Disease drives decline of another familiar garden bird

Disease drives decline of another familiar garden bird - British Trust for Ornithology (BTO)

A parasitic disease is responsible for the severe declines seen in both UK Greenfinch and Chaffinch populations, say scientists in a new open-access paper in Scientific Reports, a Nature Publishing journal.
The research was carried out by the British Trust for Ornithology (BTO) and the Zoological Society of London (ZSL)’s Institute of Zoology (IoZ).

Chaffinch numbers dropped by 29% in the UK between 2008 and 2018, while we lost 67% of our Greenfinches over the same period. As a result, Greenfinch was moved on to the UK Red List in the most recent Birds of Conservation Concern assessment, published last year.

This new study uses large-scale citizen science data and reveals that both species have declined in ways consistent with the impact of trichomonosis, a disease that was first detected in UK finches in 2005. Earlier work by the same organisations demonstrated that the disease had driven the decline in Greenfinch populations – but the cause of the Chaffinch’s decline, which began several years later, was unknown before now.

Scientists looked at patterns of change in Greenfinch and Chaffinch populations since the emergence of trichomonosis. They found that the declines were driven by a reduction in the survival of adult birds, a pattern that corresponds with the high levels of disease observed. A shift in recent years has seen trichomonosis diagnosed most often in Chaffinches - previously it was diagnosed most often in Greenfinches. This led the researchers to conclude that it was probably behind that species’ decline, too.

The joint BTO/IoZ study showed that survival rates of Greenfinches and Chaffinches were lowest in human-associated habitats. Disease transmission may be higher in these environments, where birds congregate at garden feeders. Trichomonosis makes swallowing difficult and can cause birds to regurgitate food, contaminating shared sources of food and water with their saliva.

Read the full open-access paper here.