lchunfai

Extensive scaling on rear flanks and vent, looks like a heavy and tame pigeon, long undertail coverts and large wing coverts.. these make me think of ryukyu..

Also, there was a Ryukyu Green Pigeon recorded in Tainan few days ago, I see no reasons why there should not be one on Po Toi too.^^

lchunfai

By the way, there was a Vivid niltava appeared in Taiwan but out of its normal range at the same time, similar to the one on Po Toi, both birds look untidy.
An experienced birder in Taiwan, SteveM, suggested that these two niltavas may escape from the same ship.

It sounds possible to me.