Higher rates of prebreeding condition gain positively impacts clutch size: A mechanistic test of the condition‐dependent individual optimization model

A combination of timing of and body condition (i.e., mass) at arrival on the breeding grounds interact to influence the optimal combination of the timing of reproduction and clutchsize in migratory species. This relationship has been formalized by Rowe et al. in a condition‐dependentindividual optimization model(American Naturalist, 1994, 143, 689‐722), which has been empirically tested and validated in avian species with a capital‐based breeding strategy. This model makes a key, but currently untested prediction; that variation in the rate of body condition gain will shift the optimal combination of laying date and clutchsize. This prediction is essential because it implies that individuals can compensate for the challenges associated with late timingof arrivalor poor body condition at arrival on the breeding grounds through adjustment of their life history investment decisions, in an attempt to maximize fitness. Using an 11‐year data set in arctic‐nesting common eiders(Somateria mollissima), quantification of fattening rates using plasma triglycerides (an energetic metabolite), and a path analysis approach, we test this prediction of this optimization model; controlling for arrival date and body condition, females that fatten more quickly will adjust the optimal combination of lay date and clutchsize, in favour of a larger clutchsize. As predicted, females fattening at higher rates initiated clutchesearlier and produced larger clutchsizes, indicating that fattening rate is an important factor in addition to arrival date and body condition in predicting individual variation in reproductive investment. However, there was no direct effect of fattening rate on clutchsize (i.e., birds laying on the same date had similar clutchsizes, independent of their fattening rate). Instead, fattening rate indirectly affected clutchsize via earlier lay dates, thus not supporting the original predictions of the optimization model. Our results demonstrate that variation in the rate of condition gain allows individuals to shift flexibly along the seasonal decline in clutchsize to presumably optimize the combination of laying date and clutchsize. A plain languagesummary is available for this article.