The total volume of eggs was measured for each female by the sum over clutches of clutch volume, the latter being estimated by the product of egg number by mean egg volume.
Clutch volume was calculated as the product of mean egg volume and total fecundity.
These 20 measurements were then averaged to obtain one estimate of mean egg volume per female.
Plasma carotenoid concentrations were also higher in females with higher mean egg volume but unrelated to clutch volume (Pearson correlations; mean egg volume, r = 0.50, P = 0.008, N = 27; clutch volume, r = 0.31, P = 0.120, N = 26; Fig. 4).
In addition, mean egg volume was higher in individuals with higher tarsus R/G values (Table 2 and Fig. 1a).
In our system, we found that chicks hatched from nests with higher mean egg volume had higher early body condition (Gladbach et al. 2010 ).
We compared egg traits (mean egg volume, clutch volume, and ovary weight) across years using ancova, with year as a categorical explanatory factor, and somatic body weight as a covariate.
The results remained qualitatively the same after controlling for hatching date (partial correlations; mean egg volume: r p = 0.46, df = 23, P = 0.020; clutch volume: r p = 0.22, df = 23, P = 0.290).
Here, we show that correcting egg number and mean egg mass by female weight at egg laying allowed the detection of negative associations between egg quantity and quality and between egg quantity and egg care.
It is common, however, that individuals do not only differ in their absolute trait value (like mean egg size) but also in their slopes of response to some factor or covariate (like change of egg volume over the laying sequence).
