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Willow Warblers Linger Longer

…summer migrants are staying in Britain longer…

Willow Warblers Linger LongerA new report in BTO News, the newsletter of the British Trust for Ornithology, has the first tentative evidence that summer migrant birds are staying in Britain longer in the autumn than they used to. Many migrants are arriving earlier in the year and both residents and migrants are laying their eggs earlier in response to global warming. Now the Willow Warblers leaving Britain through Dungeness in Kent have been shown to be leaving five or ten days later than they did 30 years ago.One consequence of this change is that the birds may be able to fit in extra breeding attempts and both young and old birds may have longer to prepare for their journey south. The records from the daily counts of Willow Warblers at Dungeness were computerised from the written records for 1960 to 2000. The summary of the records for the autumns 1962-68, compared with 1994-2000 showed exactly the same sort of pattern but displaced by five to ten days later for the more recent period.The results could be explained by a change in the numbers of birds coming through Kent from different parts of Britain ? early birds originate in southern Britain but later birds are from all over, including northern Scotland and Ireland. It is known that the southern British populations suffered a decline some ten years ago whilst the northern ones were untroubled. However, the fact that the shape of the accumulated graphs are very similar makes this unlikely.Since there are a good number of records of the birds having two broods in southern Britain in one year, but not from the north, these results might indicate that the area where they are able to be double brooded will be moving northwards. The BTO has other data sets, which may show whether this is happening. Further work is needed to determine whether these results are part of a general trend.In two months time real-time maps of the returning Willow Warblers will be available through Migration Watch. This ambitious new scheme will log the records of thousands of birdwatchers through the BTO website http://www.bto.org/migwatch Anyone can participate and see the results.For further information contact:
John Marchant 01842 750050 or
E-mail: john.marchant@bto.org during office hours
Chris Mead 01760 756466 or
E-mail: chris.mead@zetnet.co.uk anytime

4th July 2014