That is just the way nature intended it to be. The barn owl population crashed around here a few years ago and it was fairly noticeable that that the vole population was very poor at the same time. Last year the vole population exploded here and it was one of the most successful breeding seasons for the barns for years. Looking at that is it any wonder that we now have so many buzzards about these days, to them we just keep filling the bird feeders only not with peanuts but pheasants.