Long-distance dependencies in birdsong syntax

Songbird syntax is generally thought to be simple, lacking in particular long-distance dependencies in which one element affects choice of another occurring considerably later in the sequence. Here we test for long-distance dependencies in the sequences of songs produced by song sparrows (Melospiza melodia). Song sparrows sing with eventual variety, repeating each song type in a consecutive series termed a “bout.” We show that in switching between song types, song sparrows follow a “cycling rule,” cycling through their repertoires in close to the minimum possible number of bouts. Song sparrows do not cycle in a set order but rather vary the order of song types from cycle to cycle. Cycling in a variable order strongly implies long-distance dependencies: choice of the next type must depend on the song types sung over the past cycle, in the range of 9-10 bouts. Song sparrows also follow a “bout length rule,” whereby the number of repetitions of a song type in a bout is positively associate...