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Sparrowhawks don't affect prey populations because they only take birds that would otherwise die of starvation.
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They are generalist predators that only prey on the commonest species, so can't drive down the population of any one species in particular.
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Sparrowhawk populations can only survive where there is a thriving population of potential prey species, and would quickly starve if they depressed the numbers of prey available.
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Repeated studies have shown that the presence of Sparrowhawks has no effect on prey populations.
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The numbers of Sparrowhawks in Britain has increased approximately four-fold since the early 1960s. Prior to this the species had undergone its own catastrophic decline, caused by the residues of pesticides introduced just after the Second World War.
No census data are available earlier than this, so we have no idea how many Sparrowhawks there were in Britain, but we do know that they were heavily persecuted, partly because they became noticeably more common in wartime when gamekeeping declined.
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 Sparrowhawks were protected by law in the early 1960s in response to their decline. Withdrawal of the offending pesticides enabled recovery to begin, and by the early 1970s they were increasing rapidly in the countryside (green line in the plot). By the mid-1970s rural House Sparrow populations (red line) were undergoing a corresponding decline, but urban sparrow populations remained relatively stable until the early 1980s, after which they declined rapidly (orange line). This also corresponds with the lag in colonisation of urban habitats by Sparrowhawks, which only began in earnest during the 1980s (blue line).
Coincidence perhaps? |
Sparrow populations in the south and east lived for a minimum of several decades without any experience of predation by Sparrowhawks. Under these conditions the most successful individuals would have been those that were less vigilant, and more inclined to take risks to secure resources, such as feeding out in the open away from cover. When Sparrowhawks finally returned, therefore, they found sparrow populations that had lost their predator averse behaviour, and were therefore all too easy to pick off. This explains why the steepest Sparrow declines have occurred in arable areas, because it is these areas in particular from which Sparrowhawks were extirpated by heavy use of pesticides as seed dressings. The sudden and rapid recovery of Sparrowhawk populations in these areas meant that Sparrow populations had no time to re-adjust to their presence before suffering significant declines.
This applies even more to urban Sparrow populations. Before the post-war crash in Sparrowhawk populations, cities were heavily polluted and may have been unsuitable for the species, which in any case would have been wary of human contact because of trapping and shooting in the countryside. Urban House Sparrows had probably been free of Sparrowhawk predation for hundreds of years rather than just decades. When Sparrowhawks started to colonise urban areas in the 1980s, they discovered a depauperate prey fauna (there are relatively few kinds of birds in cities compared to the countryside) but one that included large numbers of an ideally proportioned species with little or no predator averse behaviour.