Measurements

Sexual dimorphism slight; males slightly larger in most dimensions; no significant geographic variation. At fledging, mean wing length of juveniles (425 mm [range 405–440, n = 18]) 82–89% of mean adult length (493 mm [range 482–518, n = 18]); apparently reaches mean adult length 7–10 d after leaving nest (Wanless 1984). At Bass Rock, e. Scotland, mean culmen length of males 100.1 mm (range 93.5–110.0, n = 66), females 99.2 mm (range 92.5–104.0, n = 66); mean culmen depth of males 35.5 mm (range 33.0–40.0, n = 27), females 34.3 mm (range 32.0–39.0, n = 36); mean length of tarsus of males 50.2 mm (range 48.5–51.0, n = 3), females 50.8 mm (range 47–59, n = 12); and mean wing length of males 513.0 mm (range 500.0–535, n = 9), females 510.2 mm (range 484.0–522.4 [n = 14]; Nelson 1978a).

No information on seasonal variation in mass; adult females weigh slightly more than males; at Bass Rock, average mass of male Northern Gannets 2,932 g (range 2,470–3,470, n = 27), females 3,067 g (range 2,570–3,610 [n = 27]; Nelson 1978a).

Prefledging and fledging juveniles heavier than adults; at fledging at Ailsa Craig, w. Scotland, ju-veniles weigh significantly more (3,543 g ± 281.0 SD; n = 31) than adults (3,286 g ± 226 SD [n = 23, p <0.001]; Okill and Wanless 1986, Wanless and Okill 1994); 7–10 d after leaving nest, body mass about the same as adults (Wanless 1984).