In answer to Geoff's comment about the difference between Dusky, Yellow-browed and Pallas's seasonality:
I think that Pallas's, unlike the other two, is geniunely a winter visitor with little passage. Note that the arrival in autumn is later, and departure in spring earlier than for the others. Hong Kong lies more-or-less at the southern end of the wintering range for this species so there is little passage through HK to locations further south.
The reason for the difference between Dusky and Yellow-browed is less obvious. The passage of Dusky in Deep Bay is also very obvious in autumn but less so in spring. I wonder whether this represents a difference in migration routes, with Dusky taking a more inland route than Yellow-browed in spring (rather like the 'continental' autumn migrants such as Yellow-rumped Flycatcher).
