Wild bird indicators: using composite population trends of birds as measures of environmental health

World leaders have set global and regional targets to reduce the rate of biodiversity loss by 2010, and their relative success, or failure, in meeting these aims will be measured against a set of indicators. For such indicators to be effective, they need to meet a range of practical and scientific criteria. Their development is often driven pragmatically by the information available. One such biodiversity indicator that has proven highly effective and influential in Europe is the wild bird indicator. This is based on the composite population trends of birds combined using a geometric mean and derived from national breeding bird surveys. Recent work has emphasized the importance of common species to ecosystem functioning and suggested that the depletion of their populations might significantly affect ecosystem services. National governments and the European Union are increasingly using these measures to assess sustainable development strategies, environmental and ecosystem health, as well as in the fulfillment of biodiversity targets. Equivalent indicators have been published in North America. There are a number of reasons to believe that birds might be useful indicators of biodiversity. They are sensitive to anthropogenic changes, they are well known, excellent time-series exist, and they have a resonance and connection with people and their lives. Yet, there are counter arguments and some risks in using birds in this way. Our work provides a blueprint for others to follow using similar data on birds or other taxa, and in other countries and regions. In the discussion, we review the strengths and weaknesses of using bird population trends as biodiversity indicators, and look forward to how this work might be developed. Wild bird indicators only measure a component of biodiversity change and need to be used carefully to assist policy makers and land managers in managing the natural resources and conserving nature.