That Britain has an important role in the wider pattern of bird movements is particularly evident at this time of the year, as many species – both rare and common – are noted arriving and departing by birdwatchers. As the first of our wintering geese arrive, so the last of our summer-visiting warblers depart. The scale of these movements is sometimes more difficult to gauge, however, particularly when it comes to common and familiar species like Greenfinch, Goldfinch and Chaffinch.
It is easy to assume that these finches, together with our other ‘resident’ species, are here all year; after all, they fill our hedgerows and woods with song in the summer and make use of our garden feeders come winter. Thanks to the efforts of licensed bird ringers we know that many of our wintering Chaffinches and Greenfinches arrive here from elsewhere, and that some of our breeding Goldfinches leave Britain for a winter spent in France and Spain.
The degree to which our finch populations are connected with those elsewhere in Europe was brought home to me recently through a piece of work with which I was involved. For several years now, I have been studying the impacts of an emerging infectious disease in finches. This disease, caused by a protozoan parasite, is known as trichomonosis and was first seen in wild finches in 2005. During 2006 we had our first outbreak, centred on the West Midlands and the Southwest of England, but in 2007 we saw it hit East Anglia. Our work has shown the disease to have reduced the Greenfinch population in affected regions by a third and the Chaffinch population by a fifth. This is the first time that anyone has been able to document a population level impact in a widespread European bird resulting from an infectious disease.
Worryingly, in 2008 a case was diagnosed in Sweden, followed by others the same autumn in Finland and Norway. Working with vets at the Institute of Zoology in London and molecular biologists at UEA, we have found that the trichomonad parasites taken from these birds are identical. This implies that the parasites arrived in Sweden, Finland and Norway via birds that had migrated from Britain. By examining bird movements we have been able to demonstrate that the most likely candidate is the Chaffinch, individuals of which migrate from eastern England to these countries come spring. Wintering Greenfinches, on the other hand, tend to migrate between eastern England and western Norway, away from where recent cases have been seen. With a foothold in these northern European populations we may see the disease emerge elsewhere in Europe, as migratory individuals mix over time. More on the disease can be found at www.bto.org/gbw