Outside the breeding season, Snowy Owls are less dependent upon rodents as a source of food than during breeding. They hunt a wide range of prey species, and can take birds as large as Capercaillie Tetrao urogallus and Herring Gull Larus argentatus. A male Snowy Owl in Boston, USA, was observed to take birds as large as Canada Goose Branta canadensis and herons. Snowy Owls in the same area regularly prey upon American Black Duck Anas rubripes, a species the size of Mallard Anas platyrhynchos. Fish, frogs and other aquatic animals also form part of the diet (Potapov & Sale 2012). Grouse are probably important prey for Snowy Owls which winter in the Arctic. Even though little is known about winter diet of Snowy Owls, there is reason to believe that those owls that winter in mountainous and tundra areas survive upon grouse if there are few rodents. A male Snowy Owl that was fitted with a satellite transmitter in Finnmark in 2007 spent parts of four consecutive winters on the Kola peninsula in western Russia. According to satellite pictures, the undulating landscape used by the bird is dominated by birch and willow scrub. This suggests that the area is good habitat for grouse. The fact that grouse may be important winter food for Snowy Owls in some areas has been described previously (Portenko 1972; Mikkola 1983; Mehlum & Gjertz 1998; Rogacheva 2005; Potapov & Sale 2012; Frank Doyle pers. comm.). The occurrence of grouse in the Fennoscandian uplands may therefore be crucial in determining whether or not Snowy Owls can overwinter. Snowy Owls can occur near open channels in the ice where they hunt upon seaducks and other seabirds. The owls take seaducks both when resting on the ice as well as when on water (Hagen 1952; Parmelee 1992; Gilchrist & Robertson 2000; Robertson & Gilchrist 2003; Therrien et al. 2008, 2011). A positive correlation has been found between the distribution and density of Snowy Owls and concentrations of seaducks (Common Eider Somateria mollissima and Long-tailed Duck Clangula hyemalis) in Hudson Bay (Robertson & Gilchrist 2003). Studies using satellite transmitters in Canada have revealed that exploitation of marine environments may be a main strategy for some adult Snowy Owls in certain wintering areas. Some open channels in the ice are as far as 160 km from the coast, with many overwintering seaducks (especially Common Eider and Long-tailed Duck) and Guillemots. Climate change may lead to these channels becoming so expansive that prey become dispersed over a wider area, and this may in turn affect hunting success of Snowy Owls wintering in such areas (Therrien mfl. 2008, 2011). Snowy Owls can even prey on their conspecifics. It is assumed that this happens in winter when food is in short supply, and that owls that are killed are in very poor condition (Potapov & Sale 2012). However, a good proportion of Snowy Owls migrate southwards for the winter. This might be to the bushy tundra in Russia or to the steppes in Kazakhstan (Dementiev & Gladkov 1951, Portenko 1972), or the Great Plains in North America (Kerlinger et al. 1985). Snowy Owls may also winter in other open landscapes such as the coast, islands, saltmarshes, rivers, lakes, mires, moorland or golf courses. Snowy Owls are often seen near human settlements, and they will often perch on artificial structures (buildings, poles, fences) (Nagell & Frycklund 1965; Cramp 1985; Parmelee 1992). The species generally avoids forest areas, although it may be encountered at the perimeter (Glutz & Bauer 1980). Recent data on Snowy Owl males marked in Saskatchewan, suggest that they migrate via coniferous forests to and from their summer quarters in arctic Canada.
Snowy Owls lead a nomadic lifestyle and there is evidence to indicate that they can cover large distances during early spring in order to find suitable nesting sites with sufficient prey resources. Snowy Owls appear to be dependent upon rodents (particularly lemmings Lemmus og Dictrostonyx) when establishing a breeding territory. Studies in Greenland have shown that Snowy Owls do not attempt to breed unless there is a density of about two lemmings per hectare when the snow melts (Gilg mfl. 2003). The most important prey species in early studies from Hardangervidda was Root Vole Microtus oeconomus (see Hagen 1952), whereas Norway Lemming Lemmus lemmus was the most important prey item in the study area in Finnmark in both 2007 and 2011. In addition to Norway Lemming, Grey-sided Vole Myodes rufocanus was also recorded at several nests. At an abandoned nest in 1987 in Finnmark, Grey-sided Vole dominated over both Root Vole and Norway Lemming in snowy owl pellets from the nest site (Solheim 1989). Despite the formidable availability of rodents in 2011, prey remains from several bird species were also found, including Purple Sandpiper Calidris maritima, Rock Ptarmigan Lagopus mutus and Snow Bunting Plectrophenax nivalis. Even though rodents are very important in determining whether Snowy Owls attempt to nest, they may also supplement their diet during the breeding season with other prey items up to the size of hares. Remains of Arctic Fox Alopex lagopus have been recorded in the diet of Snowy Owls from both Wrangel Island and from Greenland (Krechmar & Dorogoy 1981, Gilg mfl. 2006). The energy requirements for a Snowy Owl family have been estimated (Potapov & Sale 2012). The parent birds will each consume 240 grams of rodents, whereas each of the young will eat 250-340 grams. Snowy Owls only lay large clutches in years where the average weight of rodents (mainly lemmings) is over 60 grams, and they only nest in years where the average weight of prey items is over 40 grams. If the average weight of prey items sinks during the breeding season, then the owls will begin to hunt alternative prey, often with a resultant decline in nestling survival. Adult Snowy Owls often cache prey close to the nest for later consumption. This usually happens at night when activity around the nest has calmed down, and prey items are then collected the following morning (Potapov & Sale 2012). Such behaviour was also observed during fieldwork in Finnmark in 2011.