Test of the variation in yolk progesterone (in pg/mg) concentrations and total yolk progesterone (in ng).

Results of GLMs with egg category (A- or B-eggs) and as a fixed factor and female body mass and date of laying onset as covariates. n = 120 for both dependent variables. Only significant interactions are shown in these models, while other non-significant ones were removed from the model during the backwards-stepwise procedure. As a measure of effect sizes we used partial Eta-Square values (ηp2; i.e. the proportion of the effect + error variance that is attributable to the effect) for the factors and covariates tested with a GLM.