Markov dependencies in the song syntax of Hermit Thrush (Catharus guttatus)

Abstract

The sequences in which songbirds deliver their songs may help serve song’s primary functions related to territorial defense and mate attraction but determining that requires a clear understanding of the syntax of the species in question. Hermit Thrush (Catharus guttatus) males sing with a degree of predictability, cycling up and down the frequency spectrum with immediate variety. The present work used two complementary approaches to better understand Hermit Thrush song syntax by investigating the degree to which song type choice relies upon the identity of preceding songs within large (> 1000) song sets from 12 males. In one analysis, Fisher’s tests were used to assess whether the song immediately before the next song—or two, three, or four spots before it—influenced the next song. Results indicated that the next song is influenced by both the previous song and the one before it, and less so by those three or four songs back. In addition, simulated sequences were generated that reflect Markov models from zero-order (the next song depends on no preceding songs) through fourth-order (it depends on the previous four songs), and then compared back to the bird’s observed song sequence. This analysis revealed that naturally occurring sequences are best described by second-order Markov models, indicating a reliance upon the previous two songs. Together, the results indicate that Hermit Thrush song sequences are determined mainly by preceding two songs. Future work will further explore the presence of higher level dependencies, along with the role of syntax in territorial defense and aggression.