The size of waterbird populations continuously change. Counts of waterbirds describe these changes. If long-term and/or steep declines are detected, this should signal the need for conservation measures. However, conservation actions will only be effective if they tackle or mitigate threats that negatively impact population growth at the scale of the flyway. Identifying where and when in the annual cycle of a flyway population declines are caused could be a first step towards identifying which ecological factor is responsible for the decline and how this could be reversed. This entails demographic monitoring, i.e. the investigation of spatial and temporal variation in survival and reproduction. This is exemplified with a case study in which observations of individually colour-marked Sander[1]lings Calidris alba along the coasts of Europe and Africa were used to estimate temporal and spatial variation in the probabilities of annual adult survival and the age of first reproduction. Combined with estimates of clutch survival from the Greenlandic breeding grounds, it could be shown that the growth of the Sanderling flyway-population is cur[1]rently limited by annual variation in clutch survival and adult survival in W Africa. Despite its potential to effectively target conservation action, demographic monitoring is not the standard practice and we are often in the dark about the causes of population change. Increased and continued long-term and flyway-wide efforts to monitor survival and reproduction of waterbird populations could considerably improve this situation