The Slovenian bird ringing database holds significant potential for assessing the population dynamics of bird species, as data have been consistently collected since 1927. However, due to the non-standardized nature of data collection, the primary aims of this research were to reduce bias and enhance statistical inference from heterogeneous data for studying population trends and irruptive movements of birds. We analysed population trends for 15 mostly passerine species during autumn migrations between 2000 and 2016. To obtain more homogeneous and representative samples, we combined different data selection approaches to develop 20 candidate models. Among them, we selected the best model for each species, i.e., a species-specific model, and in addition, identified the universal model, which best fitted the largest number of studied species. Using the universal model, we further analysed 74 passerines, ringed during autumn migrations between 2000 and 2016. The trend directions were largely consistent with broader European and Slovenian breeding population trends. To identify the factors most influencing population trends, we clustered species by the following attributes: geographic origin, migratory behaviour, ecological preferences, and life history traits. Most bird populations, and almost all groups, showed declining trends, indicating a widespread and extensive impoverishment of avifauna and biodiversity in Slovenia and across Europe. Time series data also enabled the identification of outliers, which may arise due to unique events, such as bird irruptions. We identified outlying years (irruptions) for fringillid species over an 87-year period (1935–2021) by intersecting various outlier detection methods. According to our findings, irruptive movements were observed in all the species studied, suggesting a gradient of irruptive tendencies, with irruptions being more frequent in boreal seed-eating species. In highly irruptive species (> 11% irruption years during the study period), populations generally exhibited larger scaled mass index during irruptive years, which was most likely associated with short-term stress in northern wintering areas. During irruptions, birds often form larger flocks; however, these are highly variable. Therefore, bird ringing remains one of the most effective methods for detecting irruptions. To fully realize the potential of ringing data for research and conservation, future bird ringing activities should be conducted under a standardized protocol.
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