The strong winter winds on the Barrens affect the Caribou in still another way. While they pack the snow so firmly that man may dispense with snowshoes, this condition naturally increases the difficulty that the Caribou experience in pawing through the snow to reach the lichens that are covered by it. The limited grazing capacity of such areas as are laid bare by the wind may force a reduction in the wintering population. Although the snow in the timbered regions to the south covers virtually the whole surface of the land, it is evidently less compact and so offers more favorable feeding conditions than the areas of hard-packed snow on the Barrens. (Charles Schweder states that Willow Ptarmigan will fly out of the Barrens to spend the night in tracts of timber, where the snow is softer and thus more suitable for the nocturnal burrows of these birds.) Another apparent inducement for resorting to the tracts of timber in winter is the abundance there of tree lichens, such as _Alectoria_ and _Usnea_ (_cf._ Richardson, 1829: 243; J. B. Tyrrell, 1894: 441; Dix, 1951: 287), upon which the Caribou may feed without regard to snow conditions. (See also _Retrograde autumnal movement_.) Reindeer lichens (_Cladonia_ spp.) and doubtless other lichens are of such slow growth that forest fires may deprive the Caribou of this indispensable food for a period of years. According to Mr. G. W. Malaher, the recent burning of a large area north of The Pas may have deflected the animals toward the southeast, causing them to extend their migration to an abnormal distance in that direction. For a similar reason in years past, according to Pike (1917 [1892]: 50), they avoided “great stretches of the country” near the Mackenzie River, and also on the south side of Great Slave Lake. A quarter of a century after Pike’s time, Dogribs reported that Caribou had not come to the lower Taltson River for several years, “because the timber had been burned off” (Harper, 1932: 30). Some years ago, extensive fires in Manitoba were said to have been deliberately set by prospectors with the aim of exposing the underlying rock. Charles Schweder believes that the Caribou show a certain predilection for rocky places, owing to the more luxuriant growth of lichens there. _References._--Richardson, “1825”: 328-329; Bompas, 1888: 24; Pike, 1917 (1892): 50; Wheeler, 1914: 60; Blanchet, 1930: 52; Jacobi, 1931: 192, 194, 195; Harper, 1932: 30; Ingstad, 1933: 34, 161, 163; Hornby, 1934: 105; R. M. Anderson, 1938: 400; Clarke, 1940: 100, 106-107; G. M. Allen, 1942: 299; Downes, 1943: 261; Porsild, 1943: 389; Wright, 1944: 186; R. M. Anderson, 1948: 15; Banfield, 1951a: 5, 11, 27-29. _Influence of insects on distribution_ It is quite possible that the blood-sucking mosquitoes (_Aedes_) and black flies (_Simulium_) and the parasitic warble flies (_Oedemagena_) and nostril flies (_Cephenemyia_) have a definite and important influence on the extent and dates of caribou migration.