The IBA encompasses the core use area of the largest known communal roost of wintering Rough-legged Hawks in the world. Location of the core area (in east 1/2 of sect. 5, T20N, R19W) was determined during a graduate study of radio-tagged hawks over several consecutive winters. The roost is in a stand of foothills conifers on the edge of the Mission Valley, which was formed by glaciers in the Pleistocene and is now dominated by irrigated farmland and pastures.
Hawks are attracted to the Mission Valley because of high numbers of voles that occur in some winters. The communal roost offers thermal (and presumably energetic) benefits by being warmer and more protected from wind than are foraging areas in the valley.
During winters with high numbers of voles, more than 300 Rough-legged Hawks use the roost each night, fanning out from the roost in the morning to hunt in pastures and grasslands in the Mission Valley. In some years, more than 1,000 raptors (mostly buteos, eagles, and Short-eared Owls) winter in the valley. This is by far the largest known communal roost of Rough-legged Hawks in the world.
A low density of residential homes is scattered throughout most of the core roosting area. The potential for human disturbance exists, but such disturbance appears to be low given that hawks continue to use the roost despite the presence of homes. Disturbance potential could increase if the density of homes increased, and if timber harvest was initiated. Urban-interface fire prevention is increasing the pressure from neighbors to reduce Douglas fir densities. Shooting of hawks in the Mission Valley continues to be a problem; some of the birds that have been killed are Rough-legged Hawks that were using the roost.
The core area of the roost identified as an IBA is on private lands within the Flathead Indian Reservation of the Confederated Salish and Kootenai tribes. Ownership tends to be in parcels of 1 to 2 ha.
The roost area is a mixed conifer forest consisting of Douglas fir (69% of trees), ponderosa pine (23%), and western larch (8%). Houses are scattered throughout the roost area at low density.
The roost and the hawks using it were the focus of a senior thesis and master's thesis by Chad Olson, University of Montana, during the winters of 1994-95 through 1998-99. A low density of residential homes is scattered throughout most of the core roosting area.