Ornitho Family: two decades of citizen science in biodiversity monitoring

Over the last two decades, citizen science has profoundly transformed bird monitoring worldwide. The development of online portals and mobile applications has enabled large numbers of volunteers to collect biodiversity observations at unprecedented spatial and temporal scales. These initiatives have generated vast datasets that are now widely used to produce species distribution maps, analyze phenological patterns, and assess population trends. At the same time, the opportunistic nature of many observations raises well-known challenges related to sampling bias, variation in observer effort, and imperfect detectability, which require appropriate methodological frameworks and validation procedures. In Europe, several national and international platforms have emerged to support the collection of ornithological observations by volunteers. Among them, the ornitho.ch platform has developed into a network of country-specific portals sharing common technical foundations while remaining adapted to local contexts. Together, these platforms form the Ornitho Family, which currently supports biodiversity monitoring across a large part of Europe. The system integrates both casual observations and more structured data collection approaches, allowing a wide range of ecological questions to be addressed. A key challenge for such large-scale citizen science initiatives is to reconcile broad participation with data quality. To this end, the Ornitho Family system combines multiple validation procedures and encourages the use of structured protocols that explicitly account for observation effort and detectability. For example, in France, structured observation protocols have been introduced to promote standardized data collection, making it possible to apply statistical models that explicitly account for variation in observation effort, imperfect detection, and observer-related biases. Such approaches illustrate how citizen science data can be rendered suitable for robust ecological inference when appropriate methodological safeguards are implemented. In this paper, we provide an overview of the data types collected within the Ornitho Family network, the associated validation processes, and the methodological implications for ecological analyses based on large-scale citizen science data. Using selected examples, we illustrate how different observation protocols influence the type of analyses that can be performed and highlight both the opportunities and the limitations inherent to volunteer-based biodiversity monitoring systems. The analyses presented here are intended to illustrate methodological implications rather than to provide exhaustive ecological assessments.