Thursday, 29 April 2010

Sparrowhawks display overhead

Our local Sparrowhawks have been much in evidence over the last few days, soaring above the garden with an almost lazy ease. This suggests that these birds are breeding locally, a sure sign of their recovery from the effects of persecution and pesticide residues that saw their loss from much of the east of England just a few decades ago. The return of the Sparrowhawk has not been welcomed by everyone, with letters to the EDP over the years testament to the range of emotions stirred by the return of this native hunter. However, its recovery suggests that we have managed to fix some things in the wider environment and improve the fortunes of previously scarce species.

The presence of this pair of Sparrowhawks above the town unsettles the local doves but the hawks are probably more interested in displaying their ownership of this particular territory than in anything else. You can tell that the two birds are a pair because the female is noticeably larger than her mate, a pattern present in a good number of raptor species and something known as ‘reversed sexual size dimorphism’. It is thought that the size difference stems from the different roles of the two sexes. The male is the hunter and main food provider while the female’s is incubating eggs or brooding young; he needs to be agile enough to catch prey and being small and light allows this. The female has to have sufficient body reserves to cope with the energetics of reproduction so she cannot be as light and, therefore, as agile. The difference in size also allows the hawks to feed on differently-sized prey, the male taking small birds, up to the size of a Blackbird, and the female taking larger prey, up to the size of a Woodpigeon. By preying on slightly different prey the two birds reduce the amount of competition between them. Reversed sexual size dimorphism is more pronounced in raptors that pursue difficult to catch prey (like birds) than it is in those species that take more slowly moving prey (like small mammals) or which scavenge.

Food availability has a tremendous influence on Sparrowhawk breeding success. Males can only successfully attract and retain a female if they can prove that they are a good hunter. The male feeds the female at the future nest site and she will only hang around if she is well-provisioned. If not, she’ll leave to find a better male. Once the birds are paired, then food supply becomes all important for the resulting chicks and breeding success will be determined by how much food the parents can ultimately provide. The presence of Sparrowhawks here suggests a healthy population of their prey.

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