Eriksson, K. “On the Occurrence of the Grasshopper Warbler (Locustella Naevia) and River Warbler (L. Fluviatilis) in Finland Related to the Bird Watching Activity”. Ornis Fennica 46, no. 3 (September 30, 1969): 113–125. Accessed April 20, 2025. https://ornisfennica.journal.fi/article/view/132768.