The tundra is a sub-Arctic zone with long, cold winters and short, warm summers. Precipitation is low. Soil a meter below ground and deeper is permanently frozen (permafrost), which does not allow water to drain easily through the soil, so it collects in shallow pools. Trees and shrubs are stunted since their roots cannot grow into the permafrost. Low shrubs, lichens, mosses, and small herbaceous plants are found instead. The most common mammals on the tundra are the barren-ground caribou, Arctic wolf, Arctic fox, Arctic hare, lemmings, and voles. Occasional sightings of muskoxen have been made near Cape Chidley, Labrador's most northerly point. The polar bear is the dominant carnivore of the tundra, but is an occasional visitor to coastal Newfoundland, especially in the spring. Many birds migrate to Labrador in spring to lay their eggs and raise their young before returning south for the winter, including the common eider, harlequin duck, Common redpoll and American pipit. The Labrador tundra is also permanent home to ptarmigan.