What are goldfinches doing on tree branches?

At this time of year I often see Goldfinches appearing to feed high on tree branches, often sycamores. It is hard to see what exactly they are doing, but definitely there are no seeds involved, it looks like they are pecking the bark or possibly lichens. I checked and they were not holding seeds under their feet (as they often do) or wiping their bills clean. They appear to peck the surface of the branch. I managed to photograph one that appears to lift and hold a bit of bark with lichen on top (top shot).

I am very curious to know what they are doing. During the breeding season Goldfinches use lichen to camouflage their nests, I only see this behaviour during winter. Has anyone seen this? Please comment! Some more photos of the behaviour follow.

3/12/2017.

25/01/2018

25/01/2018. This individual stayed pecking the end of this branch for quite a while.

16/11/2016

UPDATE 11/1/20
I found out a chorusing Goldfinch flock in a local park today. Several of them were feeding on branches in two lime trees. I managed a couple of videos showing how they target areas at the end of broken branches and deftly peel bark and wood, discarding bits. If they are feeding on the bark or wood itself or on some invertebrates living in it is unclear.