Why Are House Sparrows Declining? The Mystery Explained

How much have house sparrows declined?

Globally, the house sparrow is far from rare: the IUCN classes it as Least Concern, with a population of well over a billion birds. The trouble is concentrated in some of its most familiar haunts.

• Britain: numbers have fallen by roughly two-thirds since the early 1970s, and by around 90% in some regions. The bird almost vanished from central London, and it is red-listed as a species of high conservation concern.
• Europe: a 2021 analysis of bird monitoring across the European Union and the UK estimated that the house sparrow had lost about 247 million birds since 1980, the largest decline of any species, roughly halving its population there. In the Netherlands it has been placed on the national Red List.
• North America: Breeding Bird Survey data show a fall of roughly 80% since 1966 in this introduced population.
• Asia: declines have been reported from many large cities, especially in India.

The decline was steepest in city centers and suburbs, while some rural populations have since stabilized. That pattern is an important clue.

Not enough insects for the chicks

The best-supported explanation for urban declines is a shortage of insects. Adult house sparrows eat mostly seeds, but their chicks need a diet of insects and spiders for their first couple of weeks: caterpillars, aphids, beetles, crickets and similar prey.

Research in suburban Leicester, England, in the early 2000s found that many sparrow chicks starved in the nest and that breeding success was linked to the amount of insect prey, especially aphids, near nest sites. Where insects were scarce, parents raised fewer and lighter young, too few to replace adults that died.

Many changes in towns reduce insect life: paving over front gardens for parking, replacing native shrubs and hedges with tidy lawns, decking or non-native ornamental plants, and widespread use of pesticides. In farmland, the switch to autumn-sown crops, the loss of winter stubble fields and herbicide use have cut both insects and the spilled grain that sparrows once relied on.

Fewer places to nest

House sparrows nest in holes and crevices in buildings: under loose roof tiles, behind fascia boards and in gaps in old walls. Modern houses are built tightly, and older homes are often renovated with sealed roofs and soffits, leaving no gaps. Losing nest sites does not kill adult birds, but it can stop a colony from breeding.

Because sparrows nest in loose colonies and rarely move far, a neighborhood that loses its nest holes may lose its sparrows altogether, and the gap is slow to refill.

Predators, disease and pollution

Other factors may add to the problem in some places:

• Predators: sparrowhawks have recovered in Britain since the 1970s, and domestic cats kill large numbers of small birds. Predators probably matter more where sparrows lack dense shrubs to hide in, but studies have not found them to be the main cause.
• Disease: a study of London sparrow colonies published in 2019 linked declines to avian malaria, a blood parasite spread by mosquitoes, with infected colonies more likely to shrink.
• Pollution: air pollution from traffic may harm sparrows directly or reduce the insects they need. A link to additives in unleaded petrol was proposed in the early 2000s, but remains unproven.
• Mobile phones: often blamed in news reports, but there is no convincing evidence that phone masts caused the decline.

Most scientists now think the decline has several overlapping causes, which differ between countries and even between neighborhoods.

Has this happened before?

Yes. In the late 19th and early 20th centuries, city sparrows fed heavily on spilled grain and undigested oats in horse dung. When cars replaced horses, urban sparrow numbers dropped sharply. Today’s decline is different in scale and timing, though, and it has also hit suburbs and farmland.

In 2000 the London newspaper The Independent offered a prize of 5,000 pounds for the first scientific paper to explain the house sparrow’s disappearance. The prize went unclaimed for years, a sign of how puzzling the problem is.

The house sparrow’s history elsewhere is also one of boom rather than bust. After it was introduced to North America in the 1850s, it spread across the continent in a few decades (see how house sparrows got to America).

How can people help house sparrows?

Conservation groups such as the RSPB and BTO suggest some simple measures that help sparrows find food and nest sites:

• Grow native plants, hedges and shrubs that support insects and give sparrows cover from predators.
• Leave some wild corners, long grass and seed heads in gardens and parks.
• Avoid pesticides that kill the insects chicks depend on.
• Put up nest boxes, including multi-chamber “sparrow terraces,” on the walls of houses, since sparrows like to nest close together.
• Keep gaps in eaves and roofs for nesting birds when renovating, where it is safe and legal to do so.

To raise awareness, World Sparrow Day has been held on March 20 every year since 2010. House sparrows remain common enough to see in many gardens, and they are among the species described in our guide to garden birds in the UK.