Experimental manipulation of female reproduction reveals an intraspecific egg size-clutch size trade-off

A negative relationship, or trade-off, between egg size and clutch size is a central and long-standing component of life-history theory, yet there is little empirical evidence for such a trade-off, especially at the intraspec ific level. Here, I show that female zebra finches (Taeniopygia guttata) tr eated chronically during egg formation with the anti-oestrogen tamoxifen la y smaller eggs (by 8%) but produce larger clutches (on average two eggs mor e) than controls. Decreased egg mass in tamoxifen-treated females was assoc iated with a 50% decrease in plasma levels of the two yolk precursors, vite llogenin and very-low-density lipoprotein. Although tamoxifen-treated femal es laid more; smaller eggs (and had a higher total expenditure in their clu tch), they did not differ from controls in the number of chicks fledged: th e mass or size of these chicks at fledging, or the chicks' egg-production p erformance at three months of age. However, tamoxifen-treated females had l ower relative hatching success: they laid more eggs but hatched the same nu mber of chicks. Among individual tamoxifen-treated females, birds that laid the smallest eggs early in their laying sequence laid the largest number o f additional eggs, that is, there was a negative correlation, or trade-off between egg size and clutch size.