reindeer (Rangifer tarandus) body parts brought to nests and owls following polar bears to secondarily feed on their kills. Even huge marine mammals such as walrus (Odobenus rosmarus) and whales can be fed upon by these owls when the opportunity occurs. Snowy owls produce a pellet that in different areas averages a median of about , averaging up to in length as in Europe. Prey spectrum The snowy owl is primarily a hunter of mammals. Most especially, they often live off of the northerly lemmings. Sometimes other similar rodents like voles can also be found frequently in the snowy owl's foods. It is R-selected, meaning that it is an opportunistic breeder capable of taking advantage of increases in prey numbers and diversity, despite its apparent specialization. Birds are commonly taken as well, and may regularly include passerines, northern seabirds, ptarmigan and ducks, among others. Sometimes infrequent consumption of other prey such as beetles, crustaceans and occasionally amphibians and fish is reported (of these only fish are known to have been identified to prey species). All told, more than 200 prey species have been known to be taken by snowy owls around the world. Generally, like other large owls (including even bigger owls like the Eurasian eagle-owl), prey selection tends toward quite small prey, usually small mammals, but they can alternate freely with prey that is much larger than typical given the opportunity or even bigger than themselves, including relatively large mammals and several types of large bird of almost any age. One study estimated for the biomes of Alaska and Canada, mean prey sizes for snowy owls were , in western North America, the mean prey size was and in eastern North America was , while the mean prey size in northern Fennoscandia was similar (at ). The mean number of prey species for snowy owls per biome ranged from 12 to 28. The opportunistic nature of snowy owls has long been known during their primarily winter observed feeding habits (leading to their unpopular nature and frequent persecution well into the 20th century). Summer diet The snowy owl's biology is closely tied to the availability of lemmings. These herbivorous rodents are largish members of the vole clan that are the predominant mammal of the tundra ecosystem alongside the reindeer and probably make up the majority of the mammalian biomass of the ecosystem. Lemmings are key architects of the soil, microtopography