Parasitic disease poses threat to greenfinches

A greenfinch, photographed by Simon Dell.
A greenfinch, photographed by Simon Dell.
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The most troubling development in the world of garden birds in recent memory has been a steep decline in numbers of greenfinches caused by a disease known as Trichomonosis.

Pigeon fanciers call it “canker”, while to falconers the condition is “frounce” but whatever the species the effect is identical.

The parasite Trichomonas gallinae causes a swelling at the back of the throat which leads to a progressive difficulty in swallowing and breathing by inflected birds. You know you are looking at a tricho bird if it is lethargic, gapes frequently and has fluffed-up plumage.

I first noticed the condition in Yorkshire greenfinches round about the year 2000, and by 2006 a full scale UK epidemic was declared by the British Trust for Ornithology. Its most recent annual BirdTrends report, published just before Christmas and so given little publicity, highlights that the country’s greenfinch population has suffered a “rapid and alarming decline” of 59 percent.

Yet many birds survive with no sign of the disease in some areas. Last year, I became aware that I was seeing more greenfinches around my garden above the Aire Valley, and hearing their familiar long drawn-out wheeze at favourite walking haunts in Wharfedale. Then I saw a social media post by Linda Jenkinson, founder of the Leeds Bird Fair who runs birdwatching classes in the city under the name of Start Birding.

She wrote on Facebook: “Greenfinch numbers are really good in my garden at the moment with about a dozen around the feeders each day. It’s taken a few years to get the flock to this size after a bout of Trichomonas gallinae a few years ago.”

So is the greenfinch making a comeback?

“Sadly, not everywhere,” Linda told me.

“Talking to the people who attend my bird classes, what seems to be happening is that it’s recovering in some very localised places around Leeds and presumably elsewhere.

“Where I live in North Leeds I’m picking up the song at the moment, but I know people over in Gledhow and Roundhay who haven’t got any greenfinches in their gardens at all.”

The disease is supposed to have affected chaffinches too, but I have seen no evidence of this. One reason might be that the disease was thought to be spread by garden feeders used by greenfinches (thus the advice to regularly sterilise your feeder), while chaffinches are content to feed on the ground.

According to a popular online birdwatching magazine, the bird’s conservation status of “green” - not in critical danger - may soon have to be radically revised.

“Should the current rate of decline continue, it could be moves straight to the red list in the next update,” it said.

The red list includes birds which are globally threatened or, in the UK, have suffered a severe decline in their breeding populations. It includes at-risk species like hen harrier, skylark and yellowhammer.

So be thankful if you are seeing greenfinches in your garden. And be sure to clean your bird feeder.