ajohn

I don't know what the function of the dark spot on the iris is, but I do know that it is found in other species. Check these pictures of Cinnamon Bittern for example.
http://orientalbirdimages.org/se ... Family_ID=&p=25
http://orientalbirdimages.org/se ... _ID=&pagesize=1

And this isn't restricted to bitterns. Similar dark marks on the iris are found on Oystercatchers (apparently only females!):
http://orientalbirdimages.org/se ... _ID=&pagesize=1
and Black Woodpecker:
http://orientalbirdimages.org/se ... _ID=&pagesize=1

and quite possibly other species as well. I don't know whether it's significant that all are long-billed species, and the dark spot seems to be aligned with the bill on each.

ajohn

I've just remembered where I've noticed this most often - woodpigeons show the same. See:
http://mikeatkinson.net/woodpigeon.htm

So that ruins my theory about long-billed birds!

ajohn

Your comments on Yellow-fronted Canary are interesting. I have noticed that in the Deep BAy area they tend to turn up in mid/late winter, feeding on the fruits of Casuarina trees. I wonder whether they may be making localised movements within Hong Kong rather than true large-scale migration.