Wolverine (Gulo gulo) Recorded as Predator of Nestling Great Gray Owls (Strix nebulosa) in Norway

Abstract

The Great Gray Owl (Strix nebulosa) has a circumpolar distribution in the boreal forest zone (Cramp 1985) in both the Palearctic and Nearctic region (subspecies S. n. lapponica and S. n. nebulosi, respectively). The Palearctic subspecies has extended its range toward the south and southwest in Scandinavia during the last four decades (Sonerud et al. 2021 and references therein). This expansion includes most of northern Europe (Ławicki et al. 2013). Since 2009, at least 445 nesting attempts were recorded in Hedmark County, Norway (Berg et al. 2011, Berg 2016, Berg et al. 2019). The wolverine (Gulo gulo) is the largest member of the mustelid family, weighing up to 18 kg (Wilson and Mittermeier 2009). It inhabits the taiga in both Eurasia and North America (Wilson and Mittermeier 2009). The wolverine is capable of killing large ungulates such as reindeer (Rangifer tarandus), but it is generally an opportunistic feeder that regularly scavenges on carcasses (Wilson and Mittermeier 2009). Wolverines have bred in the northern half of Hedmark County since 2010, when the Great Gray Owl expansion started (Flagstad et al. 2013, Tovmo and Mattisson 2021). The southernmost known wolverine dens are in the same area as four Great Gray Owl platforms surveyed in 2021 (Fig. 1). Nest predation is a common cause of mortality for many bird species (Caro 2005 and references therein). Mustelids prey on both mammals and birds (Wilson and Mittermeier 2009), and European pine martens (Martes martes) are especially arboreal and known predators of cavity-nesting owls and ducks (Korpimäki and Hakkarainen 2012, Sonerud 1985, 2021a, 2021b). A few Great Gray Owls have made nesting attempts at ground nests in Hedmark County and all have been predated (Berg et al. 2011). Since 2011, an increasing number of artificial nest platforms (see Stefansson 1997, Solheim 2014) have been erected by local ornithologists in Hedmark County, resulting in .400 platforms installed by 2022 and 48% of nesting attempts occurring on platforms in 2018 (Berg et al. 2019). Here we report the first record of wolverine as a nest predator of Great Gray Owl nestlings on nest platforms. Four Great Gray Owl nests on artificial platforms were monitored with wildlife cameras in central Hedmark County in southeastern Norway (60851N, 11859E) in May and June 2021 (Fig. 1). The platforms surveilled were located along the Kynna watercourse in central Hedmark County, with 31.6 km between the two most-distant platforms, platform 1 and platform 4. Platforms were placed approximately 5.5 m above ground in pine (Pinus sylvestris) or spruce (Picea abies) trees. Three of the platforms (platforms 2, 3, and 4) were used by nesting Great Gray Owls in previous years. Seven wildlife cameras were erected to monitor the platforms. Camera types included Boscon Guard Entry (n1⁄4 3), Spypoint Force Dark (n1⁄4 2), Moultrie No-glow (n1⁄4 1), and Glory LTE, L4-E (n1⁄4 1). The cameras were mounted on tree trunks 2–5 m from the platforms. Cameras were set to take three images when triggered, and to pause for 1 min prior to taking the next image. Batteries and memory cards were changed at platforms 1, 3, and 4 on 31 May 2021. The cameras were collected after the nestlings had fledged, or otherwise disappeared. Three of the seven cameras either failed to trigger when the other camera captured movement at the platforms, or the batteries failed. At all four platforms, at least one camera functioned and captured images regularly. A total of 153,600 images were recorded: 7026 images at platform 1; 22,800 images at platform 2; 74,531 images at platform 3; and 49,243 at platform 4. 1 Email address: r-solhe3@online.no