They'll build their nests on the broom platform, or between the broom and the trunk. They take mushrooms or pine nuts out to brooms and cache them or eat them. You'll often find ten mushrooms cached in one broom. One red squirrel even hauled loads of elk pellets 22 meters up into a broom. Animals like brooms because they provide protection from predators and weather.
Large brooms are relatively warm in winter and cool in summer.
And some are large indeed. Hedwall found some brooms in eastern Arizona in which wood rats had built nests of sticks. One was large enough that I could have lowered myself into it. Raptors such as northern goshawks and great gray owls also make their homes there. In the Southwest, Douglas fir brooms are popular nesting sites for threatened Mexican spotted owls.