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1. Optimality theory predicts that both timing of arrival and arrival state on the breeding area will determine reproductive timing and investment in migratory organisms. We tested this idea using a condition‐dependent individual optimization model (Ardea68, 1980, 225 and The American Naturalist143, 1994, 698) in common eider ducks through descriptive data, path analyses and experimental manipulation.

2. Our results support the causal pathways drawn from the optimization model indicating that individuals adjust their reproductive decisions as a function of their arrival date and body condition at arrival.

3. Independent of body condition, early‐arriving females had a longer pre‐laying period, but still initiated their nests earlier, and produced larger clutches than late‐arriving birds. Independent of arrival date, females in good condition laid earlier than those in poor condition. Manipulation of pre‐laying female …