Forest bird abundance in Europe has remained stable overall, unlike farmland species which have declined dramatically in recent decades. However, this apparent stability may hide large variations among species and geographical regions. We aimed to determine if forest bird species with varying life histories and biome distributions show different population trends. We used functional traits and specialisation indices to study changes in abundance and distribution of European forest bird populations. For each species, we used European‐level estimates of total abundance change over the last 40 years and changes in two components of spatial distribution: range change (i.e. area shrinkage or expansion) and distribution shift (i.e. latitudinal adjustments), both over the last 30 years. We also considered specialist groups of different biomes (i.e. boreal, temperate, Mediterranean and biome generalists) separately. We showed that boreal forest species have declined in abundance and range area, while Mediterranean and temperate species have increased in abundance and range, possibly as the result of warmer temperatures and forest expansion in these regions. The decline of boreal forest species may result from changes in forest structure and composition due to forestry practices, increasing temperatures and colonisation by warm‐dwelling species. Among boreal species, mixed forest specialists (i.e. those preferring a mix of broadleaf and coniferous trees) declined the most in abundance and range and shifted northwards. In contrast, for vertebrate carnivores (i.e. birds of prey), we observed an increase in abundance among Mediterranean species and a southward expansion among all species. Our findings suggest that forest bird species in Europe may be influenced by the combined effects of land use and climate change, with these impacts varying across biomes. Our results highlight the need for maintaining and restoring key forest habitats (e.g. through increased protected areas and extensive management) and halting or limiting climate change, especially for boreal species.

The study of spatial bias in opportunistic data produced by citizen science programs is mainly approached either from a geographical angle (site proximity, accessibility, habitat quality) or from the angle of human behavior and volunteer engagement. In this study we linked both by analyzing the effect of observer profile on spatial distribution of recordings. We hypothesized that observer profile biases spatial distribution of records and that this bias can be explained by landscape naturalness. First, we established observer profiles from analysis of the temporal and spatial distributions of their records as well as record contents. Second, we mapped a naturalness gradient at regional and local scales. Using a dataset of more than 7 million bird records covering a time span of 15 years from the west of France, we defined four types of observer: garden-watchers, beginners, naturalists, and experts. We found that recording intensity could be related to naturalness at regional level; most visited areas were those where naturalness was on average lower i.e., close to population basins and highly accessible due to well-developed road infrastructure. At local level (neighborhood of recording sites), we found that experts and naturalists recorded in areas of higher naturalness index than those of garden-watchers and beginners. These results highlight how records contributed by different types of observer may lead to complementary coverage of different areas of the landscape. Future studies should therefore fully consider observer heterogeneity and how different observer profiles are influenced by local landscape naturalness.