For the Japanese Yellow Bunting, as Geoff rightly pointed out that the two exception yrs around 1994 and 1996 (very hard to see exactly which yr), the count per yr is similar and fluctuated between 5 and 15 individuals. I would say they remained low, rather than a decline.
A long term survey of several migrants at a particular site(s) with constant survey effort may answer your question. Reviewing HKBWS bird count just provide a guess for further study, rather than a scientific evidence as Tung pted out that submission of records cannot assumpt to be constant throught yrs.
