Laskemoen, T.; Kleven, O.; Fossøy, F.; Lifjeld, J. T. Intraspecific Variation in Sperm Length in Two Passerine Species, the Bluethroat Luscinia Svecica and the Willow Warbler Phylloscopus Trochilus. Ornis Fenn 2007, 84, 131–139.