$chickens--, a case study in physical security
I had thought that our fully enclosed chicken run would mean we would be free from successful fox attacks. This morning, at around 5am, I discovered that was not to be the case.

It strikes me that our situation is almost classical in the security world, except with chickens and foxes rather than machines and h4x0rs. The attacker only needs to find one hole to get through, so the defender needs to find and close all of them to be secure. When an attack is successful, it's important the vector is found and stopped, otherwise it will continue. Sometimes discovering the vector is hard.

As best as we can tell, the fox (after months of unsuccessful attacks) now has an excellent grasp of the physical properties of chicken wire. It discovered one location where the nails holding the wire to the fence were further apart, pushed its way down vertically, deforming the chicken wire. Once inside, it grabbed a chicken, and then appears to have squeezed out by pushing and deforming the chicken wire on flap designed to allow leaves to be washed off the coop's roof. No wonder we have the saying "as clever as a fox", and that foxes have caused such massive destruction to our local wildlife.

Today we're patching up the holes, adding more tensioning wire, and adding more nails and cable ties so there will be no locations that a determined fox can squeeze through. Will that fix the problem? Yes; for a while, until the fox discovers a new and completely unthought-of means of entry.

The correct solution to this would be "defense in depth", but even there we see the classical problem of security vs convenience. We could look the coop every night (as well as the run), but then we have to visit the chickens every day at sunrise to let them out. We could buy a dog that sleeps in the yard, but then we'd have to look after the dog. We could install motion-activated floodlights, but then we'd have spend quite a bit of time and money installing them, and the fox could just get used to the light. We could develop an amazing fox-killing robot warrior, but there's probably a council ruling against it. We could deploy a large moat of steaming acid, but then we'd need to refill the moat after rain and hot weather.

We could always buy our eggs from someone else, but I would hate to admit defeat.