Use a fence charger (electric fencing unit). Safer and more effective than house current.
I’m both an EE and master electrician - DO NOT try to use 240V!!!! Too dangerous......
The problem, as others have said, is not amperage, it is voltage......as others said, get an electric fence power source - or the Bird-b-gone.....they are designed to safely do the trick.
But your wires must be close enough that the birds feet touch both at the same time.
As their foot hide is probably tough, it is likely more an insulator than a conductor, but it should work.
One can run 120v (DO NOT DO THIS) between thumb and finger ONLY if both are very dry, and all you will feel is a slight tickle. This is likely what is happening with the birds.
Wet hands, go from hand to hand so amperage goes through your heart - and the same voltage will kill you quickly.......and obviously - don’t do this either.
A high impedance fence charge will work because its output is 10,000 to 20,000 volts DC with very small amperage. It will require the high voltage to get through the bird’s tough skin on its foot. Don’t need high amps unless you want to fry them. The high voltage will knock their socks off.
Hot and Ground wires will need to be not too close or it’ll spark. I’d guess 3/4” separation.
Trying this with 110 or 240 vac is damned dangerous and probably not effective on the birds.
There is your answer. I used a bug zapper, tied a wire onto each grate and waited for the grackles. Once or twice is all they need. Also works on squirrels.
PS, the rest of the setup should work, as long as the wires are far enough apart to avoid arching.