The transition patterns of syllables (syntactical complexity) in songs of the Black-browed Reed Warbler (Acrocephalus bistrigiceps) were more complex than previously measured in the domestic bird, the Bengalese Finch (Lonchura striata var. domestica). Syntactical complexity was correlated with song duration (song length), which implies that these song characteristics have a similar function, probably in sexual selection. Syntactical complexity was not correlated with repertoire size (the number of syllable types), which is another presumable sexually selected trait but may not be important in this species, as previously suggested. Future studies on birdsongs should focus on not only repertoire size but also syntactical complexity.