Studies of less familiar birds 133. Long-tailed Skua

Transcription

1 Studies of less familiar birds 133. Long-tailed Skua By D, G. Bell Photographs by J. B. ami S. Bottomley, Eric Masking, L. A. Portenko and D. A. P. Cooke* (Plates 21-24) DISTRIBUTION THE LONG-TAILED SKUA Stercorarius longkaudus is by far the rarest of the four species of skuas off the coasts of the British Isles. Yet it nests no further away than the Hardanger Vidda in south Norway, on virtually the same latitude as Shetland, and from there northwards in a wide belt embracing the highest parts of the Norwegian and Swedish mountains. North of the Arctic Circle it extends eastwards through Finnrnark the breeding headquarters in Europe outside Russia and continues right across the northern edge of the Eurasian tundra, including the arctic islands. The Nearctic race palkscens (which has been recorded in the Outer Hebrides) breeds from north Greenland and Ellesmere Island south to the shores of Hudson Bay, extending west to Alaska and east to Labrador. The distribution of the species is thus circumpolar. Besides reaching further north than any other skua, the Long-tail extends below the Arctic Circle in several parts of *Dr. D. A. P. Cooke's photographs of light and dark phase Arctic Skuas S. parasiticus (plates 23a and 23 b) are included for comparison. EDS. continued

2 BRITISH BIRDS its range. Indeed, on the west coast of the Bering Sea it nests on the same latitude as southern England. Such a wide distribution gives a misleading impression of actual numbers, however. In the breeding season this skua is largely dependent for food on the various species of lemmings ~Lemmus and Dicrostonjx creatures with notoriously fluctuating populations and Loppenthin (194}), among others, has established a close correlation between its breeding cycle and theirs. Taking records from Greenland, Finnmark and Novaya Zemlya, he showed that in seven instances peak numbers of breeding Long-tails and full clutches coincided with lemming maxima (which occur about every four years), while in 25 cases no Long-tails at all nested in years of lemming minima. That there are only five breeding records from Spitsbergen is at least partly due to the complete absence there of lemmings and any other endemic rodents; adult Long-tails are often seen and the terrain is apparently ideal, but the only rodents are a few accidentally introduced mice and rats in the mining settlements. Because of this dependence on lemmings, the Long-tailed Skuas south of the Arctic Circle are very irregular breeders. Even where the populations are densest, in the far north, the individual pairs nest further apart than do other skuas. Virtually no counts are available, but the tentative average total advanced by Merikallio (1958) of 1,500 pairs in Finland seems too high (as he himself admitted), particularly as the species does not nest south of the Arctic Circle in that country. BREEDING In Europe the breeding grounds are exposed, stony fells or tundra, with scant vegetation (typically lichens, crowberry Empetrum and dwarf birch Betula), at altitudes varying from sea-level to 4,000 feet, but in Finnmark mostly between 200 and 400 feet. There is usually water or swampy ground close by, though the actual nest-site is dry. A typical habitat is shown on plate 22b. The birds arrive in late May or early June, but the times of arrival and egg-laying vary greatly according to the weather. The clutch usually consists of two eggs (though often only one) laid in a scantily lined depression in the ground. Both sexes share incubation duties, one on guard while the other sits. The young hatch after about 23 days, leave the nest in two days and depend upon their parents until at least ten days after flying. Intruders of any kind are mobbed, sometimes determinedly, but this species is, on the whole, less demonstrative and aggressive than its relatives. When the nesting-area contains more than one pair, neighbouring birds may participate in such demonstrations or at least show their curiosity. There are several cases of a photographer working with one of these graceful birds standing on his head (plate 24c). 140

3 LONG-TAILED SKUA STUDIES MIGRATIONS It is not the very variable numbers of Long-tailed Skuas nesting in Europe which make them rare off British coasts, but their migration routes, and our knowledge of these is incomplete. Odd ones do pass along British shores and those on the east coasts of England and Scotland are likely to be of Fenno-Scandian origin, but such occurrences are decidedly uncommon in autumn and almost unknown in spring. Five experienced sea-watchers of my acquaintance have seen a combined total of seven Long-tails, out of thousands of other skuas, in the last ten to 18 years of regular watching on the north-east coast of England, mainly at Hartlepool (Co. Durham) and Spurn (Yorkshire). It is unthinkable that if most of the European population used the North Sea they would not be recorded more often, particularly at the bottleneck of the Straits of Dover. High altitude migration, possibly by some overland route, has been suggested, but the scattered and isolated records inland in Europe offer very unconvincing evidence of this. (The incidence of inland records is rather high by comparison with those of other skuas, but the actual number is still very small and may merely reflect the comparative weakness of this species in stormy conditions.) In fact, it is likely that most European Long-tails pass between Iceland and the Hebrides, far out to sea, and join those from Greenland and eastern North America to fly down the mid-atlantic. Such a route is, in fact, direct, if one accepts the hypothesis that the species winters off the southern coasts of South America. It must be more than coincidence that, apart from exceptional instances of exhausted, wind driven flocks clearly off course (Nelson 1907 gave a dramatic account of the many seen on the east coast during the famous gale of October 1879), the central and western Atlantic is one of the only areas where big numbers of migrating Long-tails have been recorded. There steady northward movement in settled weather in May has been observed by, for example, V. C. Wynne-Edwards (writing in Bannerman 1963). It is also significant that the north-west coasts of Ireland have provided over half of the few spring records in the British Isles and that Swedish and Finnish birds arrive from the west over the north Norwegian mountains, not from the Baltic. On the American side of the Atlantic, spring records extend from the Tropics to the Arctic. In winter Long-tails desert the northern hemisphere and are then presumed to be far out to sea off the southern coasts of South America, perhaps with pallescens predominating on the west side and longicaudus on the east. Wynne-Edwards (in Bannerman 1963) quoted the observations of A. Wetmore and H. H. Beck who saw thousands in this area in winter. The paucity of such records is doubtless due to the decidedly pelagic existence of this species out of the breeding season and to the confusion between it and the Arctic Skua S. parasiticus after 141

4 BRITISH BIRDS the autumn moult, when the Long-tail's diagnostic tail streamers are lost and dark streaking appears on the breast. FEEDING The food in the breeding season is very different from that in the rest of the year. In the short Arctic summer, as already stated, lemmings are the staple diet. Other rodents may also be taken then, but birds figure very sparsely. Although, for example, buntings Emberii^a spp. are sometimes caught, the skuas often take a playful rather than a gastronomic interest in these fellow inhabitants of their breedinggrounds; waders such as Turnstones Afenaria interpres and Purple Sandpipers Calidris maritima are rarely molested, though their chicks are on occasion. On the other hand, insects and their larvae, Crustacea, worms and vegetable matter (particularly crowberries) are freely taken and the young are fed on such items to a large extent during their first few days of life. Insects are caught in straight pursuit or by soaring. Away from the breeding-grounds the sea supplies the food, but Long-tails are less piratical than the other skuas. (Some experienced observers have even doubted whether they ever chase gulls and terns, but they certainly do so sometimes.) They hawk to and fro low over the water with languid flight reminiscent of ponderous marsh terns Chlidonias spp., picking up floating offal, and they will also accept such offal thrown to them from fish-quays, retrieving it by hovering, upending or wading (Bell 1964). Incidentally, they also hover when seeking out lemmings from their holes. Carrion is taken freely. IDENTIFICATION Many reports of Long-tailed Skuas are the result of misidentification. Too few observers are sufficiently aware of the considerable variation in the Arctic Skua's body size and tail projection. It therefore seems worth giving the main characters of the Long-tail in somte detail. Adult The most striking feature of the adult is the pair of excessively long central streamers (plate 24b) which typically project up to nine or ten inches beyond the other tail-feathers (themselves graduated). Besides being so long, these are remarkably flexible, moving independently of each other and even curling backwards at times. When the bird is hovering, the tips vibrate violently as though made of rubber. These feathers, however, may project as little as 6 inches even when fully grown, whereas those of some Arctic Skuas extend to as much as 4^ or 5 inches beyond the rest of the tail; they may also have been bitten off by other skuas, while young birds do not grow them fully until about their third year and adults moult them every autumn. 142

5 LONG-TAILED SKUA STUDIES When present, these streamers, together with the slender body and neck,, give the bird a characteristic 'elongated' look which is often a surer guide in identification than its build. Undoubtedly it is typically slighter than the Arctic, but individual variation in size demands great caution. The popular idea of the Long-tail's 'tern-like' appearance can be exaggerated, and refers as much to its feeding methods described above as to its build which, though slender, is by no means frail. Small Arctics approach Long-tails in bulk (just as big ones may come near to Pomarine Skuas S. pomarinus) and the sight of a small palephase Arctic Skua with tail-streamers of extreme length, particularly just after some heavy dark-phase immatures have passed by, has misled many an unwary observer. The Long-tail's wings, though thinner than those of other skuas, are still substantial; the outer edge of the hind part is convex, or paddleshaped, thus preventing the tapering effect usually characteristic of a tern. The wing length can be striking, particularly in relation to width, producing a difference from the Arctic which is discernible to a practised observer, though only really conspicuous when seen from below {cf. plates 24a and b) a view not usually obtainable of passage birds. In normal flight the slighter build of the rarer species gives it a comparatively buoyant action with a rather shallow wing-beat, but, again, direct comparison with a tern would be misleading, for there is no noticeable 'bounce' of the body on the down-stroke. Unfortunately, Long-tailed Skuas are most likely to appear when gales are blowing and then the slight difference in flight action is lost, all skuas either labouring heavily into the wind or streaking along with it. In general coloration the adult Long-tail approximates to the palephase Arctic Skua. (A dark phase of the Long-tail has been described, but is apparently known only from juveniles and is very rare; for all practical identification purposes, only the light phase need be considered.) A basic difference is that the back, mantle, rump^ base of tail and wings are noticeably paler and greyer, affording greater contrast with the blackish cap and the dark hind border to the secondaries {cf plates 21 and 22a with 23a). Instead of the shafts of all the outer primaries being white, only those of the outermost two are. Despite the inference in some standard text-books, this does not mean that this is the only paleness on the wing: underneath there is often a pale area (varying considerably, but quite extensive on some, probably subadult individuals) formed by the whitish bases to the vanes of the primaries, palest where these join the under wing-coverts, which may themselves be tipped with whitish. In addition, when the spread primaries are seen from below against a bright sky, a pale subterminal 'flash' is visible across them; this is due to the light shining through that part of each primary which is not either too dark the tip or 143

6 BRITISH BIRDS overlapped by the next one (plate 24b). The upper surfaces of the outer primaries show palish webs, but much less extensively than those of a typical Arctic Skua. It must be noted, however, that in the latter species the shaft of the fourth primary is sometimes also brownish and the wing flash inconspicuous: the distinction is not therefore conclusive. The blackish forehead and crown extend further down the nape, and tend to look darker and more sharply defined (plates 21 and 22a). The pale collar round the neck and on the cheeks is usually a deeper straw-yellow, and almost always wider. The chin, throat and breast are whitish, without the dark pectoral band often present in the Arctic Skua (but the latter may have much paler cheeks and neck than the one on plate 23a). In the European race the deep dusky grey of the under tail-coverts usually (but not always) spreads beyond the belly and sometimes as far as the breast, forming a more conspicuous and extensive dark area on the under-parts than in the Arctic Skua and making the whiteness of the upper breast and collar correspondingly more obvious. This dusky shade on the under-parts also spreads upwards on to the flanks and sides of the breast as a progressively fainter wash. The beak is thinner and usually shorter, and is all black; the tarsus is grey and the tibia, joints, toes and webs are black. In the Arctic Skua, only the end of the beak is black, while the legs and feet are completely so. The sexes of the Long-tailed Skua are similar, but females average larger, with longer tails. Immatures The sequence of plumage changes from juvenile to adult is variable and incompletely known. The bird of the year is a colder, greyer brown than the similar but darker and richer plumage of the Arctic Skua. The pale edges and tips to the upper-parts are ashy (not rich buff or rusty) and, being wider, form heavier barring. The upper breast and throat are streaked brown, as in the Arctic, but at least sometimes these streaks do not extend so far down: the lower breast and belly are often very pale. The flanks, axillaries, under-wing and under tail-coverts are more heavily barred than those of the Arctic. In the hand, or at exceptionally close range, the rectrices can be seen to have rounded tips, as opposed to the spiked tips of the Arctic's (the text illustration in Witherby et al hardly shows this at all). This distinction (which also applies to the outer three primaries) is valid for the first and second years only (Walter 1962). The third-year bird has predominantly ashgrey upper-parts, breast and flanks..the barring and pale tips to the feathers become less conspicuous with age, and the under-wing darker, and by the fourth summer the only traces of immaturity are some barred or pale-tipped axillaries, under wing-coverts and tail-coverts. The young Arctic has greyish legs with the distal portion of the webs black, 144

7 KESTREL FOOD IN NORTHERN IRELAND as in the Long-tailed, but the distinctions concerning build, flight and the white primary"shafts are the same as in the adults. REFERENCES BANNERMAN, D. A. (1965): The Birds of the British Isles. Edinburgh and London, vol. 12. BELL, D. G. (1964): 'Feeding methods of Long-tailed Skuas'. Brit. Birds, 57: 434. LOPPENTHIN, B. (1943): Systematic and Biologic Notes on the Long-tailed Skua. Copenhagen. MERIKALLIO, E. (195 8): Finnish Birds: Their Distribution and Numbers. Helsinki. NELSON, T. H. (1907): Birds of Yorkshire. London, vol.2. WALTER, H. (1962): 'Vergleichende Untersuchungen an den Raubmowen Stercorarius parasiticus und longicaudus'. J. Orn., 103: WITHERBY, H. F., JOURDAIN, F. C. R., TICEHURST, N. F., and TUCKER, B. W. (1941): The Handbook of British Birds. London, vol. 5, p

8 PLATE 2 I. Long-tailed Skua Stercorarius longhaudus standing by its eggs, Sweden, June i960 (pages ) {photo:]. B and S. Bottomley)

9 PLATE 22 Long-tailed Skua Stercomrius longkaudus on its nest, Sweden Tune the b eas^anvthe^ *.?**-*«*> well-defined cap, the 'dean' cheeksand oak ^ t l the /o^8jail-streamers (pages i 42 - I44 ). Below, the habitat of this pair, a stony fell with scant vegetation (page I4o) {photos: J. B. and S. BottoJ^

10 PLATE 23. Light and dark Arctic Skuas Stercorarius parasiticus, Shetland, June 1964, for comparison of colour, build and tail-length with Long-tailed; even the light phase has darker upper-parts, a less clear-cut cap, pectoral shading, a paler bill and, of course, a shorter tail (pages ) {photos: D. A. P. Cooke)

11 PLATE 24. Above, Arctic Skua Stercorarius parasiticus (left) and Long-tailed Skua S. longkaudus in flight, Orkney 1946 and Finland 1958, to show differences in tail-length, wing-shape and build (pages ) {photos: Eric Hosking). Below, a Long-tailed Skua standing on an intruder, U.S.S.R. (page 140) {photo: L. A. Porienko)