Leitao, A., van Dooren, T. J. M., & Riebel, K. (2004). Temporal variation in chaffinch Fringilla coelebs song: Interrelations between the trill and flourish. Journal of Avian Biology, 35(3), 199-203. doi:10.1111/j.0908-8857.2004.03295.x.