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

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).