Coming into town on the ferry this morning I saw a vast flock of small birds sitting atop of the waves. They were just before Sulphur Channel / Green island (on the Lamma side) and whenever a boat got near them they would take off and skim over the waves at high speed.

Pretty, angular-winged little things – maybe Swallows? – and very nice indeed!

What were they, oh learned ones?

I would imagine they were 'Passer Domesticus'.
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Ford model 91 picture

Can't help you out I'm afraid - only see the 50 or so Little Egrets on the island itself.

Best guess would be the "Lesser Poly Styrene" as opposed to the "Big chunks Poly Styrene" more often seen on the waves in that particular channel.

Will need to defer to Zep or Chav, as they are the bird experts.

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My inability to tolerate your ambiguity is compounding my neurosis

They were probably terns, I have seen terns in yung shue wan before.

They sound very much like terns - these migrate in April and are sometimes seen round the waters of HK Island and Lamma. If so, they were probably black-naped terns on their way to breed in the Tolo Harbour east of the NT. Here are a couple of pics, including the Singapore $1 note which features the bird.

It would be nice to have some more details - what colour were they? How many were there? Did they dive into the water and come out again (as terns do) or just skim across the surface.

The only other possibility i can think of (assuming you're not just winding up bird nerds) is a movement of red-necked phalaropes - occasionally large numbers pass through in April (see more pictures pinched from the Internet courtesy of Google's picture search) but this is extremely uncommon

No wind up. They were small - size of a sparrow I would say, and looked to be black with white. More blacker than the picture of the tern. There were alot - maybe 100. I did not see them dive into the water, only skim the surface and I think I may have seen them again this morning, on Lamma, flying around the trees in YSW, near the grave sites at near the back path of PYY.

As I say angular small darting things. Very pretty! I keep thinking Swallows but then I know nothing...

There are certainly a lot of swallows around at the moment, and those would very likely be the ones at the land sites. But on water it would have to be a different species - still a mystery. But terns are very "angular" in shape and very nice looking birds - so that's still a possibility

I vote this as 'Thread of the year'. Fascinating, insightful and not full of gibberish.

Vote?
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Lamborghini espada

And I would like to nominate Volley's latest contribution as "Post Of The Year".

I looked in a bird book and now conclude Zep is right: they must have been terns. In the book, there I found a smallish one (can't remember: white winged / black tipped or something) that migrated through HK in April, sat on water and looked something like the birds I saw. BTW, when they give a size in cm, is that wing span or body lenght?

Yes, the details for white-winged black tern fit just right - migrates in large flocks ... feeds by fluttering over the water surface ... often look patchily black and white. If so, you are privileged to have witnessed a rare sight.

Those birds atop the waves are back. Saw them twice in the last few days, both times from the ferry. Every time I have seen them they have been (if heading from YSW) about half way between Lamma and Green Island, just after the big red shipping buoy, bobbing around on the Island side of the boat. Must be seasonal visitors.

Interesting - what time of day? Red-necked phalaropes are also migrating in numbers at this time of year.

Late afternoon, I would say...