Strandberg, Anne, and Roland Asteling. “Polygyny, Hybridisation, Double-Brooding, a Case of Site Fidelity and Time of Laying of the Chiffchaff Phylloscopus Collybita in Southern Sweden”. Ornis Svecica, vol. 8, no. 3, July 1998, pp. 125-8, doi:10.34080/os.v8.22951.