For birders all over the world

(1) Ornithology Group, DEEB, University of Glasgow G12 8QQ.
(2) Old Beach Farm, Green End, Landbeach, Cambridge, CB4 8ED
(3) Shetland Biological Record Centre, Shetland Amenity Trust, Lerwick, Shetland.

SUMMARY

This short note examines the plumage of a skua that turned up in the Scillies in October 2001, which is inconsistent with our understanding of Great Skua Catharacta skua. The use of primary shape to separate juvenile from older skuas is challenged. The use of moult to identify Catharacta skuas is also discussed, in particular the generally accepted model that South Polar Skua C. maccormicki always moults its primaries rapidly, growing 3 feathers simultaneously is also questioned.

The lab at Glasgow University have successfully extracted mitochondrial DNA from a feather shaft (collected by Martin Scott) and are currently sequencing the genes that will hopefully resolve some of the uncertainty surrounding the identity of the Scilly skua.

BACKGROUND

There have been a number of confusing skuas that have turned up in recent years which can be seen here http://www.magikcircle.com/birds/image.asp?title_id=574. The identification of Catharacta skuas anywhere is, at best, tricky and this is particularly so in the Northern Atlantic. The following comments are intended to provoke continued discussion on this complex and contentious topic. However, this is by no means a comprehensive identification article.

Using the Scilly skua specifically, we will consider ageing and plumage variation of Catharacta skuas in general. We shall then go on to discuss moult strategies and in particular how this may be applied to the identification process. While moult may be an important component of any accurate identification process, it is our opinion that this is still poorly understood for all species of Catharacta, particularly with respect to non-adult birds. Therefore at present we are in a "Catch-22" situation, we cannot use moult to identify birds, so we don't know which birds to fit to which model of moult strategy. In this respect, analysis of mitochondrial DNA from the "Scilly Skua" could prove vital if we are to establish a set of workable identification criteria for separating Catharacta skua species from one another, therefore we eagerly await news of this.

These comments are primarily based on our experience of large numbers of Great Skuas Catharacta skua that we have examined on breeding grounds in Shetland, Orkney & St Kilda, a group of juvenile Great Skuas held in captivity (under licence) and a large number of Catharacta skuas observed and photographed off the Senegal coast (Newell et al 1997).

THE SCILLY SKUA

A Catharacta skua was picked up injured on the Isles of Scilly during October 2001 and taken into captivity, where it was rehabilitated and finally released into the wild. A number of photographs and comments are posted at: http://www.magikcircle.com/birds/image.asp?title_id=452. These are now part of a substantial collection of pictures of large skuas that can be used for reference.

The bird's identity has been the source of much debate, varying from dark Great Skua to possible South Polar Skua, or simply unidentified.

Ageing & Identification

Ageing is an essential first step in the identification process, but the age of the Scilly bird is of some debate. While we are unsure of the precise age of this bird we feel, if it is Great Skua, it is almost certainly not a juvenile for the following reasons:

Bill pattern - typically juvenile Great Skua has an extensive pale base to the lower mandible, and this feature appears to hold true for captive birds up until at least February of their second calendar year, and some individuals beyond. Although there is also evidence that some young great skuas may develop darker bills at an early age, making this a rough guide only. However the bill of the Scilly bird (if it is a Great Skua) is very uniform, almost jet-black, therefore it seems likely to belong to a bird in at least its second calendar year. Juveniles of both South Polar and Brown Skua also show a pale base to the lower mandible (Malling-Olsen & Larson 1997). The time of year that the Scilly bird appeared means that if it were a South Polar or Brown it would be approaching 10 months old. At this age it is likely that both species should have developed an all-dark bill.

Bill structure - the prominent rhampotheca (the distal nail) on the bill of the Scilly skua appears pronounced and well developed, a feature more akin to adults rather than juveniles. Juvenile Great Skuas typically have a slender bill tip, giving them a more benign look compared to the axe-wielding bills of adults. There is good evidence that bill structure develops progressively with age. Despite the bill appearing to be fully developed it does lack the very pronounced gonys, typical of most adult Great Skuas

Plumage features - all of the juvenile Great Skuas that we have closely examined (probably 1500+ birds) show a rather distinctive internal pattern to the mantle, scapulars and wing coverts, although this may be restricted to the median and lesser coverts of the darkest birds. The pattern consists of a series of complex fine pale speckles, usually just short of the tip of the feather (e.g. see http://www.magikcircle.com/birds/image.asp?title_id=390). These can be variable in extent or patterning, large and richly coloured in "rufous" young or small and grey brown on the darker birds (e.g. see http://www.magikcircle.com/birds/image.asp?title_id=391 and http://www.magikcircle.com/birds/image.asp?title_id=475 ). None of these feathers are present in the Scilly skua, providing further evidence that this bird is not a juvenile.

The pale, albeit very fine hackles on the rear nape of the Scilly bird are another feature not shown by juvenile Great Skuas. The birds in captivity showed no evidence of hackles until they entered their third calendar year. However these were less extensive, broader and warmer than those of the Scilly bird.

It is also worth noting that the plumage of almost all-dark sub-adult Great Skuas noted on club sites in Shetland showed a small amount of paler mid-brown edging to the greater coverts and also a few paler smudges along the flanks (note that second calendar year birds are never seen at the breeding colonies). This is supported by close examination of the known-age captive skuas which, when in their second and third calendar years, revealed that even the darkest birds showed some signs of pale feather centres, usually on the lesser coverts and rear scapulars. Even though the Scilly skua was examined in extremely close detail, there did not appear to be any of these paler patterned feathers which we typically associate with Great Skua.

Primary shape - Baker (1993) states that the shape of the outermost primary in Great Skuas is a useful ageing character, pointed in juveniles and more rounded in subsequent generations. The Scilly bird did show pointed primaries, but a number of less subjective features (see earlier discussion) indicate that this bird is not a juvenile, and one is drawn to the conclusion that this may not be a dependable ageing criterion.

MOULT

Previously moult has been advocated as a useful tool for separating South Polar from other skuas. The Scilly skua showed no signs of moult throughout October and the flight feathers appeared reasonably fresh. In the following we consider moult strategies of Catharacta skuas and their application to the identification of this bird. However, the primary literature, containing original data, is limited and rather confusing. While much has been written about the rapid moult of South Polar compared with Great and Brown Skuas, moults in Catharacta skuas are still poorly described, particularly outside adulthood.

Great Skua

Adult moult - The complete post-breeding moult commences in August-early October, being completed between January & mid March-April (BWP, Malling Olsen & Larsson 1997) or February & mid-March (Baker 1993).

Post-juvenile moult - BWP, Malling Olsen & Larsson (1997) and Baker (1993) state that the post juvenile moult starts with a body moult in December/February that is completed in March. The flight feather moult starts (with the innermost primary) in March-April (as the body moult is completed) and is completed in June-late August, taking 150-180 days.

Second post-juvenile moult - The second feather moult commences in the skuas' third calendar year. This moult generally commences later than in juvenile birds, therefore becoming more akin to the timing of (but still earlier than) adults. However, two major references provide different assessments of the timing of this moult. Malling Olsen & Larsson (1997) state the moult commences in May/June, with the head and body completed in August, the flight feathers not normally completed until February/March. In contrast Baker (1993) states that the moult (again starting in May/June) is not completed until August/October (November). Olsen & Larsson (1997) state the moult takes 270-330 days which is slower than in adults, while Baker (1993) considers it takes 120-180, which is faster than in adults.

Moult in captive birds - Current information about non-adult Great Skua moult is rather conflicting. Therefore it is interesting to compare the moult strategy of 12 Great Skuas held in captivity. These birds commenced their post-juvenile moult in December (body feathers) and flight feather moult began January-February. Interestingly for all of the captive birds the second flight feather moult commenced [September-October] before the first moult was completed. This resulted in the old outermost primary still being present as the innermost were dropped and commenced re-growth - the outermost primary was effectively 12 months older than the innermost. While captive birds may be behaving in a manner not completely consistent with wild birds, it is of note that all of these birds moulted in exactly the same manner, if something "odd" was happening to their moult, might we have expected more individual variation? For argument sake, if this moult strategy does indeed approximate to the moult of wild birds, then perhaps immature Great Skuas have a continuous moult of feathers in the manner of large raptors.

Brown Skua

Adults - Falkland and lonnbergi Brown Skuas commence moult in February to April, completing in June-August, but occasionally October (Malling Olsen & Larsson 1997), or even as early as April (HANZAB). However lonnbergi from Pacific & Indian Oceans start primary moult by March-May, completing by October (HANZAB). With such variation one can imagine a situation where you could encounter two individuals at the same time of year, one of which had completed flight feather moult with the other yet to begin.

Juveniles/immatures - Primary moult appears to begin in September to January and ends March/June.

South Polar Skua

Adults - In contrast to Great Skua, South Polar Skuas are said to exhibit a very rapid flight-feather moult, taking just 45-60 days and growing up to three feathers at once (Ginn & Melville 1983, Malling Olsen & Larsson 1997). However this evidence does not appear to come from adult birds and derives from presumed immature birds in the Northern Hemisphere (Salmonsen 1976). Moreover, given that moult has been recorded from May to September, once again there is clearly considerable individual variation as to the exact timing of this process.

Immatures - Most of the evidence is that immature South Polar Skuas moult in the northern hemisphere replacing three or four flight feathers simultaneously, which does differ somewhat from the conventional wisdom of Great Skua moulting strategy. However, a number of South Polar Skuas photographed in the northern hemisphere do not appear to show a rapid moult of three flight feathers at all. Alan Brady photographed a South Polar Skua at Hudson Canyon, New Jersey in May 1976 (figure 45 in Malling Olsen & Larsson [1997]; plate 22-23 in Gantlett & Harrap [1992]) which, according to Malling Olsen & Larsson (1997) draws the reader to the "simultaneous moult of inner three primaries". In our view this bird is only moulting 2 primaries (assuming large skuas have 10 primaries), and there appears to be a distinct difference in the length of the two growing feathers. Furthermore an apparent South Polar Skua photographed off Senegal in October 1999 shows no evidence of a simultaneous replacement of three flight feathers, it appears to be growing only 1 or 2 feathers (P5 & 6 or just P6 http://www.magikcircle.com/birds/image.asp?title_id=375 )

Further evidence that moult is extremely variably not only in extent, but also timing, comes from two immature birds collected on a Southern Hemisphere island on 14th February (HANZAB). One individual was replacing its two innermost primaries with the remaining eight old, the other had virtually completed primary moult with P10 being the only feather in growth.

Conclusion

It is clear from the evidence presented here that timing of moult in all Catharacta skuas is confusing and the use of moult status at a particular time of year may not be a reliable method of identification. Moreover, the evidence for the rapid moult strategy of South Polar Skuas is, in our opinion, somewhat conflicting and requires further study. Therefore at present we feel that the state of moult in the Scilly bird does not provide us with any conclusive information as to its identity. Thus, a conclusive diagnosis of the identity of this bird from its DNA will provide a very valuable piece of this confusing puzzle and it could help to resolve the identity of other confusing skuas

ACKNOWLEDGEMENTS

Many thanks to Martin Scott for his help with obtaining feathers and to Dave Hatton and Simon Stirrup for providing good photographs of the Scilly bird.

REFERENCES

Baker, K. (1993) Identification guide to European Non-passerines. BTO Guide, London.
Furness, R.W. (1987) The Skuas. T & A D Poyser, London.
Ginn, H.B. & Mellville, D.S. (1983) Moult in birds. BTO Guide 19.
Gantlett, S.J.M. & Harrap, S. (1992) Identification forum: South Polar Skua. Birding World, 5, 256-270.
Malling Olsen, K. & Larsson, H. (1995) Terns of Europe & North America. Pica Press, Sussex.
Malling Olsen, K. & Larsson, H. (1997) Skuas & Jaegers. Pica Press, Sussex.
Newell, D., Porter, R. & Marr, T. (1997) South Polar - an overlooked bird in the eastern Atlantic. Birding World 10: 229-235.
Salomosen, F. (1976) The South Polar Skua Stercorarius maccormicki Saunders. in Greenland. Dansk orn. Foren. Tidsskr. (1976) 70: 81-89.
Handbook of Australian, New Zealand and Antarctic Birds (HANZAB), Volume 3, Oxford University Press, 1996