Huhtala, K., E. Pulliainen, P. Jussila, and P. S. Tunkkari. “Food Niche of Gyrfalcon Falco Rusticolus Nesting in the Far North of Finland As Compared With Other Choices of the Species”. Ornis Fennica, vol. 73, no. 2, July 1996, pp. 78–87, https://ornisfennica.journal.fi/article/view/133440.