Saturday, February 14, 2009

12th February 2009: Pelagic trip off Tanjung Dawai, Kedah Part 2

From the time the sun rose above the horizon, it was clear that there were a lot of terns about!

We counted about a thousand terns around our boat, and similar numbers around all the other boats we could see, so we estimated that there were at least 10,000 in our immediate vicinity, and possibly many more.

The bulk of the birds were Common and White-winged Terns, the latter outnumbering the former about 60:40.

Oddly, Commons outnumbered White-winged in the morning, with the reverse being true in the latter half of the day.

Same prey, different strategy. This lucky shot shows how Common Terns (and other so-called 'Sea Terns') catch their food - by plunge diving; 'Marsh Terns', like the White-winged Tern above, pick their prey from the surface of the water.

Because 'Sea Terns' need to plunge below the surface of the water, they tend to fly higher; when they spot prey they hover, and then dive.


Not the best picture, but this shows how 'Marsh Terns' pick their prey from the water surface.

Compared to each other, Common Terns (left) are larger, longer-bodied and longer-billed than White-winged (right), with much more black on the nape.

Common Terns preferred to rest on the sea, but this one sat alongside a couple of White-winged so we could take some nice comparison pics!

My guess is that this Common Tern is in its first year; the very dark primaries and secondaries are last year's juvenile feathers,and the fresh grey ones are 'first summer' feathers.

Adults can have very long tail streamers, and the many show 'suspended' primary moult, with the older outer feathers subtly darker than the newer inners.

However, some adults seemed to have an almost entirely fresh set of primaries, giving their wings an unfamiliar silvery appearance.

This bird had a severely discoloured and deformed lower mandible, similar to birds I saw last April here and here. There seems to be a high incidence of this particular deformity in Common Terns here. I wonder what causes it?

Limited parking space! White-winged Terns queuing up to land on the net floats.

There was a wide variety of plumages, with some birds already in partial summer plumage.

Compared to Common Tern, White-winged has a very compact body-shape and short bill.

Some birds, like this one, were very piebald - perhaps 1st summer birds, with pale fresh feathers contrasting strongly with very worn juvenile ones.

Black Tern is a vagrant to Sout-east Asia. One of its distinguishing features in non-breeding plumage is a dark mark at the base of the wing - like on this bird! However, the whitish new wing coverts and short bill confirm that this is simply a White-winged Tern with some black summer-plumaged feathers in a confusing place!

A few Little Terns were also present to take advantage of the ikan bilis harvest!

Adults of the nominate race in breeding plumage are really fine looking birds - with long tail streamers and silvery upperparts.

All the birds we saw in non-breeding plumage had solidly grey rumps and central tail, a feature that is sometimes (mistakenly) said to be diagnostic of Saunder's Tern.

The bill of this bird is turning to breeding colour.

Black-naped Terns were also present in small numbers. These are residents here, and in my view, one of the most beautiful terns there are.

The shimmering white and jet black plumage is enhanced by a delicate salmon-pink flush to the underparts, which, when set against the blue-green of the ocean, is just stunning!

First summer birds can be distinguished from adults by the dusky tips to the outer primaries and the slightly ragged bandit's mask.

That was it for the day. Slightly disappointing not to find anything unusual among all those thousands of birds, and surprising not to find any jaegers taking advantage of them. But we still have the months of March and April to look forward to, when birds wintering south of here will be on the move northwards.

No comments: