Kentbloke – Your Catford anecdote is germane to the discussion. Sparrowhawk populations were indeed at a historical high in the post-war period because of the decline of ‘keepering during the war itself. There were breeding attempts on Hampstead Heath and Holland Park, but they didn’t get as far as establishing themselves in the central London parks as happened in the early 1990s. You also ask why other species are not declining – but they are! Not Blue Tit, but Starling yes, along with a whole range of other species. Also, Bkrownd does have a point: House Sparrows are still one of the commonest species in Britain. They haven’t been ‘wiped out’, simply reduced to a level of abundance more in line with that of many other species.
Dantheman – Yes, I agree that taking ‘all’ factors into account would be impractical, but the more important point is that many potentially influential variables simply cannot be quantified. The argument that a conclusion is invalid because it fails to account for unmeasurable variable ‘x’ is tantamount to saying that a problem cannot be addressed scientifically.
Himalaya – It is true that there have always been cats in urban areas, and Kestrels have been urban breeders in Britain since at least the 1960s, but the threat from Sparrowhawks is of a different order, since they are specialist bird predators. Kestrels will take Sparrows if they can, but they are much less adept than Sparrowhawks because they are basically mousers.
Jos Stratford – You would be right to be sceptical if all we had done was demonstrate a broad correlation between prey and predator, but we did much more than this. One one level our paper is an in depth interrogation of the very question your raise – Is the negative correlation between Sparrows and Sparrowhawks a spurious correlation? We demonstrate differences in Sparrow population trends in urban and rural habitats and in different regions of Britain, and show that these differences can be explained by parallel variation in Sparrowhawk population trends. We also demonstrate that variation in the timing of House Sparrow decline among 200+ Garden Bird Feeding Survey sites can be explained by variation in the timing of Sparrowhawk appearence in the same sites. We also show that between year variation in Sparrow abundance across the whole sample is more negative when Sparrowhawk is present than when it is absent, and that the difference produces precisely the observed trajectory of decline.
Also, to be fair, the full quote from the abstract is “
We argue that urban House Sparrows populations’ long-term release from predator pressure made them especially vulnerable when urban habitats were colonized by Eurasian Sparrowhawks.’ We don’t claim we have evidence for this, but put it forward as a hypothesis in the paper’s discussion. Importantly, it’s a hypothesis that could easily be tested by observational and experimental studies on Sparrow populations in cities with and without Sparrowhawks/Sparrow declines, but don’t hold your breath for the professionals to sanction funding for such a study. In the meantime, we are stuck with anecdote: For instance, when was the last time anyone saw
this in London?
With respect to the ‘other variables’ issue, the quality of evidence we have presented in the paper far exceeds that produced for the significance of any of the other putative causes. Perhaps we can discuss one or two of these alternative ideas? Which ones do you favour?
