Given the relative status of Swinhoe’s (rare) and Intermediate (fairly common) in Hong Kong, and given that we see birds like the one in your photo throughout the year, I would, if I were sitting in the hide at Mai Po, id this bird as Intermediate without giving it any real thought.
If I saw a similar egret wading in shallow water, actively running after fish and raising its wings, then the idea of Swinhoe’s might come to mind. I would then focus on the legs and feet to see if they had a greenish or greenish-yellow tinge. If they did, then I would definitely be considering the possibility of a non-breeding plumage Swinhoe’s. Probably then I would focus on the length of the bill, comparing it with any Intermediate Egrets on the mudflats to see if it appeared proportionately longer… Not really sure where I’d go from there though.
By which, I guess I mean to say that you have a very good point re. the similarity of Intermediate & Swinhoe’s in non-breeding plumage, and I would tentatively suggest feeding behaviour and leg colour as indicators of Swinhoe’s.
As far as I know, no birds like that have been photographed at Mai Po – all the photos I can find on the website of Swinhoe’s are of adults – almost all of them of birds in shallow water! I am certain that the Swinhoe’s that you heard people referring to would have been adults also.
Going back to the photo, the fact that the egret is feeding on mud and that the legs appear to be grey, makes it hard for me to think that this could be Swinhoe’s and not Intermediate.
David
