hooooon

係咪田鷚(Richard's Pipit)亞種 sinensis？
Is it Richard's Pipit (sinensis)?
但我在高山影到牠，牠飛行時會叫兩聲，然後停一停再叫，叫聲和飛行時好似黃鶺鴒。
I took it at hill top. Its flight looked like Yellow Wagtail with two short sounds.

thinfor

I think you're right. It's a Richard's pipit.  Richard's pipit is commonly and widely spread in hilly areas in HK, especially Tai Mo Shan.  I even see Richard's pipits more than upland pipits there.