Dale, Svein. “Relative Magnitude of Two Northern Hawk-Owl (Surnia Ulula) Irruptions to Southern Norway: Comparison of Citizen Data and Survey Data”. Ornis Norvegica 45 (January 18, 2022): 1–9. Accessed March 29, 2024. https://boap.uib.no/index.php/ornis/article/view/3425.