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|a Historical perspective is important to understand the mechanisms of current environmental impacts on biodiversity. It may be achieved through long-term monitoring schemes aiming to record biodiversity changes over time. However, even the monitoring of bird populations, a taxon with the broadest spatial coverage and the longest time series among all organisms, does not usually cover more than 30–50 years. It is thus possible that the population status recorded by the monitoring schemes resulted from impacts preceding the monitoring data. In European birds for example, most studies use the beginning of 1980s as a baseline, even though major environmental changes likely acted earlier and its possible that many species have already depleted populations at that time. To fill this knowledge gap, we performed a unique survey among ornithologists of an older generation who performed bird observations in the Czech Republic from 1950s or 1960s up to now. They were asked to fill in a questionnaire to record relative abundance of each species in the region of their expertise during three time periods: 1950s/1960s, 1980s, and 2010s. Comparison of relative abundance between 1950s/1960s and 1980s should reflected population change prior to the time period typically set a as baseline, whereas population change between 1980s and 2010s aimed to assess the reliability of the ornithologists’ assessment by comparison with objective atlas mapping data. As expected, ornithologists of an older generation reported major population declines between 1950s/1960s and 1980s, especially among species depending on insects in their diet corresponding with steep intensification of land use for agriculture. Species associated with wetlands increased their populations, likely due to a higher nutrient input into water bodies. Interestingly, bird populations showed opposite pattern to the currently observed climate change impact, probably due to the modest climate change over these time periods. Importantly, population change between 1980s and 2010s estimated by the ornithologists corresponded well with the change based on atlas mapping data between the same time periods. Such a high level of congruence indicates suitability of memories of older generation naturalists for inferring indicators of historical biodiversity changes. We urge for performing of such assessments in other regions and for other taxa before this source of information will be lost forever. © 2021 The Authors
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