North Sea from Cape Stat or Bergen in Norway to the Shetland Isles and Scotland. Such a route is possibly taken by the larger birds (thrushes, Starlings, crows, etc.) which winter regularly in the British Isles, but it must be doubted whether the majority of the smaller Passerines (Phyllos- copus sp., Goldcrests, Robins, Redstarts, flycatchers, etc.) normally cross the North Sea at all. Such a crossing must be an uncertain and exhausting, often perilous passage for such feeble fliers and it is known to involve considerable loss in weight (Fair Isle Bird Ob servatory Bulletin, No. 6, 1952, pp. 30-31). Natural selection would be bound to oppose this journey when a much safer route, involving only short sea crossings, is available via the Skagerak and Kattegat to winter quarters in southern Europe and North Africa. If the North Sea crossings from Bergen to Scotland had been undertaken in September/October, 1951, it might be expected that there would be a coincidence of arrival dates at the observatories in south Norway and in Scotland, but this was not so, as 48 hours or more elapsed between the first waves at Revtangen-Lista and Fair Isle-May. On the other hand the arrival dates in N. Germany and Britain agree very closely, which suggests that the night migrants left the Continent when over the Skagerak, Kattegat and Denmark. However, although numbers were seen at Ottenby on September 27th and 29th it was two days before any were recorded to the south-west, a delay for which the weather can have accounted. There was much fog over the Baltic at the end of September, and on the evenings of the 28th and 29th the sky was obscured by 8/8 cloud in S. Norway and S.W. Sweden. Then at 1,800 hours on September 30th the sky was clear, and that night there was peak migration at Lista, while the following morning the night migrants were recorded in Germany and Britain. This suggests that clear skies provide optimum conditions for nocturnal migrants and the opposite may delay their departure, and it accords with Kramer's (1952) tentative suggestion that they may orientate themselves by a bearing on the sun in the evening. So nocturnal migrants left Scandinavia in force on the night of September 30th, following an evening of thus favourable conditions. But by midnight there was 8/8 cloud over north-western Denmark, and by 0700 hours on October 1st a pronounced fog bank off south west Norway. At the same time there was a force three (Beaufort scale) wind blowing from the east over Denmark. These overcast conditions prevailed over the Skagerak and the North Sea as far as and including; the east coast of Britain. Thus when the migrants were crossing the sea (Skagerak and/or Heligoland Bight) the sky was obscured and it is likely that they were unable to navigate. 98 BRITISH BIRDS. [VOL. XLVI. Their arrival at Cley with north-easterly and at Fair Isle with south-easterly winds suggests that when they were unable to navigate they were passively drifted across the North Sea by the prevailing air currents. The weather conditions on the nights of October 2nd and 3rd were exactly similar except that the clear skies of the early evenings were replaced by fog instead of heavy cloud, and nocturnal naviga- gation became even more difficult. The occurrence of night migrants in the early October rush to Britain may therefore be accounted for by the presence of easterly winds associated with overcast skies or fog which made navigation in the Denmark- Skagerak area impossible after the birds had started their migration. (To be concluded in the April number).