Meet the Red-eyed Vireo, renowned for monotonous singing. But that is a bum rap. Yes, he sings all day, even in the afternoon, and he sings at an even pace for all that time, doling out a compact avian haiku about every two seconds. But that is persistence, not monotony. The Chuck-will's-widow and all his caprimulgid kin are the kings of monotony. This vireo eschews monotony.
Charles Hartshorne was the chap who coined the term "monotony threshold," to capture the idea that there is a limit on the amount of repetition a singer can risk without losing the attention of his listeners. The Red-eyed Vireo must have a low threshold. The ground-breaking bioacoustician Donald Borror analyzed some 12,500 songs of 46 vireos from nine states and found song repertoire sizes ranging from 12 to 117. But, the song wasn't the basic syntactical unit. Many songs have more than one note or phrase, and these are used independently of each other to make the song-types, just as different arrangments of DNA nucleotides code for different amino acids.
Let's replay this snippet of singing and look for repeats. The first song-type is pretty distinctive. Scrolling through the cut, we find it again at 0:40, then again at 0:49, and then no more. The second song appears again at 1:30. The third one is more frequent, showing up at 0:19, 0:31, 0:42, 1:05, and 1:22, with its first half appearing at 0:51. Have you noticed yet that you're better at distinguishing shapes than sounds? It is also clear that the song-types do not come in a predictable order. That would defeat the purpose of maintaining attention by creating variety.
So, the next time you are sitting on the porch in the mid-afternoon and the only thing singing is a Red-eyed Vireo, you can let his cadence put you to sleep, or you can count song-types and stay awake until suppertime.