Sampling the same sites every year is fine for farmland that is maintained, but in forests the species composition will change in the course of 25 years due to succession. Do sampling sites have to be replaced if they change strongly?
Sites are not replaced if they change, they continue to be monitored. This is because we aim to monitor changes in bird populations 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) and if the forests in a country get older on average, specialists of older forest stages should increase. Such a phenomenon was shown for instance for forest birds in the Czech Republic (Reif et al., 2007).