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Testing the Sparrowhawk hypothesis

Still not convinced it's more than coincidence?
 
Neither was I, which is why I asked the British Trust for Ornithology to give me the data on House Sparrows and Sparrowhawks from their Garden Bird Feeding Survey. This is a record of the occurrence of birds at garden feeding stations at up to 200 sites around the country at any one time, going back to 1970. If Sparrowhawks are responsible for Sparrow declines, there should be a close temporal and geographical association between the spread of Sparrowhawks and the decline of Sparrows.
 
With some help from collaborators at Cambridge University, I analysed the data to see whether any such pattern might be present. Briefly, this is what we found.
 
 
 
  • Sparrow numbers are stable before Sparrowhawks become established at a Garden Bird Feeding Survey site.
  • They begin to decline immediately after Sparrowhawks become established, and continue to decline at a consistent rate for over 15 years, after which less than 20% of the original numbers remain.
  • Differences in the trajectory of Sparrow population decline in different parts of Britain can be explained by differences in the trend of Sparrowhawk recovery.
Still not convinced?
 
As Feynman used to say, "the test of all knowledge is experiment", and the great thing about being an ecologist is that sometimes you can put your feet up and let the experiment happen by itself.
 
Sparrowhawks nested in central Paris for the first time in 2008, almost 20 years after they first started to breed in central London. Nothing is certain in life, but if you want the full Mary Poppins next time you visit Notre Dame, don't leave it too long.
Subpages (1): 'The Independent' Sparrow Prize

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