Sunday, July 6, 2008

South Polar Skua in Bocas del Toro

George Angehr forwarded Andrew Kratter's report of South Polar Skua seen June 2 on Isla Colón:

South Polar Skua
(Stercorarius mackormickii)
3 June 2008
Panama: prov. Bocas del Toro; Isla Colon; Bocas town & STRI dock

Observer:
Andrew Kratter
Florida Museum of Natural History
Box 117800
University of Florida
Gainesville, FL 32611
kratter@flmnh.ufl.edu



This bird was first seen flying low over the water at 0900 off the STRI facility in Bahia Almirante. I did not have binoculars at the time, and I blew it off as a dark juvenile Pomarine Jaeger (S. pomarinus), not really considering a skua (South Polar Skua or Great Skua, S. skua) see below for discussion of species). The bird was dark, very bulky, with a short tail and no tail projections. It flew toward us, then right overhead, but right into the sun. It had heavy and deep wing beats, slower than typical for pomarines (or other jaegers).

At 1430h I was in Bocas town and my wife (Mary Hart) said she saw a jaeger, seeing the large white wing flashes. I saw the bird immediately, as it cruised back and forth along the waterfront, 50-100 feet out. Sun was behind, and thus we got very good looks, even though I did not have binoculars again. The first thing I noticed was its heavy build, broad wings, and short tail. I immediately thought it was s skua. The bird was dark, with browner color showing on back and ventrally; I did not notice if it was capped. The bird appeared to be the size of a large gull, but much thicker bodied, with a broader and shorter tail. The wings were very broad toward the body, unlike any gull or jaeger I have ever seen, and rather rounded tipped. It’s wing flaps were somewhat labored and quite deep. The bases of what appeared to be all or most primaries were white, showing both dorsally and ventrally. The tail was dark, broad and quite short. The bird showed very little body protruding behind the wings, and this, coupled with the thick chest and short tail gave the bird a very bulky look, quite unlike the longer bodied look of pomarines and other jaegers. It gained some elevation after about 2 minutes observation, and eventually soared next to a pale type jaeger, probably a Parasitic (S. parasiticus), although I later that day saw a pale subadult Pomarine Jaeger in similar plumage The comparison was stunning in the differences shown. The Jaeger looked much smaller and leaner, with thinner wings. A good bit of its body projected beyond the wings, and the tail was longer. I could not see any tail projections, but the bird was at a height where it may have been impossible to see the short projections in juv jaegers. I could see its white belly and lower breast, dark throat and upper breast, and distinct white flashes in the primary bases (from below). The skua was much darker, still showing no pale areas to the plumage other than the primary bases.

I cam back to town at 1515h with scope, binoculars, and camera but could nto find the bird in 1.5 hours of searching. At 1745h, My wife and I went out by boat to try to find and hopefully photograph the bird. While boating along the Bocas waterfront, the bird soared in high from the south. This time I had binoculars and my point & shoot Canon. It stayed pretty high, but I got a few distant shots, that ended up grainy when enlarged to size. Same impressions of size and shape as before. Better views of underparts, which were brown, a bit warmer in belly. Even some secondaries showed white bases from below. No pale markings in the plumage aside from the bases of the flight feathers. The underwing coverts were very dark or black. I still could not judge bill dimensions. The bird circled for 5-10 minutes, then flew off west, over Bocas town and towards the Caribbean side of Colon. Afterwards we went to Saigon Bay, where jaegers congregate at dusk. There we saw three different Pomarines (one light, two intermediate) and one unidentified jaeger, but no Skua.

The best photo was posted to identification forums on the internet (birdforum.net and Frontiers of Identification) and I sent the image to Klaus Olson, author of the Princeton Guide to Skuas and Jaegers. None of the responses favored Pomarine Jaeger over either skua species, though a few were noncommittal. The responses are attached below. The photo (attached) shows a straight on ventral view of the bird soaring, and thus is good for showing shape. As I had noted, the wings are very broad, rounded tipped, with large white flashes at the bases of the primaries and some secondaries. The underwing coverts look wholly dark. There is molt of the inner primaries on both wings. The tail is dark with no sign that the tips of the central rectrices protrude. The bird looks dark below, but if the shadows are brightened on photo imaging software (I use I-photo on a Mac), the head, breast, and belly are brown to the black underwing coverts. The bird is broad across the chest and the neck and head appear large.

Although I have never seen any skua, I have seen lots of photographs and video of them. I see Pomarine and Parasitic Jaeger nearly daily on the water in Bahia Almirante, where I have been working since late January. I was immediately struck by the large build of this bird, compared to any jaeger. When this bird soared next to a jaeger the structural differences were dramatic, with the skua showing much broader wings, a thicker body, a shorter tail, and much less body projecting beyond the wings. Although large female Pomarines may look quite a bit bigger than small male Parasitics, they would not show the scale of differences, nor the structural differences, that I saw between the skua and the jaeger. Olson (see below) notes that subadult jaegers would show some pale mottling in the underwing coverts, whereas the bird I saw had completely dark underwing coverts. Note that adult jaegers, which can have all-dark underwing coverts, would have projecting tail tips in June. I am therefore confident that this bird was a skua and not a jaeger. South Polar Skua would appear to be the most expected of the skuas at this time of the year in the Caribbean, although there are no Caribbean records for this species. It breeds in the Antarctica and South Shetland Islands during the austral summer, and occurs in northern oceans (Pacific and Atlantic) during the boreal summer. It thus migrates through tropical waters in passage. Interestingly, the only Caribbean record of a skua, is that of a Great Skua (C. skua). Northern populations of Great Skua usually winter south only to the Tropic of Cancer, but there are records (one each) in Belize (date?) and Guyana (date)? The warm brown tones to the underparts of the bird seen in Panama is more typical of South Polar Skua. Note also the comments of Klaus Olson regarding molt (appended below). He stated that northern populations of Great Skua (and also jaegers) should not show primary molt this time of year.

I do not have all references at hand, but according to the AOU check-list (1998), this would be the first record of South Polar Skua in the Caribbean. There is one record of Great Skua for Belize and another from Guyana. The status of South Polar Skua in Panama is hypothetical, with no specimens, but several sight reports from the Pacific Coast (Ridgely an Gwynne 1989)

Andrew Kratter
7 June 2008



Email from Klaus Olson


Dear Andy

That the bird dwarfened a Parasitic Jaeger indeed indicates a larger and heavier bird than even the largest Pomarine. From your photo I can furthermore see a skua-type - the tail is not so shortish looking athan in Great, but indeed shorter than in Pomarine, lacking tail elongation. There are large white flashes on a otherwise dark underwing: most Pom´s at this time of year would be immatures with extensive pale pattering in underwing coverts, which is not the case here.
Note that at least 2 inner primaries are moulting. this means active moult at a time, where the northern hemisphere jaegers and Great Skua still have not started their primary moult. This furthermore supports the ID of the bird as a Skua, with South Polar as the main candidate.

very best wishes
Klaus


Additional photos, taken on June 20–21, from Andrew Kratter's PBase gallery:

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