Saturday, September 1, 2012

Another South Polar Skua - Woodman Point

UPDATE: The identity of this bird has now been accepted by BARC.

Another intermediate-morph South Polar Skua Stercorarius (Catharacta) maccormicki has been reported in south-west WA, seen on the 3rd of May flying briefly over the end of Woodman Point, south of Fremantle. Unfortunately only the silhouette and underside of the bird were able to be photographed, but nevertheless several key diagnostic features confirm the bird’s identity:

Underbody contrast
Though the early morning light is not ideal, the bird shows the diagnostic hallmark of strong contrast between the fairly uniform, cool-toned, paleish grey-brown body, and the evenly cold black underwing coverts and axillaries. There are no rusty or rufous tones in the plumage as is frequent on Brown Skua [1, 2]. Other supporting plumage features include a fairly substantial white ‘flash’ in the primaries; a small but distinct dark loral eye-mask on the paler head; and the suggestion of a whitish grizzled area in the interramal area at the base of the lower bill.

South Polar Skua Stercorarius maccormicki (NB. subject to BARC acceptance) at Woodman Point, 3rd May 2012. Note the strong contrast between the pale, evenly grey-brown body and cold black undercoverts; whitish area at the base of the lower bill (interramal area); long narrow wings and short tail.

Rapid Primary Moult
There appears to be a large moult gap of four primaries in each wingtip (two growing, two missing), in a very symmetrical pattern suggestive of a normal moult sequence. This moult pattern alone is virtually diagnostic of South Polar Skua, which are known to have a rapid moult strategy, moulting 3-4 primaries at a time (cf. 1-2 in Brown Skua) [1, 2]. This rapid-type primary moult during passage (completed in 45-60 days) is unique among the skuas, instead having more in common with other super-long-distance migrant seabirds such as Arctic Tern and Great Shearwater [2]. In contrast, the prolonged moult of the Brown Skua group begins earlier (from the end of the breeding season), but can sometimes extend right throughout the non-breeding season [3].
Moult detail (primaries numbered). Note two primaries growing and two missing on each wing,  indicative of the rapid moult strategy employed uniquely by South Polar Skua during migration.

The upperwing shows moult of the upper greater coverts, creating an extended 'flash' from the exposed secondary bases, typical of South Polar at this stage of moult [2]. Also note the slender body, and small-headed and narrow-necked structure.

Compared to a typical local lonnbergi Brown Skua, the following structural features are supportive of South Polar Skua (though not diagnostic given the wide variation in both species):
• relatively slender body [1-3]
• small-headed and slender-necked appearance [2-3]
• notably long, narrow wings, especially ‘arm’ portion; narrow wing base [2-3]
• short tail; minimal projection of central retrices [1]

South Polar Skua passes overhead at Woodman Point. Note the 'big-eyed' impression created by a dark loral eye-mask on the paler head, typical of the species.

The bill is difficult to assess given the ventral flight shots, but appears typical of South Polar Skua: it is relatively modest in size and length, fairly narrow and tapering laterally; the upper bill ‘nail’ is modest and not particularly bulbous or hooked; the gonys (lower bill tip) is fairly short and the gonydeal angle is shallow and inconspicuous [1-3].

Bill detail of the Woodman Point skua

A very similar-looking intermediate-morph South Polar Skua (NB. subject to BARC acceptance) was seen at Bremer Bay in April, and was apparently resident inshore for at least a week (see previous post). This Woodman Point sighting is then the second inshore/land-based record of this rarely-sighted species in southern WA in the space of a month, perhaps challenging the view that the species normally migrates via the continental shelf and open ocean.

These sightings may also shed light on movements of South Polar Skua in the Indian Ocean, which are poorly known. The timing of this sighting in early May fits closely with published sightings of South Polar Skua in WA [1], corresponding to their northward passage towards (largely unknown) wintering areas. Woodman Point, which projects some 2km into Cockburn Sound, is a well-known local site for ‘jaeger-watching’ to view the similar autumn passage of the smaller skuas (jaegers) - but it obviously pays to check the larger skuas carefully as well!

[1] Higgins PJ & Davies SJJF (1996) Handbook of Australian, New Zealand & Antarctic Birds (HANZAB), vol. 3: Snipe to Pigeons. Oxford University Press. 
[2] Olsen KM & Larsson H (1997) Skuas and Jaegers. A guide to the skuas and jaegers of the world. Pica Press.
[3] Shirihai H & Jarrett B (2008). The Complete Guide to Antarctic Wildlife: Birds and Marine Mammals of the Antarctic Continent and the Southern Ocean (2nd ed.). Princeton University Press.

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