The Gyrfalcon (Falco rusticolus) is a very large falcon, being about the same size as the largest hawks but probably slightly heavier. Males are 19 to 24 inches long, with average weights reported as 39 or 41 oz and have a wingspan from 43 to 51 in. Females are bulkier and larger. An outsized female from eastern Siberia was found to have scaled 92 oz. The gyrfalcon is larger, broader-winged and longer-tailed than the peregrine falcon, which is known to compete with (and occasionally hunt). It differs from the hawks in general structure, having pointed wings.
The gyrfalcon is a very polymorphic species, so its plumage varies greatly. The archetypal morphs are called "white", "silver", "brown", and "black", though they can be colored on a spectrum from all-white to very dark. The brown form of the gyrfalcon is distinguished from the peregrine by the cream streaking on the nape and crown and by the absence of a well-defined malar stripe and cap. The black morph has a strongly black-spotted underside, rather than finely barred as in the peregrine. White form gyrfalcons are the only predominantly white falcons. Silver gyrfalcons resemble a light grey lanner falcon of larger size. The species shows no sex-based color differences; juveniles are darker and browner than adults.