Resource Allocation within the Replacement Clutch: do Female European Starling (Sturnus vulgaris Linneus) Adjust their Reproductive Strategy after a Full Clutch Loss?

An important factor in the evolution of reproductive strategies in birds is the loss of a clutch or a brood. Many avian species produce a replacement clutch following the loss of the first clutch, but additional breeding effort carries physiological costs and can also reduce female fitness. Thus, egg production in replacement clutches is usually reduced. In contrast, European starling mothers seem to invest equally in their first and replacement clutches. This study investigated whether female starlings use differential allocation of resources to offspring, rather than egg production, to strategically balance investment and returns from the replacement clutch. By removing the first clutch, we induced females to lay a replacement clutch, and focused on covariation of egg size, embryo sex and laying order in both clutches. Results suggest that starling mothers created a pattern of resource allocation with laying order, but not with offspring sex, in the replacement clutch; whereas no pattern of resource allocation existed in the first clutch. This finding indicates that although starlings do not reduce egg production between first and replacement clutches, they may use other mechanisms to fine-tune their investment and maximize their fitness gains from the replacement clutch.