Friday, December 11th, 2015
Return of the Greenfinch
In recent years you may have noticed that there are far fewer Greenfinch Chloris chloris around, or in fact a distinct lack of them. The population has seen something of a crash both within the Bird Observatory and nationally. Greenfinch numbers ringed at Filey have always been relatively chaotic showing rapid peaks and troughs in relatively quick succession from year to year. This may be attributed to ringing effort or more likely associated with environmental conditions such as breeding success, migration conditions and disease. However around 2011 the number caught showed a slightly different trend to the crash recovery sequence we are used to seeing from year to year. This period saw numbers crash then remain low with no recovery between 2011 and 2014. In 2015 we have noted a slight recovery in local numbers of birds rung within the observatory.
One of the reasons for this recent crash and then subsequent inability to peak is very likely linked to Trichomonosis. Trichomonosis is a protozoan parasite called Trichomonas gallinae, this tends to occur within the oral-nasal cavity and to a lesser extent the digestive and respiratory tracts of birds. Once the host is infected it rapidly increases through simple cell division. This parasite quickly dies upon leaving the host, however whilst it remains alive it is incredibly contagious. It is spread via a range of situations, predominantly through infected adults feeding young, contaminated drinking water and infected foraging sites (i.e. droppings on food). Trichomonosis is principally a disease of young birds, however the severity of the disease depends on the susceptibility of the individual bird, species of bird and on the pathogenic potential of the strain of the parasite. Adult birds that recover from the infection may still carry the parasite, but show a higher resistance to re-infection, without showing any obvious signs of infection. Interestingly, infection and mortality rates are not closely linked. The disease varies from a mild condition to a rapidly fatal one with death in 4–18 days post infection.
In juvenile “susceptible” birds, the early lesions appear as small white to yellowish areas within the mouth, especially the soft palate. The lesions consist of inflammation and ulceration of the mucosal surface. The lesions increase in size and number and extend to the oesophagus, crop and proventriculus. Eventually these lesions may even develop into large, firm necrotic masses that eventually block the oesophagus and trachea resulting in emaciation and asphyxiation. Birds like this often sit fluffed up struggling to breathe, drink or feed. Occasionally, the disease has been known to spread into the liver and other organs.
Greenfinch appear to be more susceptible to this parasite than other species, which is likely linked to feeding and roosting ecology. Interestingly similar species like Goldfinch Carduelis carduelis do not appear to be prone to regular infections. As well as Greenfinch the disease is often recorded in Chaffinch Fringilla coelebs, house sparrow Passer domesticus, great tit Parus major, dunnock Prunella modularis and siskin Carduelis spinus. Interestingly national greenfinch declines were first noted around 2006 and 2007 a period in which numbers within the bird observatory were also very low before the 2011 decline which is likely to be attributed to this. Nationally numbers then increased as we also saw in the bird observatory. There would appear to have been a secondary peak in the prevalence of this disease in Greenfinch since 2011 which may have influenced our results.
As with a lot of the species we ring in the bird observatory most of our Greenfinch are migrants. It is possible that this second crash is as a result of the disease spreading to different populations of greenfinch for example Scotland or Scandinavia. With local birds perhaps building up a resistance to the disease, but other more distant populations only just becoming infected. Interestingly though the population has always been quite chaotic in Filey which is suggestive that this disease (or others like it) are not a new phenomenon as one would expect and have always caused population fluctuations in this species. It has been notable that the population has shown a slight recovery in 2015 compared to the previous four years, although numbers remain far lower than you would expect from this species.
As a gregarious bird which often uses feeding stations in groups, it is important to follow sensible hygiene precautions as a routine measure when feeding garden birds and handling bird feeders and tables. Feeders should be cleaned and disinfected regularly. Suitable disinfectants that can be used include a weak solution of domestic bleach (5% sodium hypochlorite) or other specially-designed commercial products. Always rinse feeders thoroughly and air-dry before re-use to remove residues. Feeders should be Rotated around the garden to prevent the build up of contamination in any one area of ground below the feeders. Empty and air dry any bird baths on a daily basis. You may wish to consider stopping feeding if you have an outbreak of the disease at your feeding station, in an attempt to force the birds to feed elsewhere at a lower density (although in reality they may end up visiting another feeding station and possibly one where no hygiene measures are in place).