A morph is a colour form within the normal population - similar to the 'Dusky' morph of Long-tailed Shrike, or the colour forms of certain skuas/jaegers. It occurs within the range of the normal colour form and can interbreed with the normal colour form. This is unlike a subspecies, which refers to two forms separated geographically. Lugens morph BtLt turn up from time to time in HK, and for some reason seem to be seen more often at this time of year (perhaps suggesting an age-related or seasonal difference rather than a morph?)
I actually had a BtLt on Lantau a few weeks ago which I thought was of the Hainan subspecies (although views not great). Similar to this bird, but browner on the body (with virtually no grey on the underparts, unlike a typical BtLt). A description is currently being prepared for assessment.
