Egg viability in birds declines with increasing length of the delay in the start of incubation (i.e., incubation delay) and may be influenced by microbial infection and exposure of eggs to temperatures above physiological zero (>24°C). Onset of incubation during egg laying results in developmental asynchrony of embryos but may help to maintain viability of early-laid eggs. We manipulated incubation delay of Wood Duck (Aix sponsa) eggs and tested for effects of incubation delay and ambient temperature on egg viability and microbial infection. We also examined onset of incubation by egg-laying females. Hatching success declined slowly with increasing length of incubation delay and was very weakly affected by increasing exposure to temperatures >24°C. On average, females began incubation at night 7 days after nest initiation, but egg viability had declined only 8.6% (95% confidence interval: 4.3–15.2) by day 7. Microbial infection of eggshells was not related to length of incubation delay or mean daily temperature of the exposure period. Clutch size declined as the breeding season progressed, and nocturnal incubation began earlier in the laying sequence with declining clutch sizes. Laying females, regardless of clutch size, incubated for 3 nights before starting full incubation. We suggest that early onset of incubation may be more important in reducing incubation period and predation risk than in maintaining viability of early-laid eggs.