Grey wagtails depend on fast-flowing, invertebrate-rich streams and rivers, making them sensitive indicators of water quality deterioration from agricultural runoff and sewage.
Grey wagtails experiencing prey collapse from pollution events face acute food shortages, particularly during the breeding season when energy demands are highest. Pairs that lose territory following pollution may fail to find suitable alternatives and miss breeding entirely. Their sensitivity to water quality makes them welfare indicators for the millions of aquatic invertebrates that are also affected by pollution before their effects become visible at the bird level.