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REPORTS ON THE MOVEMENTS OF SOME COMMONER SUMMER MIGRANTS AT BRITISH BIRD

OBSERVATORIES IN 1951. As already mentioned, these reports are confined to the spring migration. The expression " Bird/Days," as used last year, represents the sum of the birds recorded each day, and as individuals may stay for more than one day it does not necessarily represent the number of different individuals seen.

YELLOW WAGTAIL. THE spring passage of the Yellow Wagtail {Motacillaflavaflavissima), began on April 2nd with the arrival of one bird at Cley. From that date the northward progress of the species up the east coast of England and Scotland was quite regular, until the first arrival at Fair Isle on May 3rd. As will be seen from the accompanying table, the only irregularity was the date of May 4th for the first bird at the Isle of May. The west coast passage began at Jersey on April 15th, to be followed by first arrivals at Lundy on April 19th and at Skok-holm two days later.

Following the first arrivals, the periods of maximum movement did not fall into such a regular pattern on the east coast. This suggests that detailed investigation of the migration routes across the south coast and overland through the inland counties might provide some interesting data. Spurn Observatory offers the additional information that the Yellow Wagtail was first observed at Ryhul reservoir on April 14th and continued to be seen in that area until mid-May, with c. 40 on May 6th; also, at Dewsbury sewage farm c. 100 were noted on April 19th. The species is not thought to round Spurn Pdint, but rather to advance towards the district from the south-west. Peak periods at the three western observatories followed a progressive pattern.

Information concerning the arrival of females is unfortunately scarce and has therefore been omitted from the table. Jersey recorded two among their first arrivals on April 15th, and during the remainder of the spring passage a proportion of females was noted with every group of birds observed. At Monks' House, the first female was seen on May 7th, 13 days after the arrival of the first male. Two females were observed with the first cock at Fair Isle on May 3rd, but the subspecies of these birds is uncertain.

On the east coast, last arrivals at Gibraltar Point, Spurn, Monks' House and the Isle of May were progressively earlier towards the north, whilst at Fair Isle the date of June n t h is the latest on record for that observatory. On the west coast, last observations for the species again follow a regular pattern.

In both groups of observatories the total number of observations of the Yellow Wagtail tends to diminish towards the north. For the sake of uniformity " Bird/Days " have been chosen for the units in tabulating these figures but, from examination of the actual

246 BRITISH BIRDS. [VOL. XLV.

data submitted by the Observatories, it is probable that they approxi­mate fairly accurately to the number of birds observed.

The weather during the first half of April was unsettled over most parts of the British Isles, with rainfall above average. This was followed by a fine, dry spell with exceptional sunshine during the last two weeks, but temperatures were below normal. May was generally cool; cloudy and wet in England and sunny in Scotland.

Whilst it would be premature to draw any far-reaching conclusions from this, the first, co-ordinated report on the Yellow Wagtail by the British Bird Observatories, many lines of enquiry are at once apparent from the general pattern of the observations. It is to be hoped that further and more detailed work will be devoted to the species, which lends itself so admirably to research of this nature. SPRING PASSAGE OF YELLOW WAGTAILS AT THE B R I T I S H OBSERVATORIES, 1951

Observatory

E. Coast Cley

Gibraltar Point

Spurn ...

Monks' House

Isle of May ...

Fair Isle

W. Coast jersey

Lundy.. .

Skokholm

First Arrivals

April 2nd

April 15th (1)

April 23rd (2)

April 24th (<=• 3°)

May 4th (2)

May 3rd (1)

April 15th (12 )

April 19th (1)

April 21st (1)

Peak of Migration

April 14th (several)

May 2nd-3rd (30 Bird/days)

May 12th (4 birds)

April 24th (c- 3°)t

Not Appreciable

Not Appreciable

April 2ist-22nd (c. 60 Bird/days)

April 2ist-25th (6 Bird/days)

May io th- i5 th (11 Bird/days)

Last Arrivals

Not Recorded *

May 30th (3)

May 27th (2)

May 15th (2)

May 12 th (1)

June n t h (1)

May 10th (20+)

June 2nd (1)

June 3rd (2)

Total Bird/days

Not Estimated *

77

13

37

3

4+

107+

1 0

26

*At Cley in the latter part of April local breeding birds were inseparable from migrants. fAt Monks' House the initial wave on April 24th was the largest number recorded. E . D . H . JOHNSON.

RED-BACKED SHRIKE. The records. THE first record of a Red-backed Shrike (Lanius collurio) at a bird observatory in 1951 was a $ which spent May I2th-i5th at Spurn.

VOL. XLV.] REPORTS ON SUMMER MIGRANTS, 1951. 247

The first at Fair Isle was also a $, May 17th, and on the same day Gibraltar Point recorded a <J. From May 19th there was a steady movement of Red-backed Shrikes at Fair Isle, with occasional records by L. S. V. Venables in south Shetland from the same date. There were three peaks in this movement, a small one on May 21st (2 (JcJ, 1 $) and bigger ones on 26th (5 S3, 2 $$) and 30th ($33, 2 <j>, plus 1 $ Shetland). This series of daily records at Fair Isle was broken on June 2nd, and at Spurn Point a $ which had been present since May 30th was last seen on this day.

Reckoning on the purely arbitrary basis of Bird/Days Fair Isle had 41 records, Shetland seven, Spurn Point eight, and Gibraltar Point one. Cley and Isle of May on the east coast, and Lundy, Skokholm, Great Saltee and Jersey in the south-west, did not record shrikes. As R. C. Chislett is definite that only two individuals, staying four days each, were at Spurn, it is clear that a concentration of Red-backed Shrikes was experienced only in the north. Here, records of <S£ (27) outnumbered records of $$ (17), and the distri­bution of the sexes shows no appreciable pattern over the period. The migration.

Examination of the series of " Daily Weather Reports " issued by the Meteorological Office of the Air Ministry leads to the follow­ing interpretation. The Spurn example of 12th, and the Gibraltar Point bird of 17th, might have arrived as drift-migrants from N.E. on the airflow of high pressure areas covering north Britain on n t h and 16th. It would seem more likely that the isolated Fair Isle occurrence of 17th was an early drift-migrant moving out.

The situation from May 19th onwards, however, is clear, and the Fair Isle and Shetland birds of that day were among the fore­runners of a big fall of northern summer-visitors of various species which culminated on 20th-2ist. Central Europe lay in a col on 19th between high pressure areas covering the eastern Mediterranean and western Norway, and calm or light air conditions conducive to migration existed over most of the Continent. Below the northern high an easterly airstream prevailed from the Low Countries to the Skaggerak and across the North Sea, and although the wind was light the migrational drift was on a very large scale (see p. 261). Conditions were similar on 21st, and on 22nd-23rd the drift was maintained, but at a lower intensity, by the northwards movement of a cold front succeeded by an occluded front the following day. On 25th-26th there was a renewal of the conditions at the beginning of the period, resulting in the first big peak of shrike arrivals in the north (fig. 1). Drift was still apparent on 27th on the north side of a low which had moved overnight from northern France to east Anglia, but was much reduced on 28th by an extension of the low pressure system to Denmark and southern Norway.

An anticyclone, approaching from the Iceland-Faeroes area, was centred over the north of Britain on May 30th, bringing calm weather (fig. 2). The second big peak of shrike arrivals at Fair Isle and south

248 BRITISH BIRDS. [VOL. XLV.

Shetland, therefore, is attributable to the outward passage of birds which had been deflected to north Britain during the previous week. This high remained for several days, and shrikes were passing intermittently till June 2nd. The records of June 5th, 10th, 20th and 29th, in all probability, can be ascribed to outgoing birds. The Weight Records.

Given a sufficient number of records which could be assigned unequivocally to one or the other of these categories, it ought to be possible to test the validity of the above interpretation by a com­parison of the weights of trapped birds. Drift-migrants which had just done a long oversea flight and sustained abnormal weight-loss due to the utilisation of their store of muscle and liver glycogen and body-fats, would be noticeably lighter than outgoing passage

To be continued...

VOL. XLV.] REPORTS ON SUMMER MIGRANTS, 1951. 249

migrants which had had a short period in which to recuperate (see Fair Isle Bird Observatory Annual Rep., 1949, p. 15. idem. 1950, p. 6, and Isle of May Bird Observatory Rep., 1950, in Scot.

250 BRITISH BIRDS. [VOL. XLV.

Nat., 63, pp. 56-63, for examples of gains in weight on the part of resting migrants).

There are, however, certain snags which militate against a clear delimitation of the two categories : there is, for instance, a diurnal cycle of weight variation, so that early morning weights of birds in the passage group, before feeding had begun, would not show the sharp distinction from drift-arrival weights that one might expect to find later in the day. There may be a sex difference ; and it is obvious that a drift-migrant trapped after it had been a day or more resting on the island would give an inflated figure for its group. Despite these complications, an examination of the weights of Red-backed Shrikes trapped at Fair Isle in May and June does lend support to the migration analysis offered above. Captures in each of the two groups are set out below:—

A. Drift arrivals. PJ 908 $ May 26th, 17.00 hrs. G.M.T. 30.20 gm, PJ 908 $ May 27th, 09.00 hrs. G.M.T. 30.59 gm. PJ 909 $ May 26th, 17.00 hrs. G.M.T. 25.03 gm. PJ 910 (j May 26th, 19.30 hrs. G.M.T. 25.98 gm. PJ 912 $ May 28th, 19.00 hrs. G.M.T. 27.31 gm.

B. Passage Birds. PJ 918 ? June 2nd, 14.30 hrs. G.M.T. 28.90 gm. PJ 921<j June 20th, 15.45 hrs. G.M.T. 28.80 gm. PJ 924 £ June 29th, 19.00 hrs. G.M.T. 32.92 gm.

The low weights of PJ 909-910 on May 26th, at the climax of the drift period, are in marked contrast to the weights of outward-bound birds in June. The high weight of PJ 908 is puzzling, but it may be explained by the fact that the bird was disturbed from the body of a Robin {Erithacus rubecula) before the first trapping, and was recaptured the following morning whilst devouring a Sedge-Warbler (Acrocephalus schcenobtenus). It may, of course, have arrived earlier. The comparatively high weight of PJ 912 may be due to its capture at the end of the drift period—this bird also may have arrived at Fair Isle a day or two before.

Summary. The passage migration of Red-backed Shrikes in spring, 1951, was

concentrated in the north of Britain, and an analysis in the fight of the meteorological conditions shown in the " Daily Weather Report " reveals a similar pattern to that found in the case of the Ring-Ouzel (Turdus torquatus). The main movement took place uninterruptedly between May 19th and June 2nd, culminating on May 26th, and lag­gards were passing irregularly toJIJune 29th. There were two peaks, the first resulting from migrational drift of birds which had reached the area Germany-Denmark-Skaggerak from S.E. through a col in Central Europe; and the second due to the elements of this same drift leaving the country under optimum anticyclonic conditions four days later. KENNETH WILLIAMSON AND ALEC BUTTERFIELD.

VOL. XLV] REPORTS ON SUMMER MIGRANTS, 1951. 251

RING-OUZEL. DURING April, 1951, occurrences of the Ring-Ouzel (Turdus tor-quatus) were few and widely scattered as to time and place, and the great majority doubtless represent arrivals of British breeding-birds. The earliest records are from Ross Links, Northumberland, on March 24th, and Gorple, Yorkshire, on the following day. In early April one was on the Inner Fame. A single bird was at the Isle of May from April 3rd-6th and another or others from 8th-i7th. There was a $ at Gibraltar Point on 17th, and a bird at Ross Links on 21st. Norfolk records are for single birds on 13th and 23rd and two on 24th. In the west, Skokholm had one on April 6th, a $ from I5th-i8th, and pairs on 19th and 21st, with a final bird on 24th. At Jersey, a $ was recorded on April 18th, and the Lundy records show single birds April ioth-i4th, 17th, 21st, 24th, 26th, 29th and May 1st.

None reached Fair Isle in April, nor were any noted at Spurn or Great Saltee. Thus far the records support The Handbook state­ment of immigration of British breeders from mid-March to early May (vol. ii, p. 133). At first sight it might seem that the preponder­ance of records from Lundy and Skokholm supports The Handbook view of a mainly south-west entry kito the British Isles'; it is not possible to draw any firm conclusion on this point, however, as there is a strong suggestion that the movements are influenced by the wind. Thus, the majority of the records during the first three weeks of the month, when the prevailing wind was westerly throughout, fall along the east coast; and the larger proportion of the later records are from the south-west and coincide with a period of mainly easterly weather.

During May there were no less than five separate movements of Ring-Ouzels on the east and north coasts of Britain. The first was on a large scale at the very beginning of the month, and was recorded from Fair Isle to the Holy Isle of Lindisfarne, Northumberland, but not farther south. A single bird was at Spurn on May 4th-5th. The second, a minor movement on May 7th-8th, was observed from the Norfolk coast to Spurn and Teesmouth on the Yorkshire coast, but not farther north. These movements were clearly the result of migrational drift from the Continent. (For a discusison of the concept of migrational drift and its importance in the British area, see the present author in Scot. Nat., in press.) The next two move­ments, also small ones beginning on May 10th and 17th, were observed only at Fair Isle, and represented outward passage of birds which had entered the country as drift-migrants earlier in the month. Lastly, two $$ at Fair Isle on May 24th arrived under conditions highly suggestive of trans-North Sea drift.

The movement at the beginning of May was on a comparatively large scale for this species and is worth analysing in detail. Two which reached the Isle of May on April 30th were evidently fore­runners of the influx of May 1st, when eight birds appeared there and

252 BRITISH BIRDS. [VOL. XLV

FIG. I .

a similar number (in the evening) at Fair Isle. This culminated in a rush of 25 and 20 at these places respectively next day. On May 2nd six birds were noted at Lindisfarne, and 12 were counted on 3rd. As J. H. R. BoswalL to whom I am indebted for the Holy Island records, did not arrive there until 17.00 hrs. G.M.T. on and it is likely that the count he then made is not comparable with his count during 11 hours' watching next day. This point is emphasized because both Fair Isle and Isle of May recorded decreases on 3rd, Ring-Ouzels disappearing from Fair Isle afterwards, but showing a very gradual decrease at the Isle of May to five birds on May 10th. The migration, at all three observation points, was not confined to Ring-Ouzels and several other species took part, including a fairly large number of Fieldfares {Turdus pilaris).

VOL. XLV.] REPORTS ON SUMMER MIGRANTS, 1951. 253

Two considerations suggest that this drift involved birds which were moving into Scandinavia from Central Europe, having come from the south-east. These points are, firstly, the distribution of the records on the east coast, with none in the southern section; and secondly, the development of the anticyclonic conditions which must have initiated this stage of the migration and contributed to the drift.

This anticyclone had covered eastern Europe and the Balkans since April 28th; its westwards flow was fairly rapid after 30th, and from early on May 1st conditions were unusually calm and clear and very favourable for migration throughout Germany. Drift of birds attempting to cross the Skaggerak could be expected in view of the presence over Denmark of a westwards-moving low, with strong east winds on its northern side, backing north-east over the North Sea. During the early morning an alteration occurred in the movement of the fronts associated with this low, causing a slight veer of wind in Forties and East Fair Isle—a fact which may explain why no arrivals took place at Fair Isle until evening. The heavier overnight drift on ist/2nd probably involved birds from farther south, displaced from the coasts of Denmark and North Germany (fig. 1). That this migration came from the south-east and not from France seems abundantly clear in the absence of Ring-Ouzels from Spurn and southwards on this peak day, since the overnight position of the trough of low " S " should, in the latter event, have produced drift all along the east coast.

Three birds on the Norfolk coast on 7th-8th, a <$ at Gibraltar Point on 7th, two at Teesmouth on 6th and 10th and six at Spurn between these dates follows the distribution one would expect in view of the North Sea conditions at that time, the airstream from the Skaggerak southwards being north-easterly on the north-western flank of a depression then centred over France (fig. 2).

The May 10th movement at Fair Isle is most interesting ; five birds occurred on that day, and three on n t h . Without doubt this movement was due not to drift, but to passage-migration of the birds that had been deflected to Britain earlier in the month. This was the first time that the north and west of Britain had enjoyed anticyclonic weather since the migrational drift of May ist-2nd, a belt of light airs and calms extending from Ireland across Scotland to southern Scandinavia—which outward-bound birds could hardly fail to reach successfully under these conditions (fig. 3). Other species associated with the Ring-Ouzels are highly suggestive of movement from the south-west, e.g., two Carrion Crows {Corvus corone)—rare at Fair Isle, which is beyond their normal range—• and the first passage of Greater Wheatears {(Enanthe ce. leucorrhoa). It may also be significant that the remnants of the earlier drift, five in number, disappeared from the Isle of May at this time.

Somewhat similar conditions prevailed on May I7th-i8th, an anticyclone over northern Britain again extending to southern

254 BRITISH BIRDS. [VOL. XLV.

VOL. XLV] REPORTS ON SUMMER MIGRANTS, 1951. 255

Scandinavia. Passage at Fair Isle was light but a variety of species was involved, the most abundant numerically being diumally-migrant Swallows (Hirundo rustica) and the six Ring-Ouzels. The last two Ring-Ouzels, $$ on May 24th, are more likely to have been drift-migrants carried across the North Sea on a south-easterly airstream caused by the line of occlusion of an Atlantic depression then filling up some way west of Ireland.

We may summarize the migration as follows. Scattered records during April indicate the return of the British breeding-stock. May movements on the east and north coasts are due to two factors ; (1) migrants moving into Germany and Scandinavia from the south­east being displaced by easterly winds in the North Sea area, and (2) these same migrants taking passage out of Britain as soon as optimum weather conditions permit. The peak of the return move­ment of Scandinavian Ring-Ouzels was at the beinning of May in 1951, but there were still stragglers 24 days after the main rush The very gradual decline at the Isle of May following the big drift of May ist-2nd perhaps indicates a tendency on the part of dis­placed birds to delay onward passage in a congenial haunt until suitable weather conditions act as a new stimulus to the migration urge. On the other hand, the succession of records may be due in part to northward coasting of birds which had earlier reached the Northumberland coast.

I am grateful to those in charge of the various Bird Observatories for supplying the information on which this analysis is based, and to the Director of the Meteorological Office, Air Ministry, for per­mission to use diagrams based on " The Daily Weather Report."

KENNETH WILLIAMSON.

SPRING MIGRATION OF WRYNECK AND CUCKOO AT EAST-COAST BIRD OBSERVATORIES, 1951.

THE first records of Wryneck (Jynx torquilla) and Cuckoo (Cuculus canorus) at an east-coast observatory were at Spurn on April 24th-25th, when different Wrynecks were observed on the two days and the first Cuckoo appeared. Two Cuckoos were at Monks' House and another at Alnwick, Northumberland, on 24th, the earliest in this region having been noted at Lanchester on 20th (Northumber­land and Durham Nat. His. Soc. Rep., 1951). Conditions over England were anticyclonic at this period, with light winds or calms, and as North Sea winds were westerly these records must represent the movement of British birds.

The next appearance of Wrynecks was due to trans-North Sea drift and was shared by Fair Isle, where a bird was trapped on May 2nd, and Isle of May, where one on 1st was succeeded by two on 2nd-3rd. Single Cuckoos were at both places on 3rd, and Gibraltar Point (where the first had appeared on April 30th) recorded three on this day. The weather conditions at this time have been sum­marized in the article on Ring-Ouzel (Turdus torquatus), antea p.

256 BRITISH BIRDS. [VOL. XLV.

252, where it is shown that the migration came from south-east Europe. At Spurn, the spring peak of Cuckoo migration was three days later—two birds on 5th and four on 6th—with the meteoro­logical situation not differing in any important respect.

A Wryneck was picked up in a dying condition at Lesbury, Northumberland, on May 7th, and there were birds at Spurn on io th-n th and at Cley on n t h . On these last two days an anti­cyclone had reached northern Scotland from the west, but the north­easterly airstream of this high pressure continued to affect the North Sea between the Skaggerak and Denmark and the English coast, so these records too are very likely due to drift.

In view of the anticyclonic conditions existing in the north of Scotland, Cuckoos at Fair Isle on 9th and 12th are more likely to have been earlier drift-migrants seeking passage out of the country. Similar circumstances attended the appearance of two Wrynecks at Fair Isle on May 17th and a Cuckoo at the Isle of May next day. A Cuckoo at Fair Isle on 19th may be in the same category, but it is difficult to be sure as this was a transitional period when rapid changes on the Continent and over the North Sea were responsible for the onset of a heavy migrational drift.

From May 19th to June 2nd there were daily records of Cuckoos at Fair Isle, with a single bird on June 4th (30 " Bird/Days"). One was at the Isle of May, and three at Gibraltar Point, on 20th. The weather conditions at this period have been described in pre­vious notes, antea pp. 248-255, and again the migration was pro­ceeding in a north-westerly direction through Central Europe. The peak day at Fair Isle was May 21st with seven Cuckoos and three Wrynecks, following a single Wryneck on 20th. There were also three Cuckoos on 27th, again apparently due to drift on easterly winds between an anticyclone to the north and a low covering southern England. High pressure conditions favourable for the departure of these birds developed on May 29th as explained in the note on Red-backed Shrike (Lanius collurio), antea p. 248, and persisted until June 5th. Later records of out-going Cuckoos were noted on June 18th and 25th-26th.

Summary. The migration of both species seems to have been very protracted. Thus, drift of Wrynecks from the Continent took place intermittently during the first three weeks of May, and in nearly every case Cuckoos were associated with them. This species, however, showed two well-defined peaks, on the Lincolnshire-Yorkshire coast from May 3rd-6th, and at Fair Isle a fortnight later; on each occasion, the meteorological conditions governing the drift indicate a migration proceeding S.E.-N.W. through Central Europe, the direction expected from the evidence of ringing recoveries (see antea vol. xxviii, p. 112). As in the case of the previously studied species, movements at Fair Isle are due partly to drift, partly to the departure of re-orientated birds under anticyclonic conditions. KENNETH WILLIAMSON.