Grey wagtails are indicators of clean fast-flowing rivers and are sensitive to pollution, drought, and invertebrate community decline in upland and lowland streams.
Grey wagtails in rivers affected by agricultural pollution face reduced invertebrate food supplies causing chronic nutritional stress. Flooding during breeding season destroys nests causing egg and chick mortality. Drought conditions reducing river flow create inadequate habitat for the aquatic invertebrates grey wagtails depend on. Pesticide contamination of invertebrate prey accumulates in wagtail tissues causing chronic health effects. Population declines from habitat degradation mean welfare incidents affecting individuals have disproportionate population-level consequences.