During a recent visit to Lincolnshire I was pleased to see the local chaffinches seemed to be doing well. They were feeding in gardens and their characteristic song was ringing out from every hedgerow. (The song is a series of short notes ending in a twisty trill, often described as h-e-b-r-e-w-s-hebrews.) Their presence may seem unremarkable, but these birds, and their greenfinch cousins, have been suffering from a disease which has significantly reduced their numbers.

The disease, called trichomonosis is caused by a tiny internal parasite. It particularly affects the pigeon family and birds of prey that feed upon them. It damages the birds' throat and gullet, making it difficult to swallow food. According to the British Trust for Ornithology (BTO) the strain infecting finches appeared in Britain in 2005. In 2006 in the Midlands and West of England chaffinch breeding success declined by 20% and greenfinch breeding success by 35%. It is estimated that half a million greenfinches died. Since then the epicentre of the disease in this country has moved east.

To make things worse migrating birds carry the disease with them. Whilst everyone is familiar with migrating swallows, cuckoos and warblers, and often aware that many waders, geese and ducks are winter visitors, the migrations of small birds that also have resident populations are not so well known. In this case thousands of chaffinches move to Britain from Scandinavia, the Baltic countries and Denmark, whilst many greenfinches from the same area winter in the Low Countries. It is now feared that the disease, which has appeared in Germany, will become more widespread on the European mainland.

Various bird-watching and recording schemes have helped enormously in tracking and monitoring the spread and impacts of the disease. These include the BTO's Big Garden Birdwatch and Breeding Bird Survey. By coincidence, at the time of the outbreak the BTO had 750 volunteers investigating bird diseases under what they called the Garden Bird Health Initiative. The data that they collected were combined with reports from vets and more casual observations reported to the RSPB, before being passed on to research scientists. This enabled a better understanding of the outbreak to be developed and for sound advice and information to be given to help to mitigate its consequences.

One thing to learn from this is that things are not always as straightforward as they seem. People often say to me, for example, that they are not seeing small birds in their garden 'because there are too many magpies about'. Apart from the fact that 'too many' is a very vague term, largely unrecognised by nature, we would all do well to remember that populations of any animal or bird are subject to sometimes large fluctuations caused by a wide range of factors. These may act alone or, more usually, in concert with each other: simple cause and effect is rarely the answer.