BIRDS ON THE EDGE has highlighted the threats to our finches and other songbirds from diseases such as the parasitic trichomonosis and avian pox being spread among the birds. The larger population consequences of these parasitic diseases, however, are rarely documented. A recent study in the journal Ibis has looked at the extent to which the trichomonosis epidemic affected the Finnish population sizes of European greenfinch, chaffinch and a control species, great tit, and the body condition of greenfinches.

The disease was first documented in Finland in 2008 and epidemics were observed mainly in south-western Finland. Greenfinches showed a significant decline of 47% in breeding numbers and 65% in wintering numbers in southern Finland during 2006 – 2010. Breeding chaffinch numbers showed a slight decline (4%) during the same study period that was significant only in central Finland. Great tit did not show a significant change in breeding numbers. During the initial disease epidemic the body condition of all demographic groups of greenfinches decreased equally, suggesting that the disease was not selective in respect of age or sex. There were no encounters of Finnish ringed greenfinches or chaffinches in the UK, which could indicate that the parasite has not necessarily been transferred directly from the UK, but perhaps by migrants from Sweden and Germany.

In conclusion, the Finnish greenfinch population has faced rapid and severe population decline due to an epidemic of finch trichomonosis, but the effect on chaffinches has been minor. The decline of greenfinches has been greater than in neighbouring Sweden and of the same order as in UK, where the epidemic was first observed. The study further emphasizes the importance of annual monitoring schemes now and especially in the future in order to detect rapid population changes in common bird species that may be caused by disease epidemics. Bird counts in Jersey have repeatedly shown a very rapid and dramatic decline in numbers of greenfinches in recent years.