Song Thrush: the Plot Thickens
News from the BTO
19th June 1996
A long term decline in Song Thrush numbers has been well documented by several surveys carried out by the British Trust for Ornithology. This goes back as far as the 1930s and has been a feature of the Constant Effort Site Scheme results for 16 years. Despite a slight increase in adult numbers over the last few years they remain at about half the level of 1980.
In a detailed analysis of the results from this scheme, which involves ringers in catching the birds under standard conditions, Dawn Balmer and Will Peach have confirmed that the problem is not to do with breeding failures. Taking the data for each year they have calculated the percentage of young caught and can find no clear long-term trend. Indeed the results coincide very well with those for the Song Thrush's close cousin, the Blackbird. And Blackbird numbers are not in strong long-term decline.
Cold winters undoubtedly affect the Song Thrush numbers but these results indicate that their failure does not come from lack of breeding success. This is borne out by analysis of Nest Record Cards and seems to exonerate the effects of Magpie predation as being the cause of this popular bird's demise. Both the BTO and the RSPB are working hard to try to unravel the detailed causes which seem likely to be caused by environmental change in the wider countryside.