Here we present the results of candling 258 eggs from 50 nests of Common Eiders Somateria mollissima in a colony in the central Baltic. Of these, 223 (86%) had a developing foetus. Among the 35 (14%) failed eggs, 15 were unfertilized and 20 contained a dead embryo. The prevalence of failed eggs is similar to the average proportion of failed eggs reported previously by the Christiansø Scientific Field Station during 1998–2014. The reason for the high percentage of failed eggs is unknown; however, low pre-incubation body mass and energetic stress is likely to be the main factor. In addition, we incubated 8 eggs in the laboratory from day 0 to hatch in order to follow the development and foetal morphology. This resulted in a field atlas from which it is possible to estimate date of incubation start using candling in early incubation (days 1–12). The atlas is a new possibility for field biologists to estimate the first day of incubation of breeding eiders and the prevalence of unfertilized and rotten eggs, which is important for studying their biology and population dynamics.
