occasionally with the western capercaillie (Tetrao urogallus), the spruce grouse (Falcipennis canadensis) and the rock ptarmigan (Lagopus muta). During the Pleistocene, the willow ptarmigan widely occurred in continental Europe. Authors who recognize paleosubspecies have named the Pleistocene willow ptarmigan L. l. noaillensis (though the older name medius might be the correct one). These marginally different birds are said to have gradually changed from the earlier (Pliocene) Lagopus atavus into the present-day species L. lagopus. Pleistocene willow ptarmigan are recorded from diverse sites until the end of the Vistulian glaciation about 10,000 years ago, when the species, by then all but identical with the living birds, retreated northwards with its tundra habitat. Distribution and habitat The willow ptarmigan has a circum-boreal distribution. It is native to Canada and the United States, China, Mongolia, the Russian Federation, Kazakhstan, the Czech Republic, Finland, Norway, Sweden, Estonia, Latvia, Lithuania, Germany, the United Kingdom and Ireland. It primarily occupies subalpine and subarctic habitats such as sparse pine and birch forests, thickets with willow and alder trees, heather moors, tundra and mountain slopes. In the winter, females and sub-adults may move to lower altitudes and seek shelter in valleys or in more densely vegetated areas, but adult males usually remain in the subalpine region. The red grouse is common on heather-clad moorland across the north and west of Great Britain and in localised areas of Ireland. Diet The willow ptarmigan has a varied and seasonal diet. The bird is herbivorous for most of its life and subsists on various plant materials. As juveniles, they may feed on insects due to an inability to digest plant material caused by underdeveloped cecums. In the summer, their diet is highly varied and may consist of berries, flowers, leaves, twigs and seeds. In Alaska, the main dietary item of the adults at all times of year is willows such as the Alaska willow Salix alaxensis, with leaves being eaten in summer and buds, twigs and catkins supplying the birds' main nutritional needs in winter and early spring. In the early twenty-first century, there has been an increase in shrub expansion in arctic Alaska that is thought to be greatly affecting the willow ptarmigan's winter diet. Because of the way they browse, Ptarmigan help shape the landscape of the area. After heavy snowfalls, the birds cannot access the shorter shrubs as they are blanketed with snow, so they will eat the