Effect of snow cover on seasonal changes in diet, habitat, and regional distribution of raptors that prey on small mammals in boreal zones of Fennoscandia

Abstract

Seasonal changes in spatial distribution of search effort of birds that prey on small mammals were studied in two structurally different coniferous forest habitats in the northern boreal zone in SE Norway. During the season with snow cover both the proportion of Microtus relative to that of Clethrionomys in the predators' diet, and their use of a clear‐cut relative to that of older forest were lower than during the snow‐free season. This was related to a lower relative availability of prey (Microtus agrestis and M. oeconomus) in the clear‐cut when the ground was snow‐covered than when it was snow‐free. Based on this local pattern I suggest the following explanation for differences in migratory strategy between raptors that prey on small mammals in Fennoscandian boreal zones: species that migrate to snow‐free areas in winter are either adapted to hunt by the energetically expensive method of quartering in open grassland habitats, where prey (Microtus) availability is relatively low during periods with snow cover (hen harrier Circus cyaneus, short‐eared owl Asio flammeus, and longeared owl A. otus), or by sit‐and‐wait in open grassland and forest habitats, the latter with relatively high prey availability during periods with snow cover, but unable to locate concealed prey (kestrel Falco tinnuculus, common buzzard Buteo huteo, and roughlegged buzzard B. lagopus). In contrast, species that remain in areas with permanent snow cover during winter use the energetically cheap sit‐and‐wait tactic, and are able to hunt in closed forest habitat and localize concealed prey (the remaining owl species). Interspecific differences in prey availability as determined by hunting habitat and hunting mode is probably more important in shaping the migration patterns of Fennoscandian owls than is nest site availability.

Density, numbers and origin of Northern Hawk Owls <i>Surnia ulula</i> in southern Norway during the 2016 irruption

“…In addition, a large snowfall on 5 November in parts of southeastern Norway (including parts of Oslo and Akershus) may have caused a redistribution of individuals in response to more difficult hunting conditions on clear-cuts (cf. Sonerud 1986, Nybo & Sonerud 1990. Some individuals may then have moved from boreal forest to agricultural areas in the lowlands with less snow cover, or they may have turned to hunting more within forest and thereby making detectability lower (Sonerud 1986, Nybo & Sonerud 1990).…”

“…Sonerud 1986, Nybo & Sonerud 1990. Some individuals may then have moved from boreal forest to agricultural areas in the lowlands with less snow cover, or they may have turned to hunting more within forest and thereby making detectability lower (Sonerud 1986, Nybo & Sonerud 1990). …”

“…I searched for Hawk Owls by stopping and scanning at every clear-cut along the roads surveyed. This method was chosen because hawk owls hunt from elevated perches giving a wide view such as from remaining trees on forest clear-cuts (Sonerud 1992(Sonerud , 1997) when the ground is not snow-covered (Sonerud 1986, Nybo & Sonerud 1990, and because clear-cuts were the most common open habitat in the study area. Scans were only made with hand-held binoculars (10 x magnification), and telescope was only used to confirm species identity of distant individuals.…”

Female offspring desertion and male-only care increase with natural and experimental increase in food abundance

“…The forest is heavily influenced by modern forestry, based on clear-cutting of the old forest and replanting, and is a mosaic of stands of all ages. More detailed descriptions of the flora and fauna of this area are given by Sonerud (1986) and Steen et al (1996). Tengmalm's owls are small (male body mass of approx.…”

Population fluctuations of moths and small rodents in relation to plant reproduction indices in southern Norway

“…Relative population indices of small rodents in September-October were obtained by use of snap-traps (commercial brand ''Rapp'') baited with cocoa fat (commercial brand ''Delfia matfett'') and put out 5 m apart along seven separate lines and checked for four days, giving 1002-1150 trap nights per year (Sonerud 1986). For each species, the indices used are number trapped per 100 trap nights, after having subtracted 0.5 trap-night for each capture of another species.…”