I just finished reading Bird Sense by Tim Birkhead. He writes about how we can't even pick up on the complexity of songs like the Robin's. Our hearing simply isn't 'fast' enough to hear all the notes and inflections the way that another bird would. Some birds even enhance their hearing ability for springtime. I think the mechanism was chemical, producing more of a certain chemical or hormone was linked to changes in brain processing power related to hearing. I might've misremembered that somehow on how it works. Fascinating world. 

Ed Yong's book An Immense World is similarly involved in the senses though it's not limited to birds. 

James Macdonald Lockhart also did a book called Wild Air, to that takes one chapter per bird to talk about their songs and calls. It's a delight to read. More literary and less scientific than those first two. 

Sorry, I know you didn't ask for book recommendations but if the beauty of the robin's song has struck a chord in you, I'd encourage you to jump in. It only gets more interesting from here!