A rather special warbler has recently put in an appearance on the Nunnery Lakes reserve in Thetford. I use the term “put in an appearance” rather loosely because the bird in question, a cetti’s warbler, has been rather elusive. Skulking within the thick vegetation, this particular individual has only revealed its presence through its characteristic song – a series of staccato notes and disyllabic alarm calls. These have an explosive quality and are far carrying – very useful when faced with a drab brown bird that prefers to remain low down within reed beds and damp willow carr.
The cetti’s warbler is the only British representative of a group of Asian warblers, known simply as the bush warblers. This particular species is a relatively recent addition to our breeding bird community. At the turn of the Nineteenth Century, cetti’s warbler was restricted, within Europe, to the shores of the Mediterranean. It then began a northwards range expansion that extended the breeding range to the Loire Basin in 1927, the Seine in 1932 and the Netherlands in 1968. The first confirmed breeding record from Britain was in 1973 and the species was first recorded in Norfolk in the same year, when an individual was found dead in the middle of Norwich city centre. This particular individual happened to be carrying a ring, showing that it had come from the newly established Belgian population. The bird is still held at the Castle Museum in Norwich.
A national survey carried out in 1996 put the British population at around 500 singing males, about 10% of which were recorded in Norfolk. Since then numbers have continued to increase and it is thought that the Norfolk population is now in excess of 200 singing males, most of which are located in and around the Broads at sites like Strumpshaw, Surlingham and into the Yare valley. This increase has only been possible because of the run of mild winters that we have been experiencing. Unlike other warblers that feed on insects, cetti’s warbler is a year-round resident and does not migrate to warmer climes in the winter. This means that it is extremely susceptible to periods of bad weather, when it is unable to find sufficient invertebrate food. We know, from past experience, that a particularly cold winter can dramatically reduce the numbers of cetti’s warblers in Britain. As such, the continued presence of this species within Norfolk, at the levels we have seen in recent years, is going to be very much dependent upon the pattern of future weather. Global climate change appears to be producing increasingly mild winters but it will only take one very cold winter to knock the population right back.