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British Birds | 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) h k 7! eS 4J Skylar </2 UJ Chaffinc Oct. 23 S.-W.N.W. 5-4 Cloudy until 10.00 hrs. then sunny 3 2 0 Oct. 24 S.W. 5-6 Squally, long bright periods 2 O 0 Oct. 25 W.S.W. 5-6 Mainly fine, squalls at times 93 2 36 Oct. 26 W.S.W. 4 Fine 17 O 0 Oct. 27 S.S.W. 6 Cloudy, drizzle at times 356 O 400 Oct. 28 S.W. 6-7 Cloudy, some rain 0 0 0 Oct. 29 S.W.-W.S.W. 6-8 Cloudy, bright periods 0 0 0 Oct. 30 W. 4 Fine 12 0 0 Oct. 31 N.W.-N.N.W. 6-4 Mainly fine 409 5° 0 Nov. 1 S.W.-N.N.W. 4 Low cloud, rain '3 9 0 Nov. 2 N.W.-S.W. 3 Fine at dawn, cloudy later 921 41 So Nov. 3 N.N.W.-N.W. 7-5 Fine 81 70 0 Nov. 4 S.W.-S.S.W. 4-5 Cloudy, bright periods in early morning 892 9 27 By day the main flocks travelling west appeared quite suddenly around 09.00 hours, presumably having set out at dawn. Before 08.00 hours scattered flocks going west were apparently the remnants of a much larger nocturnal movement. Southward- going birds, which had probably set out from East Anglia, were seen earlier. (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.
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