Impacts of disease become evident

Many garden feeders have been busy with young goldfinches and greenfinches over recent weeks, suggesting that these small birds might have enjoyed a good breeding season. This news has, however, been tempered by reports of fluffed up and lethargic looking individuals, indicating the presence of disease within the population. The timing of these disease reports is suggestive of finch trichomonosis, the disease that first emerged in 2006 in the West Midlands before turning up in Norfolk the following year. Figures from the BTO Garden BirdWatch (www.bto.org/gbw) show that the impacts of those initial outbreaks are still being felt, at least within greenfinches.

We know about the disease thanks to the work of the Garden Bird Health initiative, a collaborative project involving a number of different organisations, supported by bird food companies, government agencies and private individuals. By pairing BTO Garden BirdWatch volunteers with wildlife veterinarians, it was possible to set up a systematic network to record the occurrence of disease at different sites across the country. Birds found dead could then be examined post-mortem to determine the cause of death and to identify the role, if any, of a disease agent. The project revealed the impact of the trichomonosis outbreak on finches, notably greenfinch and chaffinch, identified the likely origin as spill over from woodpigeons (which carry the Trichomonas parasite) and established the likely route by which it then spread to Scandinavia and continental Europe.

The full, longer-term impacts of the disease in finches have just been published in a paper for which I was an author. Working with colleagues at the Institute of Zoology, the RSPB and elsewhere, we have revealed the extent of losses sustained by the British greenfinch population. As a result of the disease we have lost in excess of 1.5 million greenfinches, a quarter of our breeding population and something that has seen the population decline to levels that were more typical of the 1980s. What is not clear is what will happen next. As I have already mentioned, Trichomonas is present within our woodpigeon population, something that has not stopped it from increasing, so a species can live with the parasite. Perhaps, since this is a new disease in the greenfinch, it will reduce the population through this initial stage but, longer term, it will rumble on at a much lower level.

Having systematic monitoring in place, both of greenfinch populations and disease occurrence, is obviously important. The presence of a network like the Garden Bird Health initiative (which has now ceased for lack of funding) would provide an early warning system for disease in wild birds. With new and emerging diseases a real possibility, let’s hope more funding is secured soon.