The species reached its global range by following agriculture and shipping, and was for centuries a fixture of every European street. Then, from around the 1980s, monitoring in Britain and parts of continental Europe recorded steep urban declines. London lost a very large share of its sparrows, and the pattern repeated in cities including Hamburg and Prague, while some rural and suburban populations remained comparatively stable.
House sparrows are seed eaters as adults but feed their nestlings on insects, and shortage of invertebrate food in the first days after hatching is one of the better-supported contributors to poor urban breeding success. Loss of nest cavities in modernised buildings and reduced weedy ground for seeding plants are also implicated.
No single cause has been demonstrated. Proposals including air pollution, disease, predation by cats and sparrowhawks, and electromagnetic radiation have all been floated, and the last has never been supported by evidence. The global listing remains Least Concern because the species is enormously abundant elsewhere.