Monday, 30 January 2012

Kestrel springs a surprise

I was a little taken aback the other morning when a Kestrel flew up from beside the gravel drive that marks the entrance to the British Trust for Ornithology’s (BTO) offices in Thetford. Although I have seen them occasionally from the BTO grounds, it has been many years since they nested here and could be seen hunting and feeding alongside the 12th Century ruins with which we share the site.

This particular bird flew a short distance to land in one of the trees that mark the boundary of the grounds. Favouring a perch that was in part shadow, the Kestrel had something in its talons that it had clearly just taken from the ground. My immediate thought was that the bird had caught one of the Wood Mice or Bank Voles which do well here, but closer inspection through my binoculars revealed that it had, instead, caught a rather sizeable earthworm. Kestrels are opportunistic enough in their habits to take a range of prey species, from small mammals and birds through to reptiles and various invertebrates.

That Kestrels will take worms may surprise some readers; perhaps it is because these soil-dwelling invertebrates seem so inconsequential, hardly worth eating because of their small size and a belly full of decaying plant material. Yet earthworms feature in the diets of many larger birds: Common Buzzard and Tawny Owl being two species that spring to mind. Tawny Owls take earthworms on damp nights, when the worms venture from the safety of their burrows to search for a mate or for fallen leaves to be dragged back underground.

The appearance of earthworms in Kestrel diets has a strong seasonal component, with early spring the period when they are most likely to feature. In part this highlights a scarcity of the favoured small mammal prey at this time of the year but it also reflects an increase in the availability of earthworms, which tend to be more surface active when the ground is waterlogged. As you might expect, Kestrel diet varies with region. Those Kestrels breeding in the upland fringes of northern Britain take a lot of small mammals (mostly voles) with some earthworms and small birds. Those breeding in the warmer climes of southern Europe tend to take more birds, lizards and larger invertebrates. Such differences underline the adaptability of this bird, a hunter that will take whatever prey happens to be locally abundant.

It didn’t take long for the Kestrel to finish off the worm before slipping from its perch in a short glide. It was only the briefest of encounters but it was one that made an impression, one of those moments when you get a real sense of nature at work.

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