Lol on the 'deterring cows'
I'll second or third hanging dead ones up, it's the only thing I've found that works. Here it's jackdaws, they descend 50 at a time. I did make 'pretend' dead jackdaws from black bin liner, but that fooled them for about 10 minutes.
My neighbour's lad traps them, and since I have no decent air rifle, he will bring me the occasional one to hang up. So some mornings I get up and there are two dead jackdaws on my garden bench. Strange the things that count as gifts in the country