British Birds ^^

Vol. 59 No. i B 5 JANUARY 1966 -* *sS A

An analysis of recoveries of Great Skuas ringed in Shetland By A. Landsborougb Thomson

MATERIAL THE PURPOSE of this paper is to analyse the results of ringing chicks of the Great Skua Catharacta skua at its breeding colonies in the Shetland Islands, lying NNE of the mainland of Scotland. Up to and including 1964 a total of 5,628 Great Skuas had been marked there (or in a very few cases elsewhere) under the national scheme managed by the British Trust for Ornithology, using rings stamped 'British Museum (Natural History)' and with financial support from the Nature Conservancy. Up to 31st May 1965 (the approximate start of a new breeding season), this ringing had yielded 119 viable recovery records, which is at the rate of 2.1% (with some further records still to come from the younger birds—perhaps ten, which would bring the true recovery rate to 2.3%). Excluded from this total are 22 recoveries rejected because the birds had certainly or probably never flown (found dead at place of ringing after a short interval, or as remains after a longer one), and in one case because the distance travelled was negligible and no date could be assigned (skeleton only, found locally). Although the records are not numerous, nearly all of them represent movement of some kind. Moreover, the data are homogeneous in that all the recovered birds were ringed as chicks, or pulli, and within a limited area (Shetland), so that they were of known age and virtually identical natal origin. The number ringed otherwise than as pulli has been negligible, and no recoveries have resulted. Many of the individual records have already been published, with fuller details than need in every instance be repeated here, in the recovery lists and annual reports of the ringing scheme (Leach 1940, '947. '95°; Spencer 1955 et seq.).

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4->1» 1 « 1 B •6 >. 4) G U 1 Mont h 1 Q 1 •< BRITISH BIRDS BACKGROUND As is well known, this is a bipolar species and its sole Northern Hemis­ phere subspecies C. s. skua has a breeding distribution restricted to northern Scotland (mainly Shetland and other isles), the Faeroe Islands, Iceland and possibly Greenland. To these localities it is a summer visitor. Non-breeding birds range in summer over the North Atlantic from about 700 N south to about 40° N on the eastern side and 450 N on the western. In winter the range is roughly the North Atlantic between 6o° N and 300 N. Occurrences outside these limits are casual, and seldom inland. The movements are normally purely maritime—sometimes coastal, sometimes pelagic.

INCIDENCE OF RECOVERIES In all analyses of ringing data the seasonal and geographical incidence of recovery records may be heavily biased by factors that can hardly be evaluated statistically—differences in mortality itself, and differences in the circumstances that determine whether a casualty to a ringed bird is likely to be reported. In the present instance one is dealing with a largely pelagic species, and it has to be kept in mind that parts of the range will be very inadequately represented, if at all, in the ringing data. Further, the species is protected at its Scottish breeding places, so that apart from infant mortality there will be few records from there. It is relevant that the percentage of ringed individuals recovered is low for a large bird. As already stated, all the ones recovered were ringed at what may be termed 'zero age'. The recoveries can be grouped by years of life, standardised as running from June to May inclusively (the earliest chicks being hatched in June). On this basis the figures are: First Second Third Fourth Fifth Sixth Seventh 83 14 14 4 2 1 1 Most of those recovered were ringed on the islands of Unst (at Hermaness), Noss and Foula, the few exceptions being on Hascosay, Mousa and outlying Fair Isle. It seems reasonable to treat all as belong­ ing to a single population. Table 1 shows the recovery localities, arranged by years of life and calendar months.

FIRST-YEAR RECOVERIES In the case of recoveries during the first year of life the immediate point of departure is known, whereas in subsequent years there is no means of saying from what summer area the winter movement of a particular bird may have originated. It can be presumed that Great

4 RECOVERIES OF GREAT SKUAS Skuas return to the natal area for breeding; but this may not occur until they are at least three years old (so far as age of maturity can be inferred from the imperfectly known sequence of plumages), and by then the number of records is tailing off. The recovered birds were all ringed before they could fly, nearly all in July with very few in June or August; the sole viable recovery record for August is local. In September and October most of the records are from the eastern seaboard of Great Britain (from Orkney to Norfolk), from the opposite coasts of the North Sea (in an arc from southern Norway to Belgium) and from the English Channel (French coast). There are also some records from far inland in continental Europe (the furthest being from Minsk, U.S.S.R.)—mostly in one par­ ticular calendar year, as will be noted later. There are two records from the western side of the British Isles (Outer Hebrides and Northern Ireland), and three from the Atlantic coast of continental Europe (France, Spain, and Portugal); one of these last had reached north-west Spain as early as 4th September. The only record of a foreign-ringed Great Skua seems to be that of a pullus ringed at Oraefi, Iceland, on 27th July 1963 and recovered on Lough Corrib, Galway, western Ireland, on 23rd September in the same year (Hudson 1964). In November, December and January, records from the northern areas are substantially fewer, and apparently diminishing. On the other hand, records from the Atlantic seaboard of continental Europe are much more numerous, as can be seen in tables 1 and 2. One was recovered at sea in December off the west coast of Ireland (c. 5 20 45' N, 120 00' W). The picture to this date may seem reasonably clear, so far as it extends, but there are general grounds for suspecting a direct westward 'leak' to the open waters of the North Atlantic; and at this the ringing data do no more than hint.

Table a. Summary of first-year recoveries of Great Skuas Catharacla skua ringed in Shetland A local record in August is omitted North Sea Channel Continent West of Biscay to North coasts coasts far inland British Isles Portugal America

September 12 October 19 November 4 December 2 January 1 February - March - April - May -

5 BRITISH BIRDS The sole record for February is a transatlantic one, from the coast of Massachusetts. For March and April there are no records at all of birds of this age, and for May there is just a single record from Spain. This may in part reflect some easement in the rate of juvenile mortality and the reduction of the population at risk, but probably also an increas­ ing tendency to a pelagic mode of life. There is no sign of any return towards the natal locality by individuals about one year old. The figures for first-year Great Skuas are summarised in table 2; and see the maps on pages 10-11.

SECOND-YEAR RECOVERIES The paucity of records for the final four months of the first year of life is followed by a very marked drop in the number of records in the second year, compared with the first as a whole. The total for the second year is only 17% of that for the first. This is a low figure compared with, for example, 32.5% and 41% found in earlier studies of the Gannet Sula bassana and the Sandwich Tern Sterna sandvicmsis respectively (Thomson 1939, 1943). It is unlikely that this is due to an excessive first-year mortality on the part of a strong, swift, agile and aggressive species with a slow rate of reproduction. In all the circum­ stances, it seems more reasonable to suppose that the figures reflect a tendency, increasing with age, to a completely pelagic existence. Of the recoveries in the second year of life, nine are from the Biscay-Portugal seaboard (coasts of west France, north and north-west Spain, and Portugal) and are spread throughout the year; three more are from the western Mediterranean (east Spain, south France, and Tunisia) and details of these are given in table 3. That the only Mediter­ ranean records should fall in this year of life may well be fortuitous, as well as the fact that two of them were in November 1963 (the

Table 3. Mediterranean recoveries of Great Skuas Catharacta skua ringedi n Shetland Tables 3, 4 and 5 follow the usual conventions to indicate the circumstances of recovery: the ring number is printed in italics if the ring was returned; if the recovery date is unknown, the date of report is given within brackets; the symbols are explained in table 6

416234 pull. 7.7.62 Foula X -.11.63 Ponteau-St. Pierre, near Martigues: 43" 23' N. 50 02' E. (Bouches-du-Rhone) France AJ62J}} pull. 25.6.62 Noss 4- 4.11.63 Burriana: 390 54'N. o° 05'W. (Castelldn) Spain 41610} pull. 1.8.61 Foula X 10.5.63 Gabes: 330 52' N. io° 06' E. Tunisia

6 RECOVERIES OP GREAT SKUAS autumn in which unusual weather so greatly affected first-year birds). The total is completed by single records from east England in March (the only one even approaching the breeding area) and from New­ foundland in November.

THIRD-YEAR RECOVERIES The records for the third year of life are as numerous as those for the second, but they show a different picture. There are single recoveries in north Scotland in June, July and October, suggesting some approach to the breeding area at the age of two years. There are again winter records from the Biscay area, and also one in April. Finally, there are no less than seven records from Greenland, all in summer except for one in February, and details of these are given in table 4. It is remarkable that all the Greenland recoveries, six in summer and one in winter, should fall in this year of life (various calendar years); but more data are required to confirm that this is a definite tendency as compared with second-year birds.

Table 4. Transatlantic recoveries of Great Skuas Catbaracta skua ringed in Shetland

401218 pull. 8.7.59 Hermaness X 4.2.40 Swampscott: 420 22' N. 71° 02' W. (Massachusetts) U.S.A.

AJ7)049 pull. 16.7.62 Hermaness /?/ (7.II.63) BonavistaBay: c. 48° 5 5' N. 53° 10' W. Newfound­ land 416089 pull. 1.8.61 Foula 16.6.63 Jakobshavn: 690 io' N. 510 00' W. W Greenland 409690 pull. 24.6.56 Fair Isle X 28.7.58 Julianehab Fjord: c. 60° 45' N. 46° 00' W. SW Greenland 414)96 pull. 8.8.59 Foula + 7.7.61 Frederikshab: 620 05' N. 49° 30' W. SW Green­ land 41629) pull. 2.8.61 Foula 16.7.6} Frederikshab, SW Greenland 416180 pull. 28.7.61 Foula + 11.7.63 Frederikshab, SW Greenland 416162 pull. 28.7.61 Foula + 16.8.6} Quioge Island: 61° 56' N. 490 49' W. (^Frederik­ shab, SW Greenland 41606; pull. 28.7.61 Foula + 26.2.64 Narssaq: 6i° 00' N. 46° 00' W. (Julianehab) W Greenland

7 BRITISH BIRDS

LATER RECOVERIES The recoveries in the later years of life are few. There are two records from the Faeroe Islands in June, suggesting that there may be some interchange between the Shetland and Faeroese breeding popula­ tions. There are four records from Shetland itself in July, suggesting return to the natal area to breed from the age of approximately three years onwards; otherwise there is a single record from east England in August, and one from the Bay of Biscay in January (for years of life see table L—the oldest individual recovered was about six years of age). Apart from a single record, there are thus no ringing data relating to movement away from the breeding area in the fourth and subsequent years of life.

DIFFERENCES BETWEEN CALENDAR YEARS The foregoing analysis by years of life leaves out of account any differ­ ences between one calendar year and another. Some such differences are in fact apparent on examination of the recoveries of first-year Great Skuas. Apart from purely numerical differences, there appear to be some definite qualitative variations in the pattern of the records. October and November 1963, especially the first half of the former

Table 5. Far inland recoveries of Great Skuas Caibaracta skua ringed in Shetland Although in different countries, the two localities at which 421465 was recovered on the same day are less than ten miles apart; the record is assigned to Austria in table 1

416)$'/ pull. 14.7.62 Foula + 1.9.62 near Cherven: 530 41' 3SS. 280 50' E. (Minsk) U.S.S.R. 421268 pull. 27.7.6} Foula 0 2.10.6} River Odra, near Glogow: 51° 40' N. 16° 06' E. (Zielona-G6ra) Poland 42146; pull. 6.8.6} Foula V 10.10.6} Jochenstein: 48° 31' N. ij° 4}' E. (Nieder-Bayem) Germany X 10,10.6} River Danube, at Mederkappel: 480 28' N. 130 5}' E, (Muhlviertel) Austria 421592 pull. 26.8.63 F°ufc» X 18.10.63 Lake Constance (Bodensee), at Hard: 47" 29' N. 9° 42' E. (Vorarlberg) Austria HW01J45 pull. 8.7.64 Hermaness, Unst XA mid-10.64 Tauscha, mar Wechselburg: 50° 55' N. 120 44' E. (Sachsen) Germany 421209 pull. 26.7.6} Foula 0 X 9.11.63 Aix-les-Bains: 45° 41' N. 5 55' E. (Savoie) France RECOVERIES OF GREAT SKUAS month, were notable for the number of recoveries on the continental coasts of the North Sea and English Channel (Denmark, north-west Germany, Netherlands, north France). Unusually many of the recovery localities were some distance (20-70 miles) inland from the coasts, the furthest from the sea being at Oss in Noord Brabant; there were four such records in October 196}, as against three in all other years together. In addition, there were the four exceptional records from far inland on the Continent to which Spencer (1964) has already drawn attention; details are given in table 5, from which it will be seen that there are only two comparable records for other calendar years. In all, the Great Skuas born in 1965 contributed a disproportionately large share to the total of first-year records from the North Sea coasts and inland localities, and a disproportionately small share to the total of first-year records from the Biscay-Portugal area (compare maps A and B). The other notable point is that in the period i3th-22nd September 1959 birds of that year provided a large share (eight out of 15) of all the September recoveries of first-year individuals in any calendar year on the coasts of the North Sea and English Channel (Great Britain four, Belgium two, north-east France two; judging from experience with other species, it is improbable that war conditions were at that stage inhibiting reports of possible recoveries in Denmark and the Nether­ lands). In both 1939 and 1963 the recovery rate of first-year Great Skuas was unusually high. The ringing of 373 in 1939 yielded 14 recoveries in the first year of life; the ringing of 997 in 1963 yielded 26. The more usual situation in recent years has been a ringing total of about 400-5 50 and four to six first-year recoveries; in 1962 ringing reached a record total of 1,080, but yielded only twelve first-year recoveries. The com­ parison between the two bumper ringing years, 1962 and 1963, is particularly instructive; the first-year recovery rate in the latter was more than double the figure for the former. One may at least conclude that the phenomena described in the two preceding paragraphs are not due to ringing on an unusually large scale in the particular years. One naturally seeks a reason in unusual weather conditions, and some explanation on these lines can indeed be found; the following statements are based on an interpretation of the weather reports and maps for the relevant periods, kindly provided from the Meteoro­ logical Office (H. H. Lamb in ////.). On 26th September 1963 gales were widespread and severe in Scot­ land. Shetland had a gust of 87 knots and a maximum wind speed over one hour of 58 knots, and Orkney a gust of 78 knots and a maximum hourly wind speed of 53 knots; all these speeds were the highest ever recorded (i.e. since about 1920) in September. By 27th September the

9 BRITISH BIRDS RECOVERIES OF GREAT SKUAS

RECOVERIES IN THE FIRST YEAR OF LIFE OF GREAT SKUAS Catharacta skua RINGED AS PULLI IN SHETLAND A. circle indicates the ringing area, and each black spot approximately represents the locality of a recovery within the category of the particular map (A) Recoveries in October and November in calendar years other than 1963. In addition, crosses show four peripheral September recoveries; those not given are all on the coasts of the North Sea or the English Channel (B) Recoveries in October and November 1965 (there is only one recovery in September 1963, from north-west Germany) (C) Recoveries in December and January (all calendar years—but the winter 1963-64 contributed only two of these recoveries, both in the English Channel in December) Map A may be taken as representing the normal pattern of autumn recoveries, and Map B an exceptional one in a 'wreck' year. Map C represents the mid-winter pattern, the next phase after that shown in A. ' The only first-year recovery in February is transatlantic, and there has been none in March or April

gale-force winds had moved to the eastern half of the North Sea between Norway and the Netherlands and were WNW. Strong north-westerly winds continued on 28th September and, after a period of variable winds the next day, there were moderate north-westerly winds again on 30th September over the whole area. In October the winds were more variable, mainly from between west and north-west in the first five days and thereafter westerly until the middle of the

11 BRITISH BIRDS month. It seems probable that the great gale at the end of September was responsible for the unusual number and pattern of recovery records during the first half of October, with some effects showing even into November; there is a strong suggestion of a 'wreck' of young Great Skuas during that period. There is observational evidence that sea-birds of other species were simultaneously affected. After describing the immediate effects of the great storm at the end of September 1963, Davis (1964) wrotethat the first three weeks of October (during which 'unsettled westerly cyclonic weather continued') were 'outstanding for seabird activity'. Further, he cited a German correspondent to the effect that there were 'astonishing concentrations of seabirds in the Elbe estuary during the same three weeks', including various species of petrels, skuas and gulls. September 1939 was a month of good weather in the British Isles, with some unusually high pressure centred over south-west Scotland. Conditions over the North Sea were anticyclonic or controlled by south-westerly winds during the first ten days; but from nth Septem­ ber onwards, when a depression travelled east and slowed down, the winds were mostly northerly. Places on the east coasts of the British Isles reported northerly winds up to Beaufort force 6 on four days and up to Beaufort force 5 on three more days between nth and 22nd September; the northerly winds must have been stronger over the cen­ tral and eastern North Sea, probably just about gale force on four or five of these days. During this same ten-day period the winds were often westerly near Shetland and often north-easterly over the southern half of the North Sea and the English Channel. The period of very strong north winds almost coincides with that of the unusual September recovery records.

MANNER OF RECOVERY The manner of recovery is a relevant consideration in the inter­ pretation of ringing data; but obviously the facts do not reflect the pattern of mortality in the species as a whole, as there is a heavy bias

Table 6. Manner of recovery of Great Skuas Catbaracta skua ringed in Shetland The symbols shown are used also in tables 3, 4 and 5

v caught or trapped, and released with ring ('controlled') 3 -I- shot or otherwise killed by man 30 x found dead or dying 52 X A found long dead 6 ( ) caught or trapped alive and not released, or released but with ring removed 25 / ?/ manner of recovery unknown 3

Total 119

12 RECOVERIES OF GREAT SKUAS in favour of casualties of the kinds likely to be reported. These facts are given in table 6, in accordance with the categories used in the annual reports of the ringing scheme. Some of those in the second category were shot at sea; many of those in the third and fourth categories were, as one would expect, found on beaches; 17, mainly in the fifth category, were recorded as caught on fishing boats, through becoming entangled in the tackle, taking baited lines, and so on. Two ringed on Foula on the same day (2nd August 1963) were likewise caught on the same day (3rd Septem­ ber 1964), and apparently together, off the coast of Estremadura, Portugal.

CONCLUSIONS Ringing of pulli of the Great Skua in the Shetland Islands yields a percentage of recoveries that is unusually low for so large a bird; there is also an unusually large drop in the number of recoveries in the second year of life, compared with the first. The reason is presumably that pelagic casualties are not represented; and there are obviously large gaps in the picture of migration that the recovery records indicate. There are practically no ringing data relating to the winter movements of Great Skuas in their fourth and subsequent years of life. The general picture presented by the ringing data seems to be: (1) A dispersal of birds of the year, in autumn and winter, through the North Sea and English Channel, with some occurrences far inland (mainly under stress of unusual weather conditions in particular years); into the Bay of Biscay and off the west coast of the Iberian Peninsula; and into, and eventually across, the North Atlantic Ocean. (2) A largely pelagic existence in the North Atlantic, off Europe more than off North America, from the late winter in the first year of life and through the second and third years, with some penetration of the western Mediterranean (present evidence relating only to second year). (3) A tendency in the summer of the third year of life, i.e. at around two years of age, to visit higher latitudes—in Greenland and the north of Scotland (no evidence at present of a corresponding tendency when one year old). (4) A return to Shetland in the summer of the fourth and subsequent years of life, i.e. from approximately three years of age onwards, some reaching the Faeroe Islands (conceivably joining the breeding popula­ tion there).

COMPARISON WITH ARCTIC SKUA It is of interest to compare the records for the Great Skua with the corresponding data for the Arctic Skua Stercorariusparasiticus, but in the

13 BRITISH BIRDS latter case they are as yet too few for thorough analysis. The Arctic Skua, or Parasitic Jaeger in American usage, differs in being a unipolar and monotypic species, in having a wide breeding distribution in high northern latitudes, and in performing a transequatorial migration. The British breeding distribution is similar in being restricted to north­ ern Scotland, mostly the islands. All the recoveries to be mentioned below are of birds ringed as pulli in the Shetland Islands, most of them on Fair Isle. In the first place one may note the negative evidence that there are no recoveries, at any age, of Arctic Skuas in the period December- April, and only one in May (Brazil, as below); and that the only two recoveries in the second year of life were in the Southern Hemisphere. The number rises slightly thereafter, mainly because a few are recorded in the natal area in the period June-August from the third to the sixth year of life. In one instance an Arctic Skua ringed as a pullus on Fair Isle on 18th July 1956 was 'controlled' there as an adult female on 14th June i960 in its fifth year and eventually found dead on the coast of Norfolk about 19th August 1964 in its ninth year. As regards the movement of young Arctic Skuas of the year, these are on the wing earlier in the season than young Great Skuas and there is a wide dispersal in August. Before the end of that month there are records from as far to the east as west Denmark on 26th August 1962 and north Germany on 24th August 1961, the locality in the latter instance being in Mecklenburg, not far from Liibeck at the south­ west corner of the Baltic Sea; and from as far south as north Spain on 26th August 1956 and Algiers in August i960. At the beginning of the next month there is a record from 620 48' N on the west coast of Norway on 2nd September 1961; the other September record is from Belgium. The only two October records are from east England and south Portugal; the two November ones are from Belgium and west Denmark. After that, nothing. In the second year of life there are only an October record from Benguela, Angola (120 34' S), and a May record from near Maceio, Alagoas, Brazil (90 24' S). In the third and subsequent years there are the local summer records already mentioned, and a few other summer or autumn records from elsewhere on the coasts of Europe. In addition, there is an August record in the third year from Atangmik, Godthib District, west Green­ land (64° 55' N, 520 00' W), and a November record in the fourth year from Mossamedes, Angola (150 10' S).

SUMMARY An analysis is presented of 119 recover)? records of Great Skuas Catbaracla skua ringed as puili in the Shetland Islands. Attention is drawn to the special difficulties

H MUTATIONS OF PARTRIDGES of interpreting ringing data where there are large gaps attributable to a pelagic mode of life. The numbers of recoveries in successive years of life (June to May) are shown. The recovery localities arc tabulated by years of life and calendar months (tabic i); and the records are discussed by years of life separately. The distribution of first- year recoveties is summarised (table 2 and maps). Individual details are given for Mediterranean (table 3), transatlantic (table 4) and far inland recoveries (tabic 5). Differences in certain calendar years in the pattern of recoveries of first-year birds are discussed (maps A and B). Departures from the normal, shown notably in the number of far inland records, are related to unusual weather conditions. Frequency figures are given for the manner in which the birds were recovered (table G). For comparison, a brief account is given of the scantier data, under the British ringing scheme, on the Arctic Skua Stercorarius parasiticus.

REFERENCES DAVIS, P. (1964): 'Aspects of autumn migration at the bird observatories, 1963'. Bird Study, 11: 77-122. HUDSON, R. (1964): 'Recoveries in Great Britain and Ireland of birds ringed abroad'. Brit. Birds, 57: 583-596. LEACH, E. P. (1940a): 'Great Skua ringed in Shetland reported in U.S.A.' Brit. Birds,

(1940b, 1947, 1950): 'Recovery of marked birds'. Brit. Birds, 34: 36-43, 61-64; 39: 360-368; and 43: 313-327. SPENCER, R. (1955 el seq.): 'Report on bird-ringing for 1954'. Brit. Birds, 48: 461-498; and subsequent annual reports. THOMSON, A. LANOSBOROLGH (1939): 'The migration of the Gannet: results of marking in the British Isles'. Brit. Birds, 32: 282-289. - - (1943): 'The migration of the Sandwich Tern: results of British ringing'. Brit. Birds, 37: 62-69.

15