What do birds hear in bird song?

A female (left) and male (right) zebra finch, Taeniopygia guttata

This week's paper is a review in Animal Behaviour by Robert Dooling and Nora Prior on differences in human and bird perception of birdsong. It is broadly understood that we don't perceive the world and same way that other animals do. It's is hard for us to even wrap our heads around what it would be like to echolocate like a bat, or use electroreception like an electric fish. Even within the senses that are familiar to us, we know that we see flowers differently from bees, and dogs smell all kinds of things that we don't. So what about bird song? Apparently what birds are better at is what Dooling and Prior call "extremely fine temporal processing". In particular, this paper focuses on zebra finches which are the model organism for development and processing of bird song. In one of the studies they discuss Dooling tested "fine temporal processing", by creating artificial stimuli (called Schroeder complexes) composed of repeated harmonics that are either rising or falling. Then they created multiple stimuli in which the time interval between each rising or falling harmonic was shorter and shorter and shorter. Dooling then tested how short the intervals had to be before humans or birds could no longer distinguish the rising from the falling harmonics. 

I really wish that they had provided some audio files of the test stimuli in the supplement, because it is hard for me to grasp how different these positive and negative Schroeder complexes sound. Regardless, Dooling found that at short intervals zebra finches were much much better at distinguishing these two stimuli than humans. This review paper discusses other experiments that also demonstrate superior distinguishing of sounds over very short time intervals in birds compared to humans. These studies indicate that birds are likely able to detect stimuli, or variation in stimuli, in bird song that we simply cannot hear.