Richard Collins: Disappearing garden birds caused by an ancient parasite
Our European Greenfinch (Carduelis chloris) population has suffered a dramatic decline, caused by Trichomonas gallinae (the avian equivalent of Black Death)
Wordsworth loved the greenfinch, but is its "joy of voice" really "far above the rest"?
The green linnet’s song is a wheezy assortment of short phrases cobbled together, their timbre giving the impression that the singer is suffering from a chest infection. In spring, however, you will hear a drawn-out wheezing call, unlike that of any other finch, which is most evocative. Alas, it is seldom heard nowadays — greenfinches have become very thin on the ground.
This seed-eater is slightly larger, and stockier, than most other finches. The large bill is conspicuous. Green-plumaged males have a bright yellow stripe along the folded wing. The tail is partly yellow. Females are duller. Oddly for a trusting bird, which seems to gravitate to city gardens and parks, greenfinches are highly-strung. You have to be extra careful when trapping and ringing them; these prima donnas go into cardio-vascular shock easily. It’s not fatal, I hasten to add... they recover quickly when kept in a bird-bag.
In the current edition of , Brian Burke examines recent travails of our six commonest finch species. Analysis of Garden Bird Survey data suggests that all have declined in numbers. Notably fewer chaffinches and goldfinches are to be seen nowadays, and greenfinches seem to have suffered greatly. Brian notes that "by 2019 our breeding population was almost half of what it was 20 years prior".
The culprit appears to be a single-celled organism called Trichomonas gallinae. This avian equivalent of the Black Death causes what pigeon fanciers call ‘canker’ and falconers term ‘frounce’. "In England, from 2005 onwards, large numbers of finches became infected." The disease spread to mainland Europe and to Ireland.
Trichonomas is a very ancient parasite indeed: a paper published in 2009 claimed that theropod dinosaurs, from which birds evolved, carried it. If that is true, this protozoan has been around for at least 66 million years.
The symptoms of infection are nasty. The parasite lodges in the bill and nasal passages. Yellow pus builds up in the mouth. The disease spreads down the throat, causing lesions and swelling. Soon, the unfortunate victim can no longer swallow.
Who or What am I? Answer: greenfinch They were one of the most common birds at garden feeders. However, because of trichomonosis, their population has declined by about 35%. It can be spread at feeding stations so it is very important to regularly clean feeders and bird tables. pic.twitter.com/ABHnJI3kNs
— Shropshire Hills Discovery Centre (@ShropsHillsDC) November 2, 2023
Are bird tables partially to blame for the greenfinch pandemic? Infection is spread through close encounters with infected birds and their poo. The greenfinch is a gregarious species, roaming in mixed flocks of finches and sparrows. It can’t resist handouts of bird-seed. Visiting tables and hanging feeders may bring individuals into close contact with disease carriers. Gardeners should keep bird tables clean.
The human population was almost halved by the plague which reached Europe in 1348. However, some people survived. They, presumably, fostered ‘herd immunity’, ending the pandemic. But, as Gerry Adams might say, ‘it hasn’t gone away, you know.’ The notorious bacterium morphed and epidemics erupted repeatedly down through the ages. It is still around today. although we are much better at treating victims. Hopefully, herd immunity will save our finches, at least temporarily.
