1. Van Noordwijk & de Jong proposed a model to predict the sign of an intraspecific correlation between two life-history traits, given that a trade-off between the two traits exists. In this paper, the model is adapted to egg size/number trade-offs.
2. The predictions of the model are tested within various clades of waterfowl. Although the relationship between egg mass and clutch size in this group has been examined previously, prior analyses have either failed to take phylogenetic structure into account or relied upon phylogenies that lack resolution. Phylogenetic structure is adjusted for using recently constructed phylogenies and the method of phylogenetically independent contrasts.
3. Van Noordwijk and de Jong's model was successful in explaining between-clade variation in the relationship between egg mass and clutch size. Even if a trade-off between egg size and number exists, a negative relationship will only be observed if the variation in the pattern of allocation (i.e. few large eggs vs many small eggs) is high relative to the variation in the amount of resources invested in the clutch. Thus, this model may be useful for understanding interspecific relationships as well as intraspecific patterns.