Syllable repetition reveals links between distant phrases in birdsong

Repeated execution of individual behavioural units is a common feature of many learned motor behaviours such as dance, music, and birdsong. Little is known about the neuronal control of such learned motor sequences, and specifically, how the number of variable repetitions is determined. The songs of Bengalese finches (Lonchura striata domestica) consist of individual syllables which can repeat a variable number of times (repeat number) to form a repeat phrase. Like vocal sequences in other animals, Bengalese finch song syntax is typically modelled as a Markov chain, where the choice to repeat the same syllable type or switch to a different one is made stochastically after each syllable, before the next syllable is produced. Here, we report that repeat numbers of adjacent and distant repeat phrases in the song can be correlated across specific pairs of phrases. These hidden links between distinct phrases challenge existing models of song syntax where the number of repetitions is independently determined for each syllable type. Instead, they suggest an organisation where a joint factor can control multiple nonadjacent phrases in a song. Repeat phrases in Bengalese finches may therefore be particularly suited to study the neuronal mechanisms underlying long-range dependencies in complex vocal sequences.