Abstract
Cover photo: Northern Hawk-Owl Surnia ulula. Photo: Terje Lislevand. 
 The Northern Hawk-Owl Surnia ulula occurs as an irruptive species to southern parts of Fennoscandia. Large numbers of individuals were recorded in the autumns of 2016 and 2020 but assessing the relative magnitude of irruptions is challenging. Systematic surveys allow direct comparisons but are time-consuming and will therefore be limited in time and space. Citizen data may provide large amounts of information for wide areas but may in particular suffer from spatial biases in the observation effort of birdwatchers. I compared the 2016 and 2020 irruptions of Northern Hawk-Owls to southern Norway, and found that citizen data indicated that numbers in 2016 were ca. 2–3 times larger than in 2020. However, the relative magnitude differed geographically, and the 2016 irruption was larger in western and southern counties, whereas the difference was smaller in eastern counties (in particular in Innlandet county). Systematic surveys in eastern regions (Oslo and Akershus) indicated that Northern Hawk-Owl densities were similar in the two irruption years. Overall, Northern Hawk-Owls were recorded at the same rate (approximately one owl per 16 km), and density was estimated to be 0.09–0.18 individuals/km2 in 2016 and 0.08–0.13 individuals/km2 in 2020. Thus, citizen data and survey data from the same geographical region concurred. However, due to the geographical variation in relative irruption size and spatial variation in observer density and biases in observation effort, the overall difference between the two years is difficult to assess. The 2016 irruption was likely larger than the 2020 irruption, but the difference was probably smaller than suggested by citizen data.
Highlights
The Northern Hawk-Owl Surnia ulula occurs as an irruptive species to areas south of the regular breeding range (Mikkola 1983, Cramp 1985, Newton 2006)
Observations reported to the website of the local branch of the BirdLife Norway in Oslo and Akershus have to a large degree been exchanged with the national online portal, but a small number of owl observations from Oslo and Akershus that were only available through www.nofoa.no were not included in the present study because the objective was to illustrate irruption patterns from the major source of citizen data
According to the online portal of the National Biodiversity Information Centre, there were more than three times as many Hawk-Owl observation reports in southern Norway in 2016 as in 2020 (Table 1)
Summary
The Northern Hawk-Owl Surnia ulula (hereafter HawkOwl or owl) occurs as an irruptive species to areas south of the regular breeding range (Mikkola 1983, Cramp 1985, Newton 2006). Hawk-Owls are scarce breeders in southern Norway (Sonerud 1994) but have occurred in large numbers in autumn and winter in certain years. Recent large irruptions have occurred in e.g. 1983– 84 (Jacobsen 1984), in 2016–17 (Dale 2017), and in 2020–2021 (this study). The number of individuals involved in Hawk-Owl irruptions have rarely been estimated, but 2,000–4,000 individuals were thought to be present in Värmland county in southwest Sweden during the 1983–84 irruption (Svensson et al 1999), corresponding to ca. Dale (2017) estimated that a large proportion of the Fennoscandian population moved south in 2016, with around 10,000– 20,000 individuals reaching southern Norway, and with densities of around 0.1–0.2 owls/km in boreal forest. Assessing the relative magnitude of irruptions is challenging because of variable sources of data, and possible biases in different sources of data
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