The paper presents a method for developing biodiversity indicators using European birds as a case study. The authors developed a classification system for biodiversity indicators to help their development and clarify their purpose. They used European birds to demonstrate how robust indicators can be constructed and interpreted. Statistical methods were developed to calculate supranational, multi-species indices using population data from national annual breeding bird surveys in Europe. These indices were constructed by combining national species indices weighted by national population sizes. Supranational, multi-species indicators were calculated by averaging the resulting indices. The study shows that common farmland birds in Europe have declined steeply over the last two decades, whereas woodland birds have not. The main driver of farmland bird declines is increased agricultural intensification. The farmland bird indicator is a useful surrogate for trends in other elements of biodiversity in this habitat. The authors define an ideal biodiversity indicator as a group of species whose population trends reflect the average behavior of the constituent species and provide insight into trends in other taxa. They propose a framework for biodiversity indicators based on their ability to generalize findings and their relationship with potential drivers. The framework includes four types of indicators: Type 1 measures specific taxa, Type 2 measures biodiversity generally, Type 3 shows how specific taxa respond to drivers, and Type 4 shows how biodiversity responds to drivers. The authors developed a European bird indicator based on breeding populations of common European birds. They used a statistical method to calculate supranational, multi-species indices using population data from national annual breeding bird surveys in Europe. The study shows that common farmland birds in Europe have declined steeply over the last two decades, whereas woodland birds have not. The main driver of farmland bird declines is increased agricultural intensification. The farmland bird indicator is a useful surrogate for trends in other elements of biodiversity in this habitat. The authors conclude that birds can act as indicators of other taxa and of environmental health. They also note that birds have strengths and weaknesses as indicators, including their ability to reflect changes in other biodiversity and their response to environmental change. However, they are less suitable as indicators due to their migratory habits and wide-ranging nature, which make it difficult to link their populations with specific drivers on the ground. The study highlights the importance of developing robust indicators for biodiversity and the need for further research in this area.The paper presents a method for developing biodiversity indicators using European birds as a case study. The authors developed a classification system for biodiversity indicators to help their development and clarify their purpose. They used European birds to demonstrate how robust indicators can be constructed and interpreted. Statistical methods were developed to calculate supranational, multi-species indices using population data from national annual breeding bird surveys in Europe. These indices were constructed by combining national species indices weighted by national population sizes. Supranational, multi-species indicators were calculated by averaging the resulting indices. The study shows that common farmland birds in Europe have declined steeply over the last two decades, whereas woodland birds have not. The main driver of farmland bird declines is increased agricultural intensification. The farmland bird indicator is a useful surrogate for trends in other elements of biodiversity in this habitat. The authors define an ideal biodiversity indicator as a group of species whose population trends reflect the average behavior of the constituent species and provide insight into trends in other taxa. They propose a framework for biodiversity indicators based on their ability to generalize findings and their relationship with potential drivers. The framework includes four types of indicators: Type 1 measures specific taxa, Type 2 measures biodiversity generally, Type 3 shows how specific taxa respond to drivers, and Type 4 shows how biodiversity responds to drivers. The authors developed a European bird indicator based on breeding populations of common European birds. They used a statistical method to calculate supranational, multi-species indices using population data from national annual breeding bird surveys in Europe. The study shows that common farmland birds in Europe have declined steeply over the last two decades, whereas woodland birds have not. The main driver of farmland bird declines is increased agricultural intensification. The farmland bird indicator is a useful surrogate for trends in other elements of biodiversity in this habitat. The authors conclude that birds can act as indicators of other taxa and of environmental health. They also note that birds have strengths and weaknesses as indicators, including their ability to reflect changes in other biodiversity and their response to environmental change. However, they are less suitable as indicators due to their migratory habits and wide-ranging nature, which make it difficult to link their populations with specific drivers on the ground. The study highlights the importance of developing robust indicators for biodiversity and the need for further research in this area.