Nybo, J. O., and G. A. Sonerud. “Seasonal Changes in Diet of Hawk Owls Surnia Ulula: Importance of Snow Cover”. Ornis Fennica 67, no. 2 (July 1, 1990): 45–51. Accessed April 5, 2025. https://ornisfennica.journal.fi/article/view/133299.