Acoustical background to the many varieties of birdsong

Birdsong can be broadly classified into two categories: biphonic in which two different notes can be sung simultaneously, and monophonic in which only one note is sung. Monophonic song can be further divided into rich harmonic song, single-frequency song, and chaotic song. While some of these distinctions arise from clear anatomical features of the bird, others are more subtle and pose problems for physical scientists working in collaboration with biologists. This paper provides information on the physics and acoustics underlying these differences and shows how quantitative predictions can be made.