Sound of a gunshot maybe? That works in movies.

I was going to jokingly suggest a scarecrow

 

I visited Romania last fall, a suburban outdoor restaurant had a feeder perfectly sized for pigeons to stand in front of a box and stick their head into a feeding hole. Once their heads were in place, a small bar would be pop down to trap them by the neck with a reasonably loud snapping noise. An employee would run out and harvest the bird. Pigeon was on the menu.

 

I visited Romania last fall, a suburban outdoor restaurant had a feeder perfectly sized for pigeons to stand in front of a box and stick their head into a feeding hole. Once their heads were in place, a small bar would be pop down to trap them by the neck with a reasonably loud snapping noise. An employee would run out and harvest the bird. Pigeon was on the menu.

Wow quite an idea, and dinner to boot, hard to beat that.

Did you ever see the video of the flipping platform made to literally 'eject' squirrels back into the forest?
The guy made a platform i guess with bait, and the squirrels would come up to eat and once they were on the platform it tilted up fast and actually threw them back into the forest where they came from. It's funny to watch but it actually works to get rid of the squirrels. I think they were eating the bird seed or something so the guy wanted to get rid of them.

 

Two adjacent wires with one grounded and the other fed by an electric fence charger should be effective, if the spacing can be set so that the birds get a serious shock. Certainly it might be too much for some birds, but it does work on crows. They also find the presence of a dead crow a bit intimidating. The same setup also intimidates squirrels.
BUT it requires a fair amount of setup work and it is not very useful if the birds are perching on part of the structure, such as loading dock steel roof truss elements.

 

I did one time design and build a bird-feeder electrifier with a manual remote control for one client. That worked quite well, shocking, but not killing the invasive squirrels. Then he did mention that while it was very effective he was cursed at by the mob of them standing on the ground below the feeder and watching him as he looked out his window. As I recall that system used about 75 volts AC from a step-down transformer. The feeder had a copper deck and the wire was, I think, spaced about an inch above the deck.