We give examples of how waterbirds can be valuable indicators of changes in aquatic systems. Eutrophication is a widespread problem that can increase the food supply for some birds and hence increase their population size (e.g. great-crested grebes). Many birds are sensitive to changes in water depth, and some can decline due to siltation of shallow lakes (e.g. red-knobbed coot) or when low rainfall leads temporary wetlands to dry out (e.g. greater flamingos). The species-richness of the waterbird community is a less reliable indicator than trends for a given species, and has a weak relationship with the richness of other aquatic communities. As well as indicating changes in conditions, waterbirds can also cause them when they reach high densities, e.g. when colonial birds import nutrients from elsewhere or when geese cause overgrazing. When using waterbirds as indicators, a thorough knowledge of their ecology and clear objectives for the monitoring programme are essential.

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