The information on the phenotypic and ecological factors that influence hemoglobin concentration in free-living birds is scarce. In order to recognize sources of variation in hemoglobin levels of migratory shorebirds, we measured whole-blood hemoglobin concentration in 553 juvenile and 166 adult common snipe Gallinago gallinago during autumn migration through central Poland. Among the intrinsic determinants of hemoglobin concentration in common snipe, we identified traits such as age, wing morphology, developmental stability, nutritional condition, and molt. We found that adult birds had higher hemoglobin concentrations than juveniles. Hemoglobin concentration was not related to body size, but it correlated with wing morphology. In adult males there was also a positive relationship between hemoglobin concentration and developmental stability, measured by fluctuating asymmetry in wing shape. The process of molt was found to affect blood hemoglobin concentration in both juvenile and adult common snipe, as the lowest concentrations were recorded in the initial stages of molt. Finally, we recorded a gradual increase in hemoglobin concentration of juvenile and adult snipe over the course of the autumn migratory season, and this trend was attributed to higher fat loads carried by late migrants. Hemoglobin concentration also correlated with other indices of nutritional state, such as plasma concentrations of proteins. All this clearly indicates that hemoglobin concentration may reflect a wide range of physiological processes, but in spite of this immense variation, it is likely to reliably indicate phenotypic quality of birds.