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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.