Traditional waterfowl monitoring includes only breeding or non-breeding population sizes, but the assessment of breeding success would provide valuable information for the interpretation of population trends, as well as for the early targeting of management measures and further studies. The relationship between breeding success and subsequent breeding population trends is not very well-known in particular for many waterfowl species. Using transfer function (TF) models, we analysed the relationship between breeding success measured as duckling numbers and post-breeding population sizes (i.e. the numbers of adults and ducklings in July), and subsequent breeding population trends for the mallard Anas platyrhynchos, common eider Somateria mollissima, common merganser Mergus merganser and goldeneye Bucephala clangula in Aasla, an island in southwestern Finland. In addition, we used data on the common eider in Söderskär, an island group in the Gulf of Finland, to analyse the extent to which fledgling numbers transform into recruitment and breeding population size. As a complement to the traditional methods of population monitoring, we present a simple and cost-effective method for the assessment of breeding success: the monitoring of post-breeding population sizes (including both adults and young). The breeding population sizes of the mallard, common eider and common merganser were positively related to their breeding success with a time lag corresponding to their recruitment age. For the common eider, the effect of the recruitment number on subsequent breeding population size seemed to last for up to three years after recruitment. There was a coupling between the post-breeding sizes and the subsequent breeding population sizes of the mallard, common eider and goldeneye with a lag corresponding to the recruitment age for the mallard and the common eider, but for goldeneye, one year later than the usual recruitment age. The chicks seem to recruit to the local breeding population to an extent that is sufficient for affecting local breeding population trends. Our results indicate that the annual breeding success of our target species can be assessed on the basis of their post-breeding population sizes. This easy and rapid monitoring method for breeding success is also suitable for voluntary bird watchers.