From the tiger's stripes, which echo the light and shade of the forest, to the bright green of the tree frog, most animals have developed color schemes that blend with their environment. In situations where that environment is subjected to extremes of seasonal change, however, extra and rather special adaptations are called for.
Like other birds living in landscapes that are snow covered for part of the year, the Scandinavian willow grouse regularly exchanges the tones and patterns of its brown summer plumage for winter whites. Not that the transition is abrupt; rather it is gradual, mirroring the drift of summer into autumn and then into winter. The willow grouse's life is constantly under threat . Terrestrial predators such as lynxes and wildcats share its mountain and tundra home, and eagles sheel overhead. Survival depends upon the grouse making the most of its colouring by gearing its movements to the state of its surroundings. Thus its greatest security is achieved when there is a good blanketing of snow and the bird is in full white feather.
During the long spring thaw, the landscape becomes a patchwork of snow, rock and exposed vegetation. While the grouse retains its winter colours, it will feed only against a snowy background. Not until its plumage is almost completely changed again will the bird move to snowless terrain. So important is the right camouflage to the grouse that it will even cut down its food intake rather than risk exposing itself against a contrasting background.