Sunday, September 25, 2016

Identification of a South Polar Skua

Skuas in the Maritimes

Most Great Skua reports in our province are of birds between August and early April (McLaren 2012), while some birds can remain in the North Atlantic throughout the year (Newell et al. 2013). Great Skuas have been recorded in summer off New England and the Maritimes (Dunne 2006). The South Polar Skua is typically seen from late May through September (Mclaren 2012). Given the timing overlap, care must be taken when identifying these birds.

Size
Those with experience seeing both skua species notice subtle differences in size and structure between the two species expected in Nova Scotia waters. In relation to Great, South Polar averages smaller and more lightly built, with more slender bill and narrow wings (Alderfer 2014). Newell et al. (2013) caution that these differences are difficult to quantify, and due to the overlap in bill, wing, and tarsus measurements, species identification usually relies on plumage features.

Adult Plumage
Adult South Polar Skuas are cold-toned, with generally plain head, neck and upperparts, except for pale buff and gold streaks on the hindneck (Newell et al. 2013). Adult Greats are warm-toned, with extensive pale buff and gold streaking and mottling on the head, neck and upperparts (Newell et al. 2013). Most, but not all, adults show a dark cap (Dunne 2006). Both of these species have light, intermediate and dark morphs, but McLaren (2012) states that the light morph South Polar is extremely rare in the North Atlantic. Behrens and Cox (2013) describe how Great Skuas typically show clear contrast between the paler wing coverts and darker flight feathers and very little contrast between the coverts and back, whereas South Polar Skuas have uniformly blackish wings.

Juvenile Plumage
Juvenile South Polar Skuas fledge in about February, and have medium to dark grey head and body plumage contrasting with dark upperparts and underwing coverts. They sometimes show a paler back and can have a slightly paler band across the nape or distinctly paler head than body (Behrens and Cox 2013). Juvenile Great Skuas fledge in about August, have variably rufous (or reddish)-toned underparts, a dark hood and contrasting dark upperparts, with broad, paler, crescent-shaped subterminal markings on the scapulars and wing coverts (Newell et al. 2013). They are overall darker and cleaner looking than adults and show two-toned wings similar to adults, but with less contrast (Behrens and Cox 2013). The leading edge of the wing is often rusty brown (Behrens and Cox 2013). Some dark juvenile Great Skuas can appear to entirely lack the pale marks on the scapulars and thus are similar to South Polar. (Newell et al. 2013)


The September Brier Pelagic Skua

On September 24, 2016, forty-one birders gathered at Brier Island to partake in a pelagic trip organized by Mark Dennis. Two skuas were observed during the 4.5 hours that we were in the Bay of Fundy. Views of the second skua prompted discussion on the boat as to its identification. Many photos were taken and it was decided that we should review the literature and photographs later to reach a confident conclusion at the species level. My photos of this bird are presented here.

Figure 1. South Polar Skua in the Bay of Fundy on Sep 24, 2016. Shape and size are hard to judge, but the head appears small, the bill slightly thinner and chest less bulging in comparison to a Great Skua. The dark portion of the face appears to form a mask rather than a cap like in Great Skua. Photo by Alix d'Entremont.

Figure 2. South Polar Skua in the Bay of Fundy on Sep 24, 2016. The pale nape is evident here, but otherwise, the upperparts are uniform dark without any appreciable mottling or streaking. Photo by Alix d'Entremont.

Figure 3. South Polar Skua in the Bay of Fundy on Sep 24, 2016. The almost complete primary moult is visible in this photo. The outer two primaries (P9 and P10) are growing. Photo by Alix d'Entremont.

Since primary moult is visible on this bird, a recent paper titled South Polar and Great Skuas: the timing of primary moult as an aid to identification (Newell et al. 2013) provides helpful details. This work presents a moult scoring system and an accompanying graph where date and moult score are the x and y axis, respectively. A moult score of 0 points represents a bird with old and unmoulted primaries, while 50 points would be given to a bird with recently and completely moulted primaries.

Our September 24 skua has almost completed moulting its primary feathers. P1-P8 are completely grown (40 pts) while P9 is almost (4 pts) and P10 is more than half grown (3 pts). This gives our bird a primary moult score of 47; this combined with the date of September 24, plots this bird in a region of the moult score graph presented in Newell et al. (2013) that is unique to older South Polar Skuas. The authors also urge that the resulting identification from the moult score graph should align with other features of the bird in question. The uniform back and wing-coverts and pale neck are supportive of South Polar Skua.

The moult score graph in Newell et al. (2013) is a very useful tool to confirm tentative skua identifications. We were fortunate that the age and moult timing of this bird resulted in only one option (adult or sub-adult South Polar Skua). Using the graph with some individuals will result in two or three options. In those cases, aging, morphology and plumage features are required to narrow down to species.


References

Alderfer, J., J.L. Dunn. 2014. (Ed). Complete Birds of North America, 2nd Edition. National Geographic Society. Washington DC, USA.

Dunne, P. 2006. Pete Dunne's Essential Field Guide Companion: A Comprehensive Resource for Identifying North American Birds. Houghton Mifflin Company, New York, NY.

McLaren, I.A. 2012. All the Birds of Nova Scotia: status & critical identification. Gaspereau Press Ltd, Kentville, N.S., Canada

Newell, D., S.N.G. Howell and D. Lopez-Velasco. 2013. South Polar and Great Skuas: the timing of primary moult as an aid to identification. British Birds Issue 106, pp. 325-346.

Behrens, K and C. Cox. 2013. Peterson Reference Guide to Seawatching: Eastern Waterbirds in Flight. Houghton Mifflin Harcourt Publishing Company, New York, NY.

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