BTO: There’s a new killer in our gardens

Sparrowhawk by Edmund Fellowes/BTO
Sparrowhawk by Edmund Fellowes/BTO
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A rattle of alarm calls and a blur of wings sees an explosion of small birds away from bird tables and hanging feeders, writes the BTO’s Mike Toms.

Seconds later and a sparrowhawk swoops through the garden, shifting its course slightly to miss the apple tree and to lift up, over the fence, and away. The sparrowhawk is a relatively new presence in this urban garden, a sure sign that the population has recovered from the impacts of agricultural pesticides, like DDT, that brought about significant population declines across the eastern counties, just a few decades ago.

You probably know the story; DDT accumulated in the bodies of the small birds that fed on treated agriculture seeds and, in turn, in the predators, like sparrowhawk, which fed on the small birds. The levels of DDT were not necessarily high enough to bring about direct mortality of the hawks, but they were sufficient to impact on their breeding ecology. The chemicals led to a reduction in eggshell thickness, making eggs more likely to break during incubation and, ultimately, reducing the size of the population through a reduction in recruitment. Subsequent to the compounds being banned, we have witnessed a recovery in the sparrowhawk population and a recolonisation of former haunts, something very evident in the figures collected by the BTO/RSPB/JNCC Breeding Bird Survey and in the maps presented in Bird Atlas 2007-11.

While this is good news, and a real conservation success story, it is not without its occasional critics. The act of predation, be it a big cat taking down an antelope in Africa or a sparrowhawk taking a blackbird in an urban garden, is difficult to witness. It is nature ‘red in tooth and claw’. Some observers have suggested a link between a recovering sparrowhawk population and the declines seen in many of our smaller bird species, even going so far as to call for a cull of sparrowhawks and other birds of prey. Such calls are made without knowledge of the science that has been done on the interactions between these predators and their prey. A series of scientific studies, some of which have used large-scale monitoring data, such as that provided by the Breeding Bird Survey, have failed to find evidence that a recovering sparrowhawk population has been to blame for the declines in songbird species. Other work suggests that the cause of these declines lies elsewhere, in the changing nature of our countryside and the resources that it contains.

There is a social side to all of this. Many of us are appalled by the sight of a sparrowhawk killing another bird and yet we barely acknowledge a blackbird tugging a worm from the lawn or a song thrush smashing open a snail on our patio. Perhaps our reaction to predation says more about the different values that we place on particular creatures.