We studied environmental and maternal factors explaining bacterial loads on eggshells in a population of a hole-nesting passerine, the Pied Flycatcher (Ficedula hypoleuca) and the possible effects of bacteria on hatching success. We collected bacteria from eggshells at early and late stages of incubation and from females' feces, as a surrogate for cloacal samples, at intermediate stages of incubation. Higher environmental temperatures during laying promoted bacterial loads on the eggshells during early incubation, supporting the fact that bacterial growth is generally favored by increasing ambient temperature. Bacterial loads on the eggshells at late stages of incubation were correlated positively with bacterial loads in the female's cloaca, suggesting possible vertical transmission of bacteria. We detected no significant differences between bacterial loads of the eggshells at early and late stages of incubation. Possible bacteriostatic effects of incubation and/or other alternative mechanisms not associated with incubation may be responsible for this pattern. Bacterial loads seemed not to affect hatching success, raising the question whether bacterial species composition rather than bacterial load affects embryo survival.