Meissner, Włodzimierz, and Katarzyna Ciopcińska. “Behaviour of Mute Swans Cygnus Olor Wintering at a Municipal Beach in Gdynia, Poland”. Ornis Svecica, vol. 17, no. 3–4, Oct. 2007, pp. 148-53, doi:10.34080/os.v17.22685.