Siivonen, L. “Does the Willow Warbler, Phylloscopus Trochilus (L.), Belong to Those Species of Birds Fluctuating Greatly in Number”. Ornis Fennica 26, no. 4 (December 31, 1949): 89–97. Accessed December 25, 2024. https://ornisfennica.journal.fi/article/view/132521.