"The word 'change' in change ringing comes from the way two bells change places in the order they sound. "

Yes we know. But applied to birds it's poetry. It's a simmily. Spell it out - birds "change their voices" like churches "ring changes". Birds don't actually pull on ropes or systematically work through 80000 possible combinations, but to the forlorn and rather guilty lover it there's a similarity between the joyous sound of church bells and the sound of birds singing. Bloody poetry? I thought folk singers knew about that crap.