Thursday, April 26, 2012

21 April 2012: Pelagic off Tanjung Dawai, Kedah - Skuas/Jaegers

This was a long and difficult trip. 14 hours on the boat, and constant swell from the storm that passed through the previous day made it difficult to stay upright. My back was in pain at the end of the day from all that swaying about!

I'm still sorting through the 800+ pics I took, but have managed to edit all the skua/jaeger pics. Birds were on view from about 10am till 6.30pm, and since we were in the same broad area the whole day, it was difficult to work out how many birds were involved. I decided to get photographs of every sighting and then try to compare plumage features when I got home.

Right click on the images and select 'Open link in new tab' to see an enlarged version.

These two adult breeding plumaged Arctic Skuas/Parasitic Jaegers can be distinguished from each other by the missing primary in the left wing of Arctic Skua 1. It's only visible when the outer 'hand' of the wing is pushed well forward, but, when the wing is in a comparable position on Arctic Skua 3, there is no gap apparent.

This moulting first summer Arctic/Parasitic is a challenging bird as it shows several characteristics of a Pomarine.

The heavy-looking body, apparently broad wings and pale bases to the under primary coverts are all good pointers to Pomarine. However...
The fresh pointed central tail feathers tell a different story!
This close-up shows that the points are not caused by feather shafts of abraded feathers, but by pristine, fresh feathers. The central 2 pairs are fresh and the 3rd pair is still growing.

So it's an Arctic, but how can the anomalous features be explained?

1. The pale primary covert bases. The Collins Guide to the Birds of Britain and Europe states that 1 out of 20 Arctics show this feature, so it is not such a "cast-iron Pom indicator: as I had realized.

2. The broad winged appearance. The bird is in wing moult, with P9-10 (the outermost primaries) and the inner secondaries still unmoulted. These old secondaries project well beyond the trailing edge outline of the rest of the wing. I don't know why this is - is it just that they look long compared to the still-growing newer feathers, or are they pushed out as new feathers emerge? In either case, these 'longer' secondaries perhaps exaggerate the apparent breadth of the wing.

3. The heavy-bodied appearance. Perhaps this bird is extraordinarily fat! Not a frivolous remark - birds are feeding up to facilitate further migration, and food is in plentiful supply here. You can see the difference in waders between freshly arrived migrants and well-fed birds - so why not in skuas too?

To me, neither the head nor the bill look heavy enough for Pom, though I can only say that with the benefit of the photos! If I'm honest, had I not snapped clear pictures of the tail, I'd be happily putting this down as a Pomarine Skua, which goes to show I should be more careful!

Compared to previous years, three Parasitic Jaegers is an exceptional day-count, and especially this early in the migration season. Most sightings are from late May into June.

A classic pale-backed adult breeding plumaged Long-tailed. The lower 3 pics were taken almost 3 hours after the first 6, but I couldn't discern any differences in plumage, so I counted them as referring to one  bird. The apparent difference in upper wing covert colour is almost certainly a product of the different lighting conditions.

The barred underwing coverts suggest that this might be a 3rd summer bird.

An intermediate morph breeding adult. 

Another breeding adult. I think the smudging on the underparts might be the vestiges of non-breeding plumage.

This seems to be an adult in non-breeding plumage

I think this is a 2nd summer bird. The white vent is an unusual feature for Long-tailed. On the other hand, a breastband seems to be quite a common feature of subadult Long-tailed - more so than I had realized.

There was almost a 4 hour gap between the first four and the last five photos, but I think they are all of the same bird, which I think (!) is a first summer. Better views would have been appreciated!

Finally, a bird which came reasonably close - an adult moulting into breeding plumage.

Finally, a bird I just can't pin down! I thought at the time it was a Pomarine, but some of the photos, especially in row 3, make me feel that it was another Long-tailed. The rounded central tail feathers reveal that it can't have been an Arctic. All in all, I can't be sure, given the distance and quality of the photos. Learning from the experience of the 1st w Arctic earlier, it seems wisest to put this in the unidentified column!

So, at least 12 skuas/jaegers in all, with 3 Arctic/Parasitic and 8 Long-tailed. An instructive bunch!

Throughout my 'dissection' of the birds in this post, I've found an online article A Review of Moult and Ageing in Jaegers by Steve Howell extremely helpful ... and challenging!


John Holmes said...

Great to see such a collection of flight shots, Dave.

There are boat trips from Hong Kong this weekend and next, and we'll be hoping for some of these, too !

Choy Wai Mun said...

Dave, this a great post. Very informative and interesting.

digdeep said...

Thanks guys! Mun - time you got out on the boat again!

John - I'll exchange a couple of Long-tailed for a couple of your murrelets!