In the boreal forest, winter temperatures routinely drop to 30 degrees below zero. Birds that spend the winter in this harsh domain rely on remarkable adaptations to survive. The Spruce Grouse is one such bird. Most Spruce Grouse remain here all year. In the snow-free summer, they forage on the ground, eating fresh greenery, insects, and berries. But in the snowy winter, the grouse live up in the trees, eating nothing but conifer needles. Lots and lots of needles. Because conifer needles are both low in protein and tough to digest, Spruce Grouse grow a bigger digestive system. A grouse's gizzard, which grinds food, may enlarge by 75%!

More info and transcript at BirdNote.org. 

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