Sammalisto, L. “Secondary Intergradation of the Blue-Headed and Grey-Headed Wagtails (Motacilla F. Flava L. And Motacilla F, Thunbergi Billb.) in South Finland”. Ornis Fennica 33, no. 1 (March 31, 1956): 1–19. Accessed April 20, 2025. https://ornisfennica.journal.fi/article/view/132588.