Sunday, 21 September 2014

A presumed South Polar Skua on the Azores August 2014

August 27th was already a good day - multiple sightings of a Swinhoe’s Storm-petrel out on the Bank of Fortune plus Bulwer’s Petrels, Monteiro’s Storm-petrels and Great Shearwaters galore. Mid afternoon, I was doing the chumming as usual when out the corner of my eye I spotted a bird coming in to investigate the boat. It was purposeful, robust – a catharacta skua (or what was formerly called a catharacta skua) – and as such I shouted to everyone to take a look. This was the first ever large skua (Great Skua/South Polar Skua) that we’ve recorded on August pelagics off the Azores, so inevitably I was a little excited. We’d seen Great Skuas in late May 2011, but with an increased understanding of South Polar Skua migration from waters in the northwest Atlantic southeastwards in early autumn, as well as recent extralimital records from Lanzarote and Madeira, this species was always going to be a possibility in Azorean waters in late August.
Fortunately, the bird came in a couple of times and it was evident in the field that we were dealing with a bird in wing moult, but in relatively dull conditions and with the bird above the boat, it was a case of take photos and look at the details from there. The identification consensus on the boat was split, with some people feeling that it felt lighter in build than your typical Great Skua while others were rather nonplussed. Nevertheless, once we got back to land and to the hotel, the first thing I did was whack an email off to one of the authors of the British Birds paper on the South Polar/Great Skua identification.
Having had the experience of the Great Skuas off Graciosa in May 2011, I knew the key to getting the identification right is getting the ageing correct. What you have to think about is when in the year birds in the southern hemisphere are born compared to Great Skuas – basically 6 months apart. Adults of both South Polar and Great Skuas have one wing moult each year, after the breeding seasons, while birds in their first 12 months have two wing moults.

Initially, ageing proved difficult and therefore has made the process more protracted. With Martin Gottschling being particularly proactive in his approach, this bird has been aged as in its 3rd calendar year or older on the basis of cold-toned, scalloped scapulars that lack any streaking or golden tones to be expected in a 2nd calendar year Great Skua in late August. Additionally primary moult would either fit a 2nd calendar year Great Skua or an older South Polar Skua – with P10 in pin (1), P9 half grown (3) and P1 to P8 fully grown (8 x 5) I’d be giving this bird a primary moult score of 44. For late August, this would indicate a South Polar Skua taking the moult score charts as red (despite a limited sample) and that the bird has been aged correctly.
The bill is also relatively long and parallel-sided (described as a meat cleaver by one of the authors of the paper!), and the underparts are smooth contrasting with a dark underwing. Although not obviously capped, there is a lack of neck speckling which again is a pro South Polar Skua feature. There has been talk of suspended moult in this bird (in relation to P1 and P2), but I have struggled to see this.

It’s also interesting to look at a confirmed South Polar Skua in the North Atlantic at a similar time of year and presumably of a similar age. A bird ringed in the South Shetland Islands in 2005 was remarkably seen in waters off Martha’s Vineyard, Massachusetts on 15th August 2007. Photos and details can be found here. The primary moult, body colouration and extent of white on the primary bases is near identical to this Azorean bird.

The process of identifying South Polar Skua is really tough, and despite what seems to clarify and categorise birds by primary moult scoring, this last few weeks has clearly shown me there is still a lot to be learnt. Identifying these birds is far from being a precise science, with a lot of subjectivity on plumage features let alone any consideration of hybrids from the southern skua complex (including South Polar and Brown Skuas). However, documenting birds such as this that seem to fit the bill based on current criteria and then reviewing them going forward on increased knowledge can only be of benefit to the whole identification process.

Many thanks to several people involved in this record, most notably Martin Gottschling who reinvigorated interest in this bird, as well as Dani Lopez Velasco, Dick Newell, Chris Batty and Peter Alfrey for providing decent discussion and airing their views. Also to everyone on the boat who experienced it in its true life glory.

No comments:

Post a Comment