Kiefer, S., Sommer, C., Scharff, C., Kipper, S., & Mundry, R. (2009). Tuning towards tomorrow? Common nightingales Luscinia megarhynchos change and increase their song repertoires from the first to the second breeding season. Journal of Avian Biology, 40(2), 231-236. doi:10.1111/j.1600-048X.2009.04500.x.