Optimal allocation of effort between reproductive phases: the trade-off between incubation costs and subsequent brood rearing capacity

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Abstract
The optimal allocation of effort during reproduction is a key component of life history theory, with trade-offs predicted to operate both within and between reproductive attempts. Experimental work in this field has largely concentrated on the latter. The need to partition investment between different phases of reproduction, and how this varies between individuals, has received little empirical investigation. In this study, the costs of the incubation phase in common terns Sterna hirundo were increased independently of those of the egg production and brood rearing phases. Incubation of a clutch of three eggs, rather than the two originally laid, reduced the subsequent capacity of parents to provision their brood of two, demonstrating an important trade-off between reproductive phases that has generally been omitted in estimations of optimal clutch size. These results show for the first time that an increase in the costs of incubation alone, which have often been considered relatively trivial, can significantly depress parental performance later in the same breeding attempt. The effect of increased incubation costs was found to be most marked in the lower quality pairs, which demonstrates that individuals differ in their capacity to compensate for deviations from their allocation of effort to different reproductive phases.