Anything mechanical is likely to fail when it rains and dirt gets washed inside.

The forestry commission regularly deals with rabbits when establishing new plantations. The normal procedure is to install rabbit-proof fencing, then eradicate all rabbits in the enclosure by means of gassing or ferreting before planting the new trees. It's a one-off procedure and as long as the fencing isn't damaged it keeps the area clear for many years until the trees are well established. Nobody gets on the "Save the Rabbits" soapbox when new trees are planted. I wonder if that's because trees are seen as eco-friendly so in that case the damned rabbits rightly deserve their come-uppance, whereas in the case of Silbury it's *only* archaeology, so the fluffy bunnies win.

Rabbits are classed as vermin like rats. It's only the fact that they have long ears, big eyes and fluffy tails, and that Beatrix Potter (et al) wrote cute stories about them, that makes people go all gooey and sentimental. They can give you just as nasty a bite as a rat and they do considerable damage to crops, trees (and archaeology) if not culled. Gassing is quick and painless, as can be testified by my ex-gamekeeper who accidentally gassed himself and only just survived because I was close by to administer the antidote.