First-winter Daurian Redstarts look very similar to winter adult males.
I believe the reason for the difference is that the core wintering areas of males and females are different. This is the case for many species of passerine. Males often winter further north, possibly because they wish to be closer to the breeding grounds and so get a head start when returning to set up territories. Thus, it is likely that the further north you go in south China at this time of year, the greater the proportion of males.
Another species that seems to illustrate this is Red-flanked Bluetail. Proportionately more males occur further north in the winter, which is why when we have very cold spells in HK late in the winter, quite often we get an influx of males. I remember this was particularly evident during one late winter influx at Ng Tung Chai about 10 yrs ago. The males seem to be able to hold out further north for the whole winter, unless there is an unusually cold spell later on.
I've also noticed that more male Grey-backed Thrushes appear midwinter. Certainly, at the moment there seems to be a lot of males around, first-winter males especially.
Geoff
