This is Bird Note.

In the boreal forest, the broad expanse of forest lying south of the Arctic, winter temperatures routinely drop to 30 below zero.

Birds that spend the winter in this harsh domain of spruce, pine, and other conifers rely on remarkable adaptations in order

to survive. The spruce grouse is one such bird. Most spruce grouse, rotund, chicken-like

birds that weigh about a pound, 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.

Simple enough, right?

Just keep eating. But conifer needles are both low in protein

and tough to digest because they're heavy in cellulose. To meet the energy demands of winter

on needles alone, spruce grouse, this may seem hard to believe, grow a bigger digestive system.

Their ventriculus, or gizzard, which grinds food, may enlarge by 75%.

So remember the hearty spruce grouse this holiday season.

As you stand back to admire a Christmas tree, somewhere in the northern forest, a grouse is nibbling away at such a tree, one needle at a time.

For Bird Note, I'm Michael Stein.