Not sure why this is in Xi'an. There was a flock of them, so not an escaped cage-bird. They are supposed to live above 800m, but Xi'an is at 405m. It is certainly a Mountain Bulbul as well, and there are no other bulbuls in Shaanxi which look like it. Maybe here they come lower down in winter, or maybe they just live abnormally low here.
That is quite an exciting discovery! Maybe they are adapting to something about the human-altered environment and expanding their geographic range in some places?
This is a species which is found in Hong Kong now as well and not even in the tallest peak (HK tallest peak is 900 sth but most observation are way further down in the forest), and according to the hong kong bird watching society, the species "is not restricted to higher altitude in mainland China where it occurs down to 200m elevation or even lower in the cool months", citing the CNG Field Guide to the bird of China.
