Bird species may be "indicators," meaning that their presence and behavior in an area indicates the area's environmental health. Certain species – the osprey, spotted owl, condor, and in particular, migrating songbirds – are examples of "indicator" species, which reveal the condition, including the environmental health, of the area in which they reside.
The reaction of bird populations to the Earth's environment presages what may happen to human populations. For example, the radioactive cloud resulting from the Chernobyl nuclear accident resulted in the breeding failure of birds in Northern California in 1986.
Because of their place in many food chains, birds reflect how the health of the environment affects the health of the insects and plants upon which they feed. For instance, the sudden drop in the number of peregrine falcons in Britain and raptors in the U.S. first warned the world in 1961 of the dangers of persistent organochlorine pesticides. It was discovered that the birds were accumulating the compounds, which caused them to lay eggs with thin shells that broke easily.
Starfish accumulate toxic substance like lead and cadmium. They offer greater accuracy and live at a wide range of depths than mussels (which are already used as bio-indicators of marine pollution).
In 1985 the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) was created. This took place within the framework of the Convention on Long-Range Transboundary Air Pollution (Geneva Clean Air Convention, UN/ECE). In 1986 the Member States of the European Union (EU) agreed on the European Union Scheme on the Protection of Forests against Atmospheric Pollution(Regulation (EEC) No 3528/86).