Incubation has a significant reproductive cost in birds that can limit both current and future reproductive success. We manipulated the incubation period of Wood Ducks (Aix sponsa) to examine how female body mass, incubation behavior, and nest temperature responded to changes in incubation costs. We found no relationships between the length of the incubation period and either the percent loss of body mass or the body mass at the end of incubation. However, females that initiated nests early in the season lost more body mass than females that nested later. The number of daily recesses increased slightly with longer incubation periods, but incubation constancy over the full incubation period and during the last week of incubation was not affected by incubation-period length. Variation in incubation constancy was explained best by nest initiation date. Incubation constancy was greatest for early-nesting females and declined for females that nested later. There also was a weak positive relationship between body mass and incubation constancy. Average nest temperature was not associated with the length of the incubation period but increased with increasing incubation constancy and as the breeding season progressed. Evidence from this study is consistent with the idea that incubation is an important reproductive cost that may constrain the timing of nest initiation in Wood Ducks. Poor-quality females, by delaying nest initiation, experienced reduced incubation costs and were able to maintain nest temperatures at levels similar to those of early-nesting females.
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Vol. 128 • No. 2