Large declines of urban and suburban house sparrow (Passer domesticus) populations have been recorded in many towns and cities across Europe. In London, sparrow numbers fell by 60% between 1994 and 2004. The cause(s) of the decline is still not resolved.
Prominent theories include:
• Reduction in the availability of favoured food, either for adults or chicks or both.
• Increased levels of pollution.
• Loss of suitable nesting sites.
• Increased prevalence of disease.
• Increased levels of predation.
with other suggestions such as:
• Mobile phone mast radiation.
• Fibreglass loft insulation.
To date, no scientific research projects have proven any of the theories - either alone or in combination with others.
Since 2013 there are reports that the rate of decline has stabilised - recent anecdotal evidence suggests that in some areas (of the UK) the numbers may be increasing again.
More details here at the British Trust for Ornithology
Note: In 2000, UK newspaper The Independent offered a prize of £5,000 for a proper scientific explanation of the house sparrow's widespread disappearance - the prize remains unclaimed.
Also see: Cuckoo decline