Källander, H. “Age-Related Distribution of Blackheaded Larus Ridibundus and Common Gulls L. Canus across the Agricultural Plains of Skåne, South Sweden, in Autumn”. Ornis Svecica, vol. 14, no. 1–2, Apr. 2004, pp. 48-52, doi:10.34080/os.v14.22799.