New Bird Atlas reveals Willow Warbler declines12 Nov 2013
Bird Atlas 2007–11 has revealed widespread declines in the proportion of tetrads occupied by Willow Warblers since the 1988–91 Breeding Atlas across much of England, contrasting with increases in most of Scotland and Ireland. This striking pattern, shown by the abundance change map, is shared with several other long-distance migrants, including Swallow, House Martin and Cuckoo. Although there has been a loss at the tetrad level, the Willow Warbler still has a very wide breeding distribution, with gaps being restricted to a few 10-km squares around the Fens in eastern England and to the Outer Hebrides and Northern Isles. There has been an increase in range of just 3% in Britain & Ireland since the 1968–72 Breeding Atlas.
Information from the Breeding Bird Survey shows a 28% decline in numbers in England during 1995–2010, contrasting with a 33% increase in Scotland, whilst in the Republic of Ireland there has been a 64% increase in numbers during 1998–2010 on Countryside Bird Survey plots. Ringing data has revealed that between the late 1980s and early 1990s, there was a decline in adult survival in southern Britain, while in northern Britain there was no evidence that survival had changed, suggesting that increased mortality had probably been a major cause of the observed population decline. Reasons for these changes are unclear but could include changing climatic conditions in Britain & Ireland, the quality of breeding habitat, and environmental changes in Africa.