Yesterday, Baz said there had been an feature on BBC Breakfast about the drop in greenfinch numbers but he'd had to leave for work before the detail came on. An article in yesterday's Telegraph gives the cause as trichomonosis, a disease which normally affects pigeons but has been affecting greenfinches and chaffinches for five or so years now. The British Trust for Ornithology, who carried out the research with help from the general public, are very concerned. On their web site they recommend, regular sterilization of feeders and, if sign of disease are noted, to stop feeding birds for a period of around 2 weeks then re-introduce food gradually, keeping an eye on the situation.
I have not noticed any sick birds but there's a clearly been an absence of greenfinches. The population is declining by 1 in 3.