Habitat-use influences severe disease-mediated population declines in two of the most common garden bird species in Great Britain

Chaffinch by Sarah Kelman / BTO

Author(s): Hanmer, H.J., Cunningham, A.A., John, S.K., Magregor, S.K., Robinson, R.A., Seilern-Moy, K., Siriwardena, G.M. & Lawson, B.

Published: September 2022  

Journal: Scientific Reports Volume: 12

Article No.: 15055

Digital Identifier No. (DOI): 10.1038/s41598-022-18880-8

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Infectious disease has been linked to population declines across multiple taxa, including birds, and it is important that we understand how anthropogenic factors, such as urbanisation and the provision of supplementary food at garden feeding stations, may influence its occurrence and impact.


The influence of supplementary feeding of wildlife on disease transmission and its consequent impacts on population dynamics are underappreciated. In Great Britain, supplementary feeding is hypothesised to have enabled the spread of the protozoan parasite, Trichomonas gallinae, from columbids to finches, leading to an epidemic of trichomonosis and a rapid population decline of Greenfinch (Chloris chloris). More recently, Chaffinch (Fringilla coelebs), has also declined markedly from the second to fifth commonest bird in Britain. Using citizen science data, we show that both declines were driven primarily by reduced adult survival, with the greatest reductions for Chaffinch occurring in peri-domestic habitats, where supplementary food provision is common. Post-mortem examinations showed a proportional increase in Chaffinch trichomonosis cases, near-contemporaneous with its population decline. Like Greenfinch, Chaffinch often use supplementary food, but are less associated with human habitation. Our results support the hypothesis that supplementary feeding can increase parasite transmission frequency within and between common species. However, the dynamics behind resultant population change can vary markedly, highlighting the need for integrating disease surveillance with demographic monitoring. Other species susceptible to T. gallinae infection may also be at risk. Supplementary feeding guidelines for wildlife should include disease mitigation strategies to ensure that benefits to target species outweigh risks.

Trichomonosis is a disease caused by the single-celled protozoan parasite Trichomonas gallinae. It has been known as a disease of captive birds for some time, and also to affect certain wild birds, including pigeons, doves and some birds of prey. The emergence of 'finch' trichomonosis in populations of garden finches, first detected in 2005, is thought to be the result of a spill-over event from Woodpigeons feeding alongside finches at garden feeding stations. Research into the emergence of finch trichomonosis supports the hypothesis that the disease was the primary driver of a concurrent decline in the UK's Greenfinch population. A more recent but equally large-scale decline is evident in the UK Chaffinch population. This study explores whether the decline in Chaffinch numbers could also be the result of finch trichomonosis.

The researchers examined monitoring data for Chaffinch and Greenfinch abundance and demography at a national scale, viewing this alongside post-mortem examination records, to assess whether the occurrence of trichomonosis in these two familiar species had changed since the disease first emerged. In addition, the study also looked for evidence of differential disease impacts in urbanised versus rural habitats.

Using data from the BTO/JNCC/RSPB Breeding Bird Survey, the researchers were able to explore the pattern of changing abundance since 1994 and how this might differ between ‘peri-domestic’ (domestic gardens around human habitation) and ‘rural’ (other landscapes, not dominated by human habitation) habitats. Post-mortem examination data came from the Garden Bird Health initiative and its successor Garden Wildlife Health, partnership projects led by the Institute of Zoology, BTO, RSPB, and Froglife. Demographic data on changing survival rates came from the BTO Ringing Scheme, and information on productivity from the BTO Nest Record Scheme.

The post-mortem examinations revealed that trichomonosis was the causal agent in nearly half of all cases, and featured for 14 species, with the majority of these being Greenfinch (52% of trichomonosis diagnoses) and Chaffinch (29%). The next three most affected species were Goldfinch (7%), Bullfinch (4%) and House Sparrow (2%). The proportion of passerines diagnosed with trichomonosis accounted for by Greenfinch fell over time, with a switch from Greenfinch to Chaffinch as the modal species occurring around 2014/15 to 2015/16.

Examination of the demographic data indicated that the large national population declines (in the order of 5 million individuals overall) of Greenfinch and Chaffinch were largely driven by lower adult survival, associated with the emergence and epidemic spread of finch trichomonosis. The timing of the declines in the two finch species differed. The population impact on Greenfinch began in 2006, around the initial outbreak of finch trichomonosis, and appears to have continued since then, whereas the impact on Chaffinch populations has been more complex. In both species, adult survival fell to a greater extent in peri-domestic than in rural habitats following the onset of the outbreak, which suggests a link to garden feeding stations.

There are several possible explanations for the observed differential impact between the two species. Greenfinch could plausibly be innately more susceptible to infectious diseases and other stressors. Alternatively, or additionally, species differences in trichomonas infection dynamics, such as differences in exposure rates, time from infection to infectiousness and intra-specific transmission rates, could influence the timing and degree of population impacts. The larger, more aggressive Greenfinch, for example, tends to be dominant over Chaffinch and most other species at bird feeders, while interacting aggressively with conspecifics, something that may increase transmission risk.

Regardless of why there were differences in the timing of population declines, it is noteworthy that abundances of both species have continued to fall. Assuming density-dependent transmission and/or demographic compensation, you would predict a swift stabilisation of population trends with reduced Greenfinch and Chaffinch numbers, as was found for the emergence and spread of the disease mycoplasmosis in North American passerines. However, if other species (such as pigeons) act as parasite reservoirs, then infection may continue to occur, particularly at garden feeding stations where greater numbers and species complements repeatedly gather within a small area. Given the ongoing occurrence of the disease in these species, there is increased potential for spill-over of Trichomonas to other susceptible species using supplementary feeding stations. Indeed, trichomonosis has already been diagnosed in a range of passerines, albeit in relatively small numbers overall.

The results of this study put a spotlight on the use of supplementary feeding, both in gardens and as a conservation management tool. While supplementary feeding has known population benefits for a range of bird species, the possibility of negative and counteracting effects on species conservation and animal welfare need to be considered, along with the promotion and employment of appropriate mitigation measures. Key actions to mitigate these negative effects should be to follow best-practice guidance for disease prevention and control, such as that produced by Garden Wildlife Health (available from www.gardenwildlifehealth.org).



We would like to thank all the volunteer bird ringers, nest records and BBS surveyors who contributed to the BTO datasets used in this research. We also thank the members of the public and participants in the British Trust for Ornithology's Garden BirdWatch (GBW) survey who reported observations of garden bird mortality and veterinarians who conducted some of the wild bird post-mortem examinations included within this study (including Katie Beckman, Lydia Franklinos, Joseph Heaver and Vicky Wilkinson). We thank Dario Massimino for providing habitat specific BBS trends.


The BTO part of this work was funded through the generosity of our members and supporters who contributed to the BTO Chaffinch Appeal, a gift from the Estate of Helga Rettke in memory of her son, Ralph Rettke-Grover and a grant by The Mitchell Trust.

Financial support for the Garden Bird Health initiative (2005-2012) came from the Birdcare Standards Association, British Trust for Ornithology, British Veterinary Association Animal Welfare Foundation, CJ Wildbird Foods, Cranswick Pet Products, UK Department for the Environment Food & Rural Affairs (Defra) and Welsh Government through the Animal & Plant Health Agency’s (APHA) Diseases of Wildlife Scheme (DoWS) Scanning Surveillance Programme (Project ED1600), Gardman Ltd, Institute of Zoology, Royal Society for the Protection of Birds and the Universities Federation for Animal Welfare.

Funding for the Garden Wildlife Health Project since 2013 came from the Banister Charitable Trust; Defra and the Welsh Government and the APHA DoWS; the Esmée Fairbairn Foundation, the Garfield Weston Foundation and the Universities Federation for Animal Welfare. IoZ staff receive financial support from Research England. The BTO/JNCC/RSPB Breeding Bird Survey is a partnership jointly funded by the BTO, RSPB and JNCC (on behalf of Natural England, Natural Resources Wales, NatureScot and the Department of the Environment Northern Ireland), with fieldwork conducted by volunteers. The BTO Ringing and Nest Record Schemes are funded by a partnership of the BTO and the JNCC, with the Ringing Scheme also funded by The National Parks and Wildlife Service (Ireland) and the ringers themselves. 

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