What did the branches of the trees enjoy earlier?

The poem states: 'Bare ruined choirs, where late the sweet birds sang.' The branches are compared to 'bare ruined choirs' (church choir lofts) where sweet birds USED to sing earlier. So earlier, the branches enjoyed the sweet songs of birds. 'Choirs' here is a metaphor for the branches themselves (now ruined/bare); the actual content earlier was sweet birdsong.