Measurements

Sexual and geographical variations are small (Table 2). Differences between sexes for adult wing length significant, with males shorter, on average, than females; other linear measurements examined similar (Engelmoer and Roselaar 1998). Juvenile wing and tail significantly shorter than adult; bill shorter to about 1 yr, and tarsus and middle toe similar to adult (Cramp and Simmons 1983). Additional material required to clarify significance of recorded sex and possible geographical differences.

See Table 2 . Body mass similar between sexes and among regions. Considerable variation within and outside breeding season, particularly pronounced prior to long-distance migratory movements and at staging areas en route. Seasonal patterns of changes of mass vary within and between different populations, as do patterns of premigratory fattening (see Summers et al. 1989 for details); e.g., ne. Canada–Greenland birds (interpres) wintering in Britain display 2 different regimes in spring: departures early May, only moderate fat levels (mean mass = 130 g), fly 1,500 km to Iceland to replenish fat reserves (arrive early May, mean = 117 g; depart late May, mean 180–190 g) before flying on to nw. Greenland and ne. Canada, whereas departures mid- to late May accumulate greater fat reserves (mean mass 150–160 g), bypass Iceland, and fly directly to ne. Greenland, a single nonstop 2,500-km flight (Clapham 1979, Wilson 1981, Summers et al. 1989); premigratory fattening on breeding grounds prior to fall exodus similar (e.g., see Thompson 1973 for Siberian/St. Lawrence I. birds passing through Pribilof Is. in preparation for 3,200-km nonstop flight to nw. Hawaiian Is.). Maximum rate of increase in body mass about 7 g/d, though rates of 3–4 g/d more usual (Morrison and Wilson 1972, Summers et al. 1989); body mass of about 165 g allows a single flight of 4,000 km. For introduction to details of complex annual weight cycles, mostly for race interpres, see Morrison and Wilson 1972; Branson et al. 1978, 1979; Summers et al. 1989; Ens et al. 1990; Zwarts et al. 1990; Gudmundsson et al. 1991; and Engelmoer and Roselaar 1998. During breeding, mean body mass in both sexes tends to decrease from just prior to laying to mid-Jul, although variation is high and data few.