The Nature Conservancy (406/443-0303) maintains a bird refuge north of Polson [1] at Safe Harbor Marsh. Reach it by turning off Highway 93 just north of town onto Rocky Point Road. More than 130 species of birds and waterfowl have been observed here.
Links:
[1] http://www.moon.com/destinations/montana/missoula-and-northwestern-montana/flathead-lake/polson