Sex and age effects of eider on uropygial glands
Males had 30% larger uropygial mass than females [GLM with normally distributed data and identity link function: males: 5.34 g (SE = 0.08), n = 116; females: 4.06 (0.09), n = 113; χ2 = 93.09, df = 1, p < 0.0001; estimate (SE) = 0.64 (0.06)]. The mass of the uropygial gland increased with body mass in males, but not in females in which uropygial mass was basically constant across the range of body masses [sex: χ2 = 89.87, df = 1, p < 0.0001, estimate (SE) = 0.612 (0.06); body mass; χ2 = 6.67, df = 1, p = 0.01, estimate (SE) = 0.91 (0.35); sex × body mass: χ2 = 12.75, df = 1, p = 0.0004, estimate (SE) = 1.27 (0.35)]. Inspection of the interaction plot revealed that the size of the uropygial gland increased more strongly with body mass in males than in females. Age classes 2 and 3 years had smaller uropygial glands than age classes 4, 5 and 6 years (Fig. 1; GLM with normally distributed data and identity link function: χ2 = 12.23, df = 4, 221, p = 0.016).
Molt and mass of uropygial gland of eider
The mass of the uropygial gland of eider decreased with the daily growth increments of feathers in a model that included wing area, age and date [Fig. 2; full model: χ2 = 156.79, df = 1, p < 0.0001; growth increment: χ2 = 9.24, df = 1, p = 0.0024, estimate (SE) = − 0.44 (0.14)].
The mass of the uropygial gland increased with the duration of the annual molt in a model that included wing area, age and date [Fig. 3; full model: χ2 = 155.83, df = 1, p < 0.0001; duration of molt: χ2 = 8.28, df = 1, p = 0.004; estimate (SE) = 0.022 (0.008)].
Mass of the uropygial gland, flight and date of eider
Wing area increased with mass of the uropygial gland of eider in an analysis that controlled for the effect of age (Fig. 4; full model: LR χ2 = 61.52, df = 1, p < 0.0001; wing area and uropygial gland: χ2 = 8.39, df = 1, p = 0.004, estimate (SE) = 6.35 (2.17); wing area and age: χ2 = 28.14, df = 1, p < 0.0001). In contrast, there was no significant effect for aspect ratio of mass of the uropygial gland (LR χ2 = 0.22, df = 1, p = 0.64).
The mass of uropygial glands fell during the year [Fig. 5; LR χ2 = 10.97, df = 1, p = 0.0009, estimate (SE) = − 0.011 (0.003)]. An analysis that controlled statistically for the effect of age and sex showed an even stronger partial effect of date [full model: LR χ2 = 144.82, df = 1, p < 0.0001; age: LR χ2 = 16.26, df = 1, p = 0.0027; sex: LR χ2 = 0.31, df = 1, p = 0.58, estimate ‒ 0.20 (SE = 0.36)].