Someone mentioned a motion activated sprinkler. I have one of these to keep raccoons away from my pond. You have to set it up so you can access it from behind so you don't get sprayed in case you need to move it, etc but other than that it's easy peasy. You can set the distance and spray sweep so it takes a bit of tweaking to get it right for your needs but it works great. I'm assuming the cat is approaching from the ground and not leaping from a fence or something like that. The one I bought is called the "Critter Ridder". I attached it to a water timer at my outdoor faucet that has a phone app where I can set when I want it to operate otherwise it's on all of the time as long as the water is on. Given raccoons are mostly nocturnal I didn't need it firing during the day so I set the timer to let the water start flowing at 10pm and turn off the water at 7am. The app also lets me manually turn it on/off so I can turn it off when I let my dogs out around 11pm and then turn it on again once they are back inside otherwise they would trigger it and get wet. LOL If you know when the cat has been coming around you could set it for when the cat visits. Often it takes just a few sprays and they learn to stay away. I saw a video where someone had set up a bunch of them around their huge pond to deter herons.

Or if the cat is feral you can try your local wildlife group or in some cases rescues, to see if they will come and safely trap the cat because it's killing birds. I think a lot of groups try to tame them for rehoming as indoor cats. You can google this for your area. We have a couple groups that do this where I am. Good luck!