Replacing a long-running common bird monitoring scheme by a new one: what predicts the correspondence of population changes between the schemes?

Abstract Long-term monitoring schemes provide crucial data on bird population dynamics and serve as an invaluable source for reaching informed conservation decisions. Although it is important to keep field techniques unchanged over time to ensure time-series continuity, methodological improvements are sometimes inevitable to track increasing demands on data quality and representativeness. If the changes are too deep, it is reasonable to open a new scheme and, after some time of overlap, to close the old one. Under these circumstances, it is important to evaluate to what extent the data on population dynamics correspond between the schemes. Here, we performed such an evaluation using data from two common bird monitoring schemes in Czechia (Central Europe): an old scheme running from 1982 to 2024 and a new scheme running since 2018. We calculated annual population indices and trends of 104 species independently for each scheme over the period of scheme overlap (2018–2024) and showed that correspondence between the schemes increased with (i) species’ commonness, (ii) detectability, (iii) preference for farmland habitat, and (iv) migration distance. Moreover, we found that the correspondence improved with the length of the overlap period, but the improvement was modest after 5 years. To our knowledge, our study is the first one testing the factors that may influence the correspondence of population measures between different long-term bird monitoring schemes. These results can be used for designing a new scheme and for the interpretation of species population measures in different schemes.