Sparrowhawks and songbirds
4 October 2007
Sparrowhawks are an emotive subject, and while it is true that numbers have recovered after decades of persecution, it is a misconception that sparrowhawks control the number of songbirds- the reverse is in fact true.
The sparrowhawk has no serious predators, although its chicks and fledglings are taken by pine martens and goshawks, which themselves are scarce in the UK.
Numbers of sparrowhawks are limited by the ‘carrying capacity’ of the habitat. This means that the amount of food and the number of nesting sites limit the number of sparrowhawks that can survive. Therefore, young birds will not pair and breed until there is a vacant territory that contains adequate food. In years of high adult survival, juvenile survival is correspondingly poor.
'...if songbird numbers increase, sparrowhawk numbers increase, and if songbird numbers go down, so do sparrowhawk numbers.'
These factors control the population so that if songbird numbers increase, sparrowhawk numbers increase, and if songbird numbers go down, so do sparrowhawk numbers. This very close and sensitive link between sparrowhawks and their prey make the hawks a monitor of the health of the ecosystem.
Some people are worried that sparrowhawks eat too many small birds and cause their population to fall or even become extinct. This is unlikely to happen, since the hawks take mainly the most common species, and since they would starve to death before a significant drop in overall prey numbers. Small birds can rear between five and 15 young in a season, while only one or two need to survive to keep the population stable. The vast majority die before the following breeding season: of starvation, illness or injuries.
So sparrowhawks may alter the way in which birds die, and perhaps also the seasonal pattern of mortality, but not the overall number that die.
The Breeding Bird Survey for 2006 showed that the UK sparrowhawk population declined by 1% between 1994-2006, whereas common prey actually increased in numbers: blackbirds by 18%, robins by 18%, great tits by 54%, collared doves by 39% and song thrush by 17%.
Deterring sparrowhawks from your garden does not stop them hunting, but will simply move the action elsewhere. Consequently, using deterrents is entirely for human benefit- it will make no difference whatsoever to the birds. Rather than deter them, try to learn to admire the skill and beauty of this very specialised hunter.
* From Sparrowhawks and songbirds, RSPB Birds, Summer 1997 (Newton, I and Perrins, C)
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