Several color phases, ranging from white to dark slate-gray, exist. White birds exhibit varying degrees of dark barring on the upper parts of the body, but the undersides may be nearly pure white. Darker birds have considerable dark barring and streaking on a light gray breast and belly. Gray birds are more commonly observed in Montana than white birds. Immature birds are similar to adults, but have blue-gray legs instead of the yellow legs of the adults. Females measure about 22 inches in length and have a wingspan of about 48 inches. Male measurements range from two to four inches smaller.
For a comprehensive review of the conservation status, habitat use, and ecology of this and other Montana bird species, please see Marks et al. 2016, Birds of Montana.
Gyrfalcons are much larger and appear stockier than either Peregrine or Prairie Falcons.
Western Hemisphere Range
During years of prey shortages in the north, large numbers of Gyrfalcons move down into the northern United States in winter.
Gyrfalcons breed in the remote Arctic and sub-Arctic zones around the globe. During the winter months, they often are seen as migrants or over-wintering birds in eastern Montana and in the western valleys, especially where waterfowl or upland game birds concentrate.
Gyrfalcons primarily eat birds, ranging from small songbirds to ducks and grouse.
Probably most records are of first year birds, that have not yet established breeding territories.
Male Gyrfalcons begin defending their breeding territories as early as January and February, and females arrive by March. Eggs are laid by late April or early May, and young hatch after 30 to 35 days of incubation. The young fly when about seven to eight weeks old.