BRITISH BIRDS NUMBER IO, VOL. XLVI, OCTOBER, 1953.

MIGRATION AT THE KENTISH KNOCK LIGHTSHIP. BY D. F. OWEN (Edward Grey Institute, Department of Zoological Field Studies, Oxford). (1) INTRODUCTION. IN conjunction with the 1952 autumn watch for visible migration organised by the Edward Grey Institute (Lack, in press)*, I visited the Kentish Knock Lightship (51 °39'24"N., OI°4.O'4.8"E.) which is situated in the Thames approaches in the southern section of the North Sea. My period aboard commenced at 12.00 hours on October 22nd and finished at the next relief at about 10.00 hours on November 5th. Eagle Clarke (1912) spent a month aboard the Kentish Knock in the autumn of 1903. As his dates were from September 17th until October 18th my visit, although nearly 50 years later, can in a sense be regarded as a continuation of his. The vessel is moored well out of sight of land, the nearest points being at North Foreland, Kent, and the Naze, Essex, both of which are 22 miles away. The Dutch coast is about 90 miles due east at the estuaries of the Rhine, Schelde and Maas, and the nearest point due north is Southwold, Suffolk, about 50 miles away. Bird observation from a lightship presents a number of diffi­ culties not encountered at shore stations. The small size of the vessel allows it to be much influenced by the continuous motion of the sea, and when rough seas combine with strong winds and driving rain, observation of small Passerine migrants, particularly single birds, is by no means easy. Large birds and flocks of small birds are not as difficult to see. When visibility is reduced to less than three miles it is necessary to sound the fog-horn, at which times I found it impossible to concentrate on watching birds, so loud was its wail. The vessel is equipped with a revolving lantern which throws out three white beams, to which under certain con­ ditions many birds are attracted. During the day the small size of the vessel does not appear to deflect the main flocks of passing migrants. Thus all directions observed are likely to be true direc­ tions, except in the case of exhausted birds which came aboard. L^nfortunately during the fortnight I was on the Kentish Knock * The present paper was written simultaneously with another by Dr. David Lack describing the whole watch of which Mr. Owen's observations formed a part. Certain details of general application have consequently been shortened here and will receive fuller treatment in Dr. Lack's paper which will be published in a forthcoming number.—EDS. 354 BRITISH BIRDS. [VOL. XLVI. the wind remained westerly and was usually of force 4 or more. These conditions prevailed over the whole of the southern part of the North Sea during this period, and the lack of easterly winds prevented an interesting' comparison from being" made. The relationship between weather and number of the three more abundant migrants, the Starling (Shirnus vulgaris), the Chaffinch (Fringilla coelebs) and the Skylark (Alauda arvensis) is shown in Table 1. In the period of the watch the wind had little effect, except on October 28th and 29th when no Passerines at all were seen during strong S.W. gales. Apart from this, rather more birds were seen on days with sunny weather in the early morning than on days with overcast mornings. Doubtless the scarcity of migrants on October 30th was due to the gales of the two previous days, which probably caused a temporary hold-up of migration in the area. Misty conditions by day did not seem to hinder the birds from moving in a fixed direction nor was there an increase in the number of birds which came aboard. TABLE I : THE INFLUENCE OF WEATHER ON NUMBERS OF STARLINGS, SKYLARKS AND CHAFFINCHES. Date Wind direction and General weather conditions Numbers passing force (morning) in b est hour (Beaufort scale)

7! eS

4J Skylar k

(2) MOVEMENTS OF STARLINGS. As at the other stations (Lack, in press) the Starling was the most numerous species, and except on the two days of strong VOL. XLVI.] MIGRATION AT KENTISH KNOCK. 355

S.W. g-ales birds were seen daily, but numbers varied consider­ ably. The general direction taken by Starling flocks was west but as shown in Table 2 there was some daily variation. These varia­ tions in flight direction perhaps indicate that different popula­ tions of birds were involved on different days, for, except on one day, the flight direction was independent of wind direction. How­ ever, on November 2nd the wind backed slowly during the day from N.W. to S.W. and the direction taken by the migrating flocks tended to change correspondingly to some extent (though not entirely, as many flocks continued west). The results of this day's observations are summarized in Table 3. The change in wind direction must have been encountered by many of the Starlings when they were over the sea and these observations perhaps mean that the birds orientate themselves partly by the wind. November 2nd was cloudy, except at dawn, and possibly Starlings were un­ able to get a bearing from the sun and therefore had to depend to some extent on wind direction.

TABLE 2 : DIRECTIONS TAKEN BY STARLING FLOCKS ON GOOD DAYS. Direction of Oct. 27 Oct. 31 Nov. 1 Nov. 2 Nov. 3 Nov. 4 Flight S.W. 0 02307 W.S.W. 0 01503 W. 23 6 8 53 11 43 W.N.W. 10 N.W. 2 N.N.W. 0

TABLE 3: CHANCE OF FLIGHT CTION OK STARLING FLOCK WITH CHANGE OF WIND DIRECTIODIE N ON NOVEMBER 2ND. Time Wind 'irection oj Flight G.M.T. (Force 3) W.N.W. W. W.S.W. S.W 09.05-10.20 N.W. 22 0 0 10.27-10.55 W.N.W. '2 0 0 "•00-11.35 W. 9 0 0 "•SS-H-oS W.S.W. 7 3 1 I4-IS-I5-i7 S.W. 3 3 1

Assuming that most diurnal migrant Starlings set off from the Continent about dawn it is possible to estimate the time spent in making the sea crossing. At the Kentish Knock the first of the Continental departures usually appeared just after 09.00 hours; therefore the time taken to make this journey of rather less than a hundred miles was about three hours, at a speed of rather more than 30 m.p.h. Wind direction and force seemed to have very little effect on the time taken to make the crossing, but the lack of tail winds during the fortnight prevented a better comparison from being made. The main movement of Starlings was usually over by 12.00 hours, but on some days, notably on October 31st and November 2nd, there were steady streams of birds flying west in small flocks until dusk, when observations ceased. 356 BRITISH BIRDS. [VOL. XLVI.

Starling's flew just above the sea in compact flocks, and dipped over large waves which would decrease wind resistance. On one occasion two birds drifted in the water past the ship. These were completely waterlogged, but still alive. The average flock-size was about 30-40, flocks of 70 were not infrequent and the largest flock was 300. Larger parties were scarcer on days with a wind of force 5 or more, suggesting that the wind broke up Starling flocks as they crossed the sea. Large flocks were more frequent in the morning when the first of the dawn departures appeared. Numbers in each party gradually decreased during the day, falling to a minimum in the late afternoon. Likewise scattered flocks and single birds were a feature of the very early morning movement. On overcast days, flocks of Starlings could be picked out up to half a mile away. There was no evidence of any being deflected by the Light­ ship, except that, often, as a flock passed, an odd bird broke away and came aboard. In most cases, after spending a short time aboard, they flew off in the correct direction. Sometimes, especially in the afternoon, single birds in an exhausted condition came aboard; these almost invariably died.

(3) MOVEMENTS OF CHAFFINCHES. Chaffinches were numerous only on October 27th, compara­ tively small numbers were seen on other days. The most important direction taken by birds passing the Lightship was W.N.W., rather fewer flew west and very small numbers flew N.W. and S.W. Birds were seen when winds were recorded in directions between S.S.W. and N.W. and there was no evidence of flight direction being influenced by wind direction. On October 27th the first flock appeared at 08.45 hours, ten minutes before the first Starling arrivals. The movement on this day continued until 12.00 hours, after which few birds were seen. At times single birds appeared on board the Lightship when none were observed passing. This was especially noticeable on October 31st when during the afternoon four individuals appeared at intervals, but no Chaffinches were seen passing the ship that day. This might mean that the birds were flying high out of sight, and agrees with observations in Holland (Deelder, 1949) and at Land's End (Lack, 1952) that this species may set out to sea at a considerable height. Chaffinch flocks seen flew low over the waves and usually comprised 30-50 birds, but flocks of 70, 150 and 200 were seen on October 27th. A few birds which flew west soon after dawn on November 4th were probably stragglers from a larger nocturnal movement. Two males collected were of the central European race F.c. hortensis.

(4) MOVEMENTS OF SKYLARKS. Skylarks were seen passing in a general direction west on eight days during my period on board the Lightship. The VOL. XLVI.] MIGRATION AT KENTISH KNOCK. 357

movement varied between S.W. and N.W., the main directions being west and W.N.W. There was also some daily variation, for on October 31st and November 3rd most birds were seen moving west and north of west, whereas on November 2nd the main directions were west and south of west. In addition to the westerly movement there was also a small passage of birds going south. This usually took place in the early morning between 06.00 and 08.00 hours. These birds had presumably departed from the Suffolk coast. The movements of Skylarks as seen from the Kentish Knock are shown in Fig. 1.

N

10 Milei J

FIG. I: MOVEMENTS OF SKYLARKS. It was impossible to obtain peak times for Skylarks flying west. Only small numbers were seen between 07.00 hours and 10.00 hours and there was not a sudden appearance of birds around 09.00 hours as with Starling and Chaffinch. On October 31st the first arrivals were seen at 10.45 hours; the movement then became fairly continuous until sunset. On days when this species was at all numerous the peak passage occurred after 12.00 hours. The maximum numbers ob­ served were on November 3rd when 70 passed in the half hour commencing 14.30 hours. There is little doubt that departure times of migrating Skylarks are highly variable and are not con­ fined to dawn and sunset. Birds which appeared at the lantern soon after dark on one night must have set out during the after­ noon. Many Skylarks setting out by day must complete their 358 BRITISH BIRDS. [VOL. XLVI.

journey after dark and birds which arrived at the lantern after 04.30 hours would normally have completed their journey by day. Unlike the Starling and Chaffinch single birds and small parties were much more numerous than larger flocks. Flocks of over 20 were unusual, the maximum recorded being of 50. Few Skylarks came aboard by day, but single birds sometimes circled the ship before continuing on. All flew very low over the sea.

(5) MOVEMENTS OF THRUSHES. Since thrushes are usually said to be primarily nocturnal migrants, those seen migrating by day at the Kentish Knock are of special interest. Four species were involved, Blackbird (Tardus merula), Song Thrush (T. ericetorum), Fieldfare (T. pilaris) and Redwing (T. musicus). None was seen before October 31st, though it is known that a large movement took place on two or three nights in the third week of October. Blackbirds were most numerous, and all flew west, except for single birds N.W. and W.N.W. and four S.W. ('with four Sky­ larks). All probably set out during the day as none were seen before .08.35 hours. Rather more passed during the afternoon than during the morning. No large flocks were recorded, the maximum number in a party comprised 7 birds (twice). Single birds often (circled the ship and came aboard, these usually departed after resting for a few minutes. The great majority of those seen were males, only one female was definitely identified during the day. Smaller numbers of Song Thrushes and Fieldfares flew west during the Blackbird migration. One party of 4 Field­ fares was accompanied by 2 Redwings and a party of Starlings contained a single Song Thrush. On November 3rd a Fieldfare came aboard during a N.N.W. force 7 gale and set off west after resting a few minutes. Two flocks of 40 and 25 Redwings flew S.S.W. and S.W., otherwise all birds of this species were odd individuals, except for one flock of 12 which flew west on November 4th. The birds which travelled S.S.W. and S.W. might, presumably, have flown direct from southern Scandinavia. As with the Skylark some thrushes must have set out during daylight and completed their journey by night.

(6) OTHER SPECIES. Other species seen from the Kentish Knock were in much smaller numbers. Of these Rooks (Corvus frugilegns) were most numerous and were seen on six days. The directions of flight varied, but the majority flew west, W.N.W. and N.W. Three flocks flew S.W. and a single bird flew S.S.E. The earliest birds appeared at 08.50 hours and a slight peak was reached at 11.00 VOL. XLvi.] MIGRATION AT KENTISH KNOCK. 359 hours—12.oo hours. The height of flight was very variable, but during strong head winds there was a tendency to fly low over the sea. Often, on reaching the Lightship they gained height and circled once or twice before moving off. A few came aboard and perched on the rigging. Single birds were most common and the maximum number recorded in one flock was 24. Probably a larger migration of Rooks took place as neighbouring lightships reported "very large numbers" on several occasions. One flock of 17 Jackdaws (C. monedula) flew W.N.W. on October 31st, these were the only birds of the species seen. Small numbers of other species took part in the diurnal move­ ment westwards. These included Meadow Pipits (Anthus pra- tensis], Tree Sparrows (Passer montanus), Goldcrests (Regulus regulus), Swallows (Hirundo rustica), a Sparrow Hawk (Accipiter nisus), a Kestrel (Falco tinnunculus), a Stock Dove (Columba aenas) and 7 Lapwings (Vanellus vanellus). The direction of flight was west, except in the case of Meadow Pipits which varied between N.W. and S.S'.W., the Sparrow Hawk which flew W.S.W. and the Kestrel S.W. On two occasions Lapwings accompanied flocks of Starlings. Tree Sparrows were possibly travelling high out of sight as on several days single birds appeared on board when none was observed passing. This agrees with the observations at Land's End that this species may travel at a considerable height (Lack, 1952). The trickle of south-going birds, usually seen in the early morn­ ing, and already mentioned under Skylark, comprised also small numbers of Meadow Pipits, Great Tits (Parus major), Swallows, Linnets (Carduelis cannabina) and a Short-eared Owl (Asio flam- meus). The Short-eared Owl and one Great Tit came aboard be­ fore moving off in a general southerly direction. It seems likely that most of these birds had departed from East Anglia. A few Meadow Pipits also flew east and north and this species was thus recorded travelling in all the major compass directions.

(7) MOVEMENTS OF SEA BIRDS. Normally only a few Lesser Black-backed Gulls (Lams fuscus), Common Gulls (L. canus) and Kittiwakes (Rissa tridactyla) were seen around the ship. There were also some Razorbills (Alca torda), Guillemots (Uria aalge), Puffins (Fratercula arctica), Red- throated Divers (Colymbus stellatus) and Common Scoters (Melanitta nigra), especially near the sand-banks to the west of the ship where the water is rather shallow. The Kentish Knock is situated too far out to sea to come within the feeding area of the Black-headed Gull (Larus ridibundus), and up to November 3rd only three were seen, two of which flew west. On November 3rd a large movement took place, commenc- 360 BRITISH BIRDS. [VOL. XLVI.

ing at 09.15 hours and reaching a peak of 30 birds an hour at 11.00 hours. Small parties and single birds, most of which were adults, flew west and slightly north of west all day. All travelled in a fixed direction low over the sea and none was seen feeding. The movement continued on November 4th but fewer birds were involved. Outward bound to the Kentish Knock in the relief ship Triton I did not see Black-headed Gulls more than four miles off­ shore. On November 5th I saw small numbers feeding in the vicinity of the Barrow Deep Lightship which is stationed eight miles off Felixstowe. Lesser Black-backed and Common Gulls moved in a general direction south on some days. On October 23rd Common Gulls flew west and Lesser Black-backs south, about 80-90 of each species passed during the morning. Parties varied from 2 to 10 and only rarely were the two species seen in company, and again the great majority were adults. Smaller numbers passed between S.W. and S.E. on other days. The most spectacular of the southward moving sea-birds was the Kittiwake, large numbers of which were seen on certain days. Only a few passed between October 22nd and 26th, more occurred on 27th, few on 28th and 29th and more again on 30th. October 31st was, how­ ever, the best day as many hundreds passed south in small parties, including a large proportion of juveniles. Few appeared on November 1st, but birds were common again on 2nd and 4th. Kittiwakes probably also move at night as the lightship-men have sometimes seen them around the lantern on misty nights. Arctic Skuas (Stercorarius parasiticus) flew south daily, usually singly, but two parties of three were seen. All were dark phase examples or juveniles. Two Pomarine Skuas (S. pomarinus) and a Great Skua (S. skua) also flew south. Gannets (S'ula bassana) passed south in small numbers, the most for one day being twelve. None was seen fishing and all except three were adults. Eagle Clarke (1912) records a similar movement of adult Gannets in September and October, 1903. The three common species of auk were seen passing south, the great majority of which were Razorbills. October 31st was a good day for auks and many hun­ dreds, in parties up to twelve, were seen. On November 4th many Razorbills participated in a great southerly movement of sea birds and outnumbered Kittiwakes. Other species were occasionally seen, including 30 Eiders (Somateria mollissima), 10 Scaup (Aythya marila), 2 Brent Geese (Branta bernicla), 4 Dunlin (Calidris alpina), a Sanderling (Cro- cethia alba), 6 other small waders that were possibly Dunlin, and 3 Fulmars (Fulmarus glacialis). All of these were moving west or south of west, except the Eiders which flew N.W. The directions taken by the more numerous species of sea birds are shown in Fig. II. VOL. XLVI] MIGRATION AT KENTISH KNOCK. 361

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FIG. II: i. MOVEMENTS OF BLACK-HEADED AND COMMON GULLS. 2. MOVEMENTS OF RAZORBILLS, KITTIWAKES AND GANNETS.

(8) MIGRATION AT NIGHT. At the Kentish Knock in late October and early November, 1952, the number of birds migrating' at night probably equalled if it did not exceed those moving by day. Migrants appear at the lanterns of lightships and lighthouses only when the weather is overcast or misty, particularly when the actual moisture content of the air is high. On clear nights many birds may migrate un­ detected, as instanced on the night of October 31st—November 1st. On October 31st Starlings, Skylarks and a few other species travelled west all afternoon until sunset, when observations had to be discontinued. Weather conditions were good, with a moder­ ating wind and clear sky. The early part of the night was clear and starry and I did not expect birds to appear at the lantern. However, at midnight the watch reported that the wind had dropped and that the sky had suddenly become overcast and that birds had already arrived at the lantern. These I found to be Starlings and Skylarks and as I watched many more arrived, with small numbers of other species. Numbers increased rapidly and by 01.30 hours the weather had deteriorated and light rain had begun to fall. By this time the rigging, masts, decks, in fact all available perching places, were covered with resting birds. 362 BRITISH BIRDS. [VOL. XLVI.

Thousands more circled the lantern and flew up the beams of the revolving- light. Many hit the lantern glass resulting in a terrific mortality. At times stunned and dead birds dropped into the sea and onto the deck continuously. Many hundreds, possibly thousands, must have been lost that night. Just before dawn birds were so numerous that as I stepped on deck from below, my head and shoulders were covered with birds seeking new resting places. The great majority were Starling's and Skylarks with smaller numbers of Blackbirds, Fieldfares and Song Thrushes, and two Short-eared Owls. Other species were probably present, but identification of odd individuals was in most cases difficult unless specimens were caught. The following morning a single Chaffinch was found on board, so presumably this species also participated in the movement. At dawn the weather became steadily worse with driving rain and a freshening westerly wind, but despite this, great numbers of birds left the Lightship and set off west, many circled several times before doing so. Condi­ tions then became so bad that it was necessary to sound the fog­ horn, which caused almost all the remaining birds to disappear. This night's observations are of particular interest since it is unlikely that many birds would have set out in the type of weather they met en route. Assuming that the origin of these birds was similar to that of those passing by day, it would appear that departures are not necessarily confined to the period around sun­ set. Birds which appeared at the lantern between 02.00 hours and dawn had presumably set out well after dark. Birds appeared at the lantern on other nights when similar conditions prevailed, but not in such large numbers. Starlings were most common and early each morning flocks set out from the lightship for the west. Skylarks passed by all afternoon on November 2nd and as soon as it became dark they appeared at the lantern. Numbers were small and probably did not exceed twelve at any one time. A few Chaffinches also appeared at the lantern on this occasion. In the early hours of October 28th during rough weather and high seas, a few Goldcrests and un­ identified finches came to the light together with about 40 Starlings. The nights of October 20th-2ist and 2ist-22nd (before my arrival at the Lightship) were comparable with October 31st- November 1st. According to the lightship-men the vessel was competely covered with birds, the majority of which were thrushes. Each morning dead or dying birds littered the deck. Some were set aside for my examination and included Redwings, Song Thrushes, Starlings, Meadow Pipits, Skylarks, a Goldcrest, a Willow Warbler (Phylloscopus trochilus) and a Water Rail (Rallus aquaticus}. There had also been many "Chaffinches," but later I discovered that these were not distinguished from VOL. XLVI] MIGRATION AT KENTISH KNOCK. 363

Bramblings (Fringilla montifringilla) a number of which were feeding from scraps in the galley when I arrived on October 22nd. These two nights and other nights were similar at all the light­ ships in the southern part of the North Sea and Channel and presumably a large movement took place. From the preceding account it will be apparent that many species of diurnal migrants also took part in the night movements and vice versa. Moreover many individuals must have set off at such a time that they would have to complete their journey after dark. Others which set off during the night would arrive in the morning. As already shown this was especially so with the Sky­ lark and Starling at the Kentish Knock. Eagle Clarke (1912) had similar experiences at the Kentish Knock and at the Eddystone Lighthouse. He records large numbers of Chaffinches at night and such birds as Redstart (Phosnicurus phmnicurus) and Wheat- ear (Oenanthe cenanthe) by day. The Chaffinch is usually said to be diurnal and the Redstart and Wheatear nocturnal when migrat­ ing. Likewise records in the B.O.C. Migration Reports (1905- 1913) show remarkably incomplete differences between nocturnal and diurnal migrants. I can only conclude that in the past too much emphasis has been placed on such differences, as many species seem to move both by day and night.

ACKNOWLEDGEMENTS. I am grateful to the Elder Brethren of Trinity House, London, for permission to spend a fortnight on the Kentish Knock Light­ ship and to the staff of Trinity Depot, Harwich, for making all necessary arrangements. I am particularly indebted to Mr. N. Brundle, Master of the ship, and his crew for much co-operation during my period aboard. Dr. David Lack has read this paper through in manuscript and I am grateful to him for many useful suggestions.

(10) SUMMARY. 1. The Kentish Knock Lightship, stationed in the southern section of the North Sea, was visited in late October and early November, 1952, for the purpose of studying the east-west move­ ment of migrant Passerines. The visit coincided with a period of westerly winds and migration was thus witnessed only during this type of weather. The effect of wind direction and force in relation to number of birds is considered. 2. Starlings were seen on all but two days and were the most numerous species. Chaffinches and Skylarks were next in order of abundance, other species being much scarcer. In addition to the main movement west a few birds flew south. These had pro­ bably originated in East Anglia. Some nocturnal species (e.g. thrushes) were seen migrating by day. 364 BRITISH BIRDS. [VOL. XLVI.

3. On certain days large numbers of sea-birds were seen migrating-. Of these Black-headed Gulls and some Common Gulls flew west, and Kittiwakes, Lesser Black-backed Gulls, Razorbills, Gannets and smaller numbers of other species flew south. 4. Many Passerines also travelled at night, but these were only seen at the ship's lantern when the sky was overcast and the moisture content of the air high. Birds might set out in clear conditions and meet unfavourable weather en route. Most of the species seen at night were the same as those seen during the day. Day and night movements showed considerable overlap and it is concluded that many late autumn migrants may be nocturnal or diurnal.

(11) REFERENCES. CLARKE, W. E. (1912). Studies in Bird Migration. London. DEELDER, C. L. (1949). "On the autumn migration of the Scandinavian Chaffinch (Fringilla c. coelebs L.)" Ardea, 37: 1-88. LACK, D AND E. (1952). "Visible Migration at Land's End." Brit. Birds, xlv: 81-96. LACK, D. (in press). "Visible migration in S.E. England, 1952." Brit. Birds. MIGRATION REPORTS (1905-1913). British Ornithologists' Club. Ed. W. R. Ogilvie-Grant.