This is the third time McCartney has named a song after a bird – the others being “Blackbird” and “Bluebird”. Paul explicitly compared this song to “Blackbird”:

To me it was just something to do with Blackbird, a wren: a wren is one of my favourite birds, little English bird, it’s the smallest English bird and I always feel very privileged to see a wren because they’re very shy and it’s just ‘Ah!’ So a combination of all of that. It’s a favourite bird for me, and then instead of making it a bird, again like Blackbird, only more definitely this time I made it a woman, you know, a girl. So it was good fun doing it.

The smallest British birds are actually the Firecrest and the Goldcrest, but we can let Paul off!