WAGNER, Emily; WILLIAMS, Tony D; Simon Fraser Univ., Burnaby; Simon Fraser Univ., Burnaby: Experimental manipulation of the egg size:offspring quality relationship

In many oviparous taxa there is a strong, positive relationship between egg size and offspring growth or survival, but in birds there is surprisingly little empirical evidence for such a relationship (in part due to the confounding effects of parental quality). Few studies have taken an experimental approach to this problem due to difficulties in manipulating egg size. Previously we have shown that treatment of laying female zebra finches (Taeniopygia guttata) with the anti-estrogen tamoxifen decreases egg size by c.10% but with little effect on offspring growth. Here we use this technique to decrease mean egg mass of individual females by between 0% and 50%. We show that there is a strong effect of the proportional decrease in egg size on embryo or chick viability. In clutches where egg size was decreased on average by c.20% embryos were not viable and/or chicks died between hatching and fledging. In contrast, chicks survived to fledging in clutches where egg size was decreased by c.10%. However, even when chicks survived to fledging, those from tamoxifen-treated eggs showed slower initial growth during the linear growth phase (5-10 days of age); though they then showed compensatory growth and fledged at similar sizes as chicks from control eggs. We show that these effects on egg quality and offspring viability are mediated via tamoxifen-induced decreases in the female�s circulating levels of yolk precursors.