Bats experience a fatality risk at wind turbines. Since some species migrate not only over land, but also over sea, offshore turbines might pose a threat. To be able to assess the impact of potential added mortality at sea, it is necessary to know or estimate the population size of bats, including the portion of the population crossing the sea. Potentially relevant species are Nathusius’ Pipistrelle (Pipistrellus nathusii), Common Noctule (Nyctalus noctula) and Particoloured Bat (Vespertilio murinus). The current study aims at developing a prototype estimator for migrating populations of bats. This is based on data, or estimates, regarding the size and bandwidth of source populations, population dynamical factors defining such populations, and factors defining migration fluxes. Acknowledging the rareness of such data, a flow model is constructed targeting a preliminary estimate for the southern North Sea (SNS). However, the approach can be adapted for use in other regions/study areas as well. The model is based on available information and data regarding bat species in the different countries bordering the SNS. This includes countries further off, which might be ‘source countries’ of bats that eventually might fly on the SNS. Currently the model focuses on the Nathusius’ Pipistrelle, because this is the species where some information and data are available. Even for this species quantitative basic estimates on source populations were only available for RO Ireland, UK (specifically for England and northern Ireland) and the Netherlands.