This morning, while I walked around the park I heard a distant loud cheerful chatter. As I approached I realised there was a flock of Goldfinches up a large horse chestnut and many of them were singing. It is the second time this week that I witness this behaviour, the first time it also happened in the morning, the flock of Goldfinches singing together from a bare sycamore. Some seemed to amuse themselves pecking the branches, but in general they were just calling and singing. Goldfinches are very social birds, they often nest in loose colonies, and, although individuals might squabble for a particular food source, they often feed together in flocks. Whereas in the summer they like the seeds of teasels, thistles and other herbaceous plants (even lavender seeds!), in the winter, they can often be seen feeding on trees, like Alder, Plane, Ash and Birch. I haven't managed to find much information about chorus singing in Goldfinches, but the behaviour has been described in the closely related American Goldfinch and Siskin, where the males in the flock joining in chorus song in winter and spring.
Goldfinch flock singing (two bulkier Greenfinches are amongst them)