A sturdy, white-bibbed bird about the size of a thrush, the dipper provides scientists with an accurate indication of the general health of the rivers and streams of Eurasia and North America.
Dippers patrol the beds of shallow, fast-flowing streams in search of insect larvae, fish fry, freshwater shrimps and molluscs such as water snails. They walk along the bottom, sometimes completely submerged, and manage to stay under water by holding their sings at an angle and clinging to pebbles with their toes. The nature of their diet makes dippers very sensitive to pollutants in the water.
As chimneys on factories and power stations grow ever higher, sulphur dioxide, nitrogen oxides and other emissions are wafted across greater areas and remain in the atmosphere for longer. They return to earth as part of acid rain, polluting trees and hillsides, and are eventually washed into waterways. Here, algae are the first to be affected, then the creatures that feed on them. If the water is polluted, it will eventually kill all the resident animal life, including the large fish. The dipper population, an integral part of this food chain, decreases, then disappears.
Scotland receives high levels of acid rain, and its underlying rocks, hard and lime free, do not buffer lakes and rivers against the acidity. A sturdy conducted there found that pairs of dippers breeding along polluted streams produced fewer and lighter eggs, and these later in the breeding season. They fed their chicks less often than usual and, as a result, the chicks had a slimmer chance of surviving.
In normal circumstances, dippers can lay two clutches of eggs a year but these parents only managed one. Moreover, it was discovered that the dipper’s breeding success has a precise cut-off point - it breeds best if the water is clean and the level of acidity measures 6.5 or above (pure water has a level of 7). But it does not breed successfully if the water is so acidic that the level is below 6.5.