Amongst the hustle of a garden feeding station the flash of olive green would alert you to the presence of a greenfinch, perching on the hanging bird feeder, using its specially designed bill to exploit the available seed. However the abundance of greenfinches has declined in many areas, certainly noticeably in gardens where I live as they have not been present now for some years. The reason though, may have in fact been linked to the garden feeding stations that they so favoured.
Greenfinches were once regularly seen in gardens across much of the UK, and were often the bird chasing others away from their prized seed. Their bright plumage with green and yellow colouring made them a favourite garden bird. However numbers of this species have declined rapidly due to a disease called Trichomonosis. 2005 saw the beginning of greenfinches being affected by this disease, as well as chaffinches.
The disease itself is actually caused by the protozoan parasite Trichomonas gallinae. This disease affects the bird’s throat, making eating and breathing very difficult, meaning they will eventually weaken and die. It is spread through contamination of feeding or drinking stations, as many birds may flock to these areas to take advantage of what they think is a valuable food or water source. Following on from the first recordings in 2005, breeding populations of greenfinches declined by 35% by 2007 in the area where the disease occurrence was highest (Robinson et al, 2010). Chaffinches were also affected, declining by 21%, meaning all in all more than ½ million birds died (Robinson et al, 2010).
The 2015 RSPB Big Garden Birdwatch continued to record an absence of greenfinches, although the better locations for this species included islands such as the Isle of Scilly and the Orkney Islands. The problem of isolation on islands is an issue for some species but it appeared to be an advantage in this case, preventing further spread of the disease to these areas. However overall since 1979 greenfinches have declined by 53% (RSPB, 2015).
It is not just finches that are affected though, as this disease is actually better known for affecting birds such as pigeons and can be referred to as ‘canker’. The disease can now sadly affect a variety of birds, such as goldfinch, siskin and house sparrow, as it appears to be able to change bird hosts. However, as it has been present in species such as doves and pigeons, it is hoped that over time the disease may eventually reduce in intensity in greenfinches. According to the BTO the British greenfinch population is currently around 1.7 million pairs in summer.
There are however ways to prevent this disease from spreading, which revolve around hygiene of garden feeding stations. These stations and bird baths should be disinfected, with a suitable product, washed and dried regularly. Advice is also often given that if you see poorly birds around your feeding stations, feeding should be stopped for a few weeks, although it is unclear how effective this is at reducing contamination problems as they may simply visit other gardens.
Robinson, R., Lawson, B., Toms, M. et al. [14 others] (2010) ‘Emerging infectious disease leads to rapid population declines of common British birds’, PLoS One, 5, (8).
RSPB. (2015) RSPB Big Garden Birdwatch Results: Fewer finches visiting our gardens [www document]. www.rspb.org.uk/news/details.aspx?id=397372