Sites are not replaced if they change. They continue to be monitored. This is because we aim to monitor bird populations’ changes across large spatial and temporal scales as a consequence of changes in habitats, and forest succession is such a habitat change. To this end, we need a representative sample of sites across a wider area (e.g. a country). If the forests in a country get older on average, specialists of older forest stages should increase. For instance, such a phenomenon was shown for forest birds in the Czech Republic (Reif et al., 2007).