The first problem I see is getting them to land where you want them to.
If you string up a wire do you think they will land on it and do you think any protected birds would land on it?
Also, if you make something powerful enough to kill a bird, it has a pretty good chance of killing a human that touches it by accident.
You might need more then 450v. An electric fence is normally 50K+ volts, but the current is very small. The huge voltage helps ensure it will shock through thick fur and such. I am no expert on this, but the worst way for a human to recieve a electric shock is from one hand to the other. The heart is in the path of the current and has a good chance of being stopped or otherwise harmed. I don't know where the heart in a Starling is, but I can't see how you could get the current to flow from leg to leg, let alone through the heart. If you had a wire with 2 conductors, insulated from eachother, but where the bird would grab both with its feet, I think you'ed get a current path across the toes and I am not sure that would be a clean, humane way to kill it.
Just a few thoughts. Have you seen any devices like this that work?