Answer:
Bare choirs.
Step-by-step explanation:
Shakespare makes note of the leafless limbs of a tree on the second verse: "When yellow leaves, or none, or few do hang". He then refers to these limbs as locations for the "sweet birds" to sing on, meaning he sees the limbs as empty, bare choirs that would normally have leaves as singers, but these are replaced in later times by the birds that perch on them.