Are birds good indicators?

Birds are valuable indicator species for biodiversity conservation for a number of reasons.

Birds have widespread popular appeal and therefore make good flagship species for mobilising volunteer-based monitoring networks (harnessing the power of citizen science), as well as for education and advocacy within civil society.
Birds occupy a very broad range of ecosystems, have varied natural histories and are widely dispersed in all regions and countries of the world. They are high in the food chain, thus integrating changes at lower levels.
They are the best known and documented major taxonomic group, especially in terms of the sizes and trends of populations and distributions, and the number of species (c.10,000) is manageable, thereby permitting comprehensive and rigorous analyses.
They are sensitive to many kinds of environmental disturbance and can be used to monitor potentially harmful changes in the environment.

Some examples

Some examples of birds as indicators of environmental change follow below. In most of the cases, changes in bird numbers and distributions were first detected by citizen-based bird-monitoring schemes:

Evidence suggests that range changes by birds may be among the first observable symptoms of human-induced climate change. For example, in Costa Rica lowland forest birds are extending their ranges up mountain slopes, apparently because the high-altitude cloud-forests are drying out as a result of global warming.
The continuing dramatic decline in numbers of farmland birds in western Europe is indicative of the continuing intensification of agricultural practices and the non-sustainability of the European Union's current Common Agricultural Policy.
Numbers of a small migratory songbird, Common Whitethroat Sylvia communis, fell very sharply across Europe in 1969. The cause was traced to the onset in 1968 of a very severe drought in their wintering grounds in sub-Saharan Africa, which also caused the deaths of up to 250,000 people by the time that the drought broke in 1973. Discoveries such as this were among the first to publicise links between issues such as desertification, climate change and biodiversity loss, well beyond the affected regions.
In the 1950s and 1960s, a huge drop in the numbers of Peregrine Falcons Falco peregrinus and other birds of prey in Europe and the USA led to the realisation that the persistent insecticide DDT was accumulating in the food chain, and that significant amounts were increasingly being found in people. As a result, in the 1970s some countries restricted or banned the use of DDT, and consequently experienced a rebound in raptor numbers and a strong decrease in human contamination.