Why is a multispecies indicator better than one based on a few species or even a single species? How many species should be included in an indicator? Should we aim for as many species as possible, or for the opposite?
The more species contribute to the indicator, the more reliable that indicator is. Many species may show interannual changes in abundance that reflect a variety of environmental factors – for example, extreme weather conditions during the breeding season, poor conditions on the winter grounds, changes in predation pressure, etc. Consequently, indicators based on one or a few species are prone to be unreliable. Indicators based on extremely rare, threatened species, will probably respond to variables specific to that species or to conservation actions targeted at them. To reduce such variability, we use a more representative ´basket´ of as many common species as possible that breed or feed in the focal habitat. Genuine directional changes in the abundance of a whole suite of birds become more apparent and a balanced picture of what is happening in the environment arises.