Why “countersing” is a great word
To vocalize a reply to the song of another, especially in the territorial rituals of birds. From the English prefix counter- (meaning 'in opposition or response') + sing. Unlike a 'duet,' which suggests a harmonious and coordinated performance, or 'imitate,' which merely denotes replication, to countersing is to engage in a pointed, responsive dialogue—a call-and-reply. It is the sharp, declarative phrase thrown back across a forest clearing, the redbreast answering his rival across the field boundary, the way two thrushes trade phrases like stones across a stream. It is an audible cartography of borders drawn in air, the sound of space claimed not by force, but by voice.
Definitions & examples from Wiktionary (CC BY-SA 3.0).