I can't explain the increase, but one factor may be significant.
My decoy bird came from a friend who lives 80 miles away, whereas in previous years I've employed locally caught birds.
Do you think this strange bird on the patch could have made any difference?
Our carrion crow population is divided into territory-holding pairs and non-breeders.
Territorial birds are dominant, with the males in turn dominant over the females.
Studies have shown the boundaries of a pair?s territory are most vigorously defended against unknown intruders, while familiar individuals from the adjoining territory are more likely to be tolerated.
This has the advantage of mutual defence against non-territory holders on the borders.
Thus it follows that a decoy bird brought in from some distance away is more likely to excite the residents than a local bird.
It would be interesting to hear from other readers whether they have noticed anything similar.
Given a choice of decoy bird I would always opt for the bird caught as far away as possible from the area in which I was trapping.