I've had them up at the car park in mid-winter, but also on another trip where I'd climbed Carn Ban Mor for dotterel, and then later went up to Cairn Gorm, and we had ring ouzel, and with snow bunting flicking around the car park.
That was in late May, and the winter trips have been Dec, Jan, so I suspect they must be possible there year round.
Maybe someone with local knowledge can comment?