On a recent trip to rural Shropshire I was shown the traditional method of deterring crows from crops. A dead crow is staked to the ground.
At home I have some bird feeders on a pole with both seeds and fat balls which are visited by a fairly large number of small birds, especially when they are feeding young. The downside is that these small birds tend to be messy feeders and drop food to the ground. This attracts Pigeons which wait on my, and neighbours, roofs waiting for the food to be dropped. One or two pigeons is not too much of a problem but over the past year the flock is now 30 which deposit shit on the roofs which not only discolour the tiles but also fertilise moss etc. I have tried various methods to try and discourage them including a plastic owl which worked for all of an hour before they were feeding within a couple of feet away.
A couple of days ago one of the pigeons was killed by a cat or fox and left on the lawn. On the first day afterwards no feeding pigeons were seen and the number waiting on the roof was down to around half a dozen. I covered the corpse overnight to prevent its removal by the prowling wild-life and today the uncovered dead pigeon seems to have prevented any pigeons returning to my roof or garden. The smaller birds are feeding as normal.
It seems that traditional methods not only work with crows!