Measurements

Mass

Largest species in genus Falco, males from 800 to 1400 g, females 1000 to 2100 g (lower values probably from individuals in poor physical condition, Cade et al. 1998). One captive male with West Nile Virus dropped from normal weight of 1150-1200 g to 750 g and survived to regain normal weight and activity (TJC). Mass of normally functioning individuals can vary by 250-500 g depending on sex and nutritional state (Cade et al. 1998). Moderate reversed sexual size dimorphism: In series of 5 geographical samples male averages for mass ranged from 64.3% to 74.0% of female averages (Brown and Amadon 1968, Cramp and Simmons 1980, Clum and Cade 1994, Cade et al. 1988, Potapov and Sale 2005). No geographic trends noted in sexual size dimorphism. Slight geographic differences in overall size: Largest birds occur in Iceland and Greenland, smallest in North America and Scandinavia. Apparent trend towards increase in size from west to east in Eurasia (Dementiev 1960, Palmer 1988, Clum and Cade 1994, Cade et al. 1998, Potapov and Sale 2005). No seasonal change in body mass documented, but data are few.

Linear

See Appendix 2. Linear measurements show slight geographic variation and parallel differences in body mass and findings of significant genetic difference between island (Greenland and Iceland) and continental (Europe and North America) populations (Johnson et al. 2007). As examples, wing length of museum skins measured flat for worldwide sample of 243 males was 368.2 mm ± 12.64 mm (SD), and for 362 females, 403.9 mm ± 12.44 mm (SD) (Potapov and Sale 2005). For Iceland, male wings of 4 live specimens measured flat averaged 373 mm ± 8.0 mm and for 23 females, 419 mm ± 5.0 mm (Cade et al. 1998); for West Greenland, 4 live males measured flat averaged 378.5 mm (range 370-386 mm), and 9 females, 414.3 mm (range 404-420) (Mattox 1970). In two estimates for North America, 20 male specimens measured flat averaged 367 mm (range 340-390 mm), and 38 females, 393 mm (range 345-410 mm) (see Appendix 2); 42 male specimens measured flat averaged 364 mm (range 340-378 mm) and 63 females, 400.5 mm (range 368-423 mm (Todd and Friedmann 1947). See other summaries in Potapov and Sale (2005).

Note: Interpretation of these small differences is often confounded by unknown variables such as method of measurement (e.g., chord or flat wing) and whether from dried museum skin or living bird.

Linear dimensions show moderate reversed sexual size dimorphism (RSD) by comparison with other falcons and accipiters. Using Storer’s (1966) Dimorphism index (mean of females – mean of males ÷ mean of males ÷ 2 + mean of females ÷ 2 X 100) for wing length: mean Gyrfalcon 9.5 (n=4), Merlin 9.6 (n=5), Saker 9.7 (n=1), Lanner 11.9 (n=1), Aplomado Falcon 11.9 (n=2), Peregrine 12.9 (n=5), Prairie Falcon 13.5 (n=3), Bat Falcon 15.1 (n=2), Goshawk 9.9 (n=3), European Sparrowhawk 16.7 (n=1); Sharp-shinned Hawk 17.1 (n=2) (data from Friedmann 1950, Storer 1966, Snyder and Wiley 1976, Cramp and Simmons 1980). Among other measurements of Gyrfalcon, the RSD index is least for length of the tarsometatarsus (4.8, 5.9) and middle toe (8.6, 8.9).

The lesser overall RSD of the Gyrfalcon compared to the Peregrine and other bird-feeding specialists, and particularly the small sex difference in size of “tarsus” and toes, may be related to the fact that both sexes feed predominantly on ptarmigan (Lagopus spp.) weighing around 500-650 g, each sex having converged toward the body and foot size best adapted to that size of prey (TJC).