any sort of loud bang would do...
I once saw a farmer with a setup that basically involved a bottle of propane, a 12v car battery, and a spark gap accross the pipe coming from the bottle of propane...
I get the impression you are looking for a bird scarer in the style of the inaudible cat/dog scarers, but i'm not sure you can, because most birds have hearing that is within the range of humans, so you would hear the noise as well (I think, but i'm sure someone will correct me)