Gram for gram, the sparrowhawk is one of our most ferocious predators, yet it is, along with the magpie, probably our most hated bird . . .
The species also has one the biggest sexual weight discrepancies. Most rapacious hawks (as opposed to carrion eaters like buzzards and kites) display reverse sexual dimorphism. In other words the female is considerably bigger than the male.
The peregrine is typical, with a British female weighing about a kilogram, while her mate is generally about 700g (in other words he is about 30% smaller which is why he is called a ‘tiercel’ – from the French ‘tier’ or ‘third’). This is even more pronounced in sparrowhawks, however, with a typical female – or spar – being twice the size of her mate (c. 290g versus c. 150g).
So what’s the point in this? Well, it allows the pair to harvest the widest of prey from their territory. In the case of the sparrowhawk the male will catch smaller birds up to the size of a blackbird, while the female takes anything up to a pigeon. This means the pair needs a territory half the size than were they the same dimensions. This has huge savings in time and effort. Not only do they spend far less time defending their patch from competitors, but calorie-burning journeys back to the nest with prey are minimised.
Despite their size, these little hawks are awesome predators. The male – or ‘musket’ (this peppery little killer gave his name to the gun) – is fast and agile enough to snatch tits and warblers from the depths of bushes, while his mate can tackle birds as big and as strong as a magpie. And this is where they provoke the ire of bird lovers. A richly-garnished bird table will attract scores of songbirds and these, in turn, will lure sparrowhawks. The sight of a favourite robin or blue tit being snatched from just outside the window is too much for many people and the fact that the hawks do not always dispatch their prey before beginning to pluck only exacerbates the loathing.
As a result, many people blame these little hawks for recent declines in songbird numbers, but in fact it is the link is the other way around. Sparrowhawks are highly specialised predators. They prey entirely on small birds and generally take only those that are below-par. In his brilliant study of the species, Dr Ian Newton found that a pair needs a territory containing 10,000 small birds. This equation is self-regulating: if the pair were to kill too many passerines, then they (or their young) will starve during the following winter. To prove the point, he found that while the chaffinch is the commonest prey species, its numbers are highest where sparrowhawk densities are heaviest.
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