Garden bird feeders are contributing to the spread of serious diseases among wild birds, scientists have warned, causing previously rare illnesses to become epidemics in some populations.

Poor garden feeder hygiene, droppings accumulations and stale food are promoting the transmission of illnesses between garden birds as the animals repeatedly congregate in the same location, coming into contact with species they would not usually interact with in the wild.

A study by the Zoological Society of London, the British Trust for Ornithology (BTO), and Fera Science analysed more than 25 years of wild bird health data, including the RSPB’s Big Garden Birdwatch, and found dramatic changes in some British bird populations, which scientists believe could have been caused by disease spread at bird feeding sites.

“We’re calling on everyone who feeds wild birds to be aware of their responsibilities for preventing disease. Simple steps we’d recommend include offering a variety of food from accredited sources; feeding in moderation, so that feeders are typically emptied every one to two days; the regular cleaning of bird feeders; and rotation of feeding sites to avoid accumulation of waste food or bird droppings,” said Kate Risely, from BTO.

The study analysed data on the protozoan parasite responsible for finch trichomonosis, which has caused a 35% drop in the population of breeding greenfinches in the British Isles, falling from 4.3m to 2.8m birds since the disease emerged in 2005.

Gardeners can combat the disease by regularly disinfecting feeders and feeding sites, and rotating the position of feeders in the garden.

Paridae pox and passerine salmonellosis were also analysed by scientists.

The study’s lead author, Becki Lawson from ZSL’s Institute of Zoology, said: “Our study shows how three of the most common diseases that affect British garden birds have changed both dramatically and unpredictably over the past decade, both in terms of the species they affect and their patterns of occurrence.

“Both finch trichomonosis and Paridae pox have emerged recently, causing disease epidemics affecting large numbers of birds, while passerine salmonellosis – previously a common condition – appears to have reduced to a very low level. These conditions have different means of transmission – so deepening our understanding of disease dynamics will help us develop best practice advice to ensure that feeding garden birds also helps to safeguard their health.”