Incubation is a period of high energetic costs and accordingly body mass losses are often detected. Why birds lose body mass during incubation is not well understood; suggestions are that it is either a consequence of energetic constraints or adaptations to an optimal mass trajectory. We studied body mass changes through the incubation period in the Common Redshank Tringa totanus, a biparental incubator, on southern Gotland in the Baltic Sea. In contrast to what has been found in other biparental incubators, body mass of both sexes decreased linearly through the incubation period. The estimated mean body mass loss was 6.7 g (SE 1.7), corresponding to ca. 5% of initial body mass at incubation start. Hatching success in males was not related to body mass and size. In contrast, reproductive success, measured as successful production of fledged juveniles, in males was negatively related to body mass during incubation and positively related to body size. This finding supports the theory that body mass loss might follow an optimal mass trajectory, possibly to increase agility through the chick-rearing stage. However, energy constraints causing body mass loss cannot be ruled out; in fact, it is not unlikely that body mass may change due to a combination of both adaptation and stress.