The Hazel Grouse (Bonasa bonasia) is a small forest grouse occurring in temperate and boreal forests from Scandinavia to the Far East. The species is assumed to have reached Hokkaido, northern Japan, via Sakhalin Island, during the last ice age about 40, 000 years ago. The subspecies occurring in Hokkaido is now recognisably distinct as B. b. vicinitas. Pairs are formed from late March to early May. During this period males whistle actively. Six to ten eggs are laid in early or mid-May and hatch in early June after incubation of 23 to 25 days. Young attain adult size by late August and have adult plumage by mid-September. The main diet consists of the leaves and seeds of herbaceous plants and trees and arthropods during late spring and summer, the buds of broad-leaved trees and vine fruits during autumn and winter, and buds and catkins in early spring. The Hazel Grouse has two large caeca supporting effective digestion of the plant fibers comprising their main diet. Hazel Grouse prefer broad-leaved and mixed forests with relatively dense undergrowth, and they avoid larch plantations in Hokkaido. Recently, the Hazel Grouse population has decreased in Hokkaido, the main cause of which is considered to be predation by the red fox (Vulpes vulpes), which increased in numbers from the early 1970s until the 1990s. Brood sizes were smaller during low population periods than during normal population periods. In order to maintain, or increase, Hazel Grouse population levels, habitat management and predator control is considered necessary.