ajohn

It's a Pallas's Leaf Warbler.
Unfortunately I think the behaviour (not very active and sitting with the head under the wing) suggests that the bird may be unwell. Maybe because it has been struggling to find enough food in the recent cold weather.

ajohn

The fluffy feathers don't indicate the age of the bird (in fact I think you can't tell the age from these photos).
It has fluffed out the feathers to help to keep itself warm.