In many birds, egg viability declines the longer that eggs remain unincubated (i.e., incubation delay), possibly because of increased levels of microbial infection. Reuse of nests is common in cavity-nesting species and may also increase exposure of eggs to microbes. Starting incubation before the clutch is complete may help protect early-laid eggs from microbial infection. In this study, we tested the effects of nest cleaning, length of incubation delay, and onset of night incubation on growth of microbes on Wood Duck (Aix sponsa) eggshells and examined the effect of microbial infection on hatching success. We used nest boxes that had been cleaned or not cleaned of previous nesting materials and sampled microbes on eggshells during early and mid egg laying and after the start of night incubation. We detected levels of eggshell microbes (10¹ colony forming units [CFU] egg^-1) lower than at more tropical locations (10^3–4 CFU egg^-1). Levels of heterotrophic bacteria were 2.2 times greater in uncleaned than in cleaned nests but did not vary with length of incubation delay. Levels of Gram-negative bacteria in cleaned and uncleaned nests did not differ but declined by 87% after onset of night incubation. We found no relationship between egg viability and infection of eggshells by either heterotrophic or Gram-negative bacteria. Bacterial infection of eggs was low at our temperate-zone study site and was further reduced with the onset of incubation. We suggest these factors contributed to the negligible effect that bacteria had on egg viability.