WILLIAMS, T.D.: Is the egg size-clutch size trade-off based on a resource allocation decision?

Life-history trade-offs have proved difficult to demonstrate using phenotypic correlations and there is still surprisingly little empirical evidence for such trade-offs especially at the intraspecific level. In contrast, experimental (hormonal) manipulation of phenotypic traits has provided support for the existence of a trade-off between egg size and egg number in lizards and, more recently, in birds. However, these studies have still provided little direct information on the mechanism(s) underlying this reproductive trade-off. Female zebra finches (Taeniopygia guttata) treated with the anti-estrogen tamoxifen during laying have 50% lower circulating yolk precursor levels and lay small eggs but larger clutches. The most parsimonious explanation for such a trade-off is that individuals have finite resources which they can divide amongst many small eggs or few large eggs, i.e., that they represent resource-allocation decisions. As a consequence this, and the ‘cost of reproduction’ trade-off, imply that individuals monitor their internal condition and level and pattern of reproductive investment, using this information to determine their total investment. Here I report results of an experiment where tamoxifen-treated zebra finches had the small eggs they actually laid replaced each day by large eggs, providing a tactile and/or visual stimulus of a “large-egg” clutch. I test whether the egg size-clutch size trade-off is influenced most by the egg size birds actually produced (i.e. ‘internal’ information) or by the clutch that they see (i.e. ‘external’ information).