This morning, I started to burn some of the dead wood that has been lying around for ages down in the donkeys' paddock.
An hour later, I was surprised to find the donkeys standing out in the sun alongside the smoldering wood, with smoke often wafting over them. I think I know what's happening. The smoke from the dry walnut wood is not particularly acrid: neither for me nor, I suspect, for the donkeys. But it seems to keep flies and other insects away from the donkeys. The proof: they're not even wagging their tails, as they normally do, constantly, to brush away flies and insects. OK, it's surely not an ideal solution, but the donkeys appear to find it efficient, at least for a while.