I have a seven year old koi pond and fortunately have never had issues with birds. This is probably because the pond is under full canopy cover. This also prevents it from overheating in the southern summers.
There are a couple of things you can do to keep birds from the pond. There is some commercial-grade bird netting you can put up, but I believe it ruins the aesthetic of a pond. Some people use owl decoys, but I don't know how effective it is. I've also seen people use motion-activated sprinklers to scare birds away. The depth of the pond also plays a part in keeping away varmints. If the pond is too deep for raccoons or heron to stand, they won't stick around for long.
When my pond was built, we put in structures for the koi to hide in overwinter. Something like large PVC pipe could be used after the fact to give your fish somewhere to hide. Live plants on the surface like lilies, water lettuce, or water hyacinth could also be used to hide the fish from predators. Plants are also great for absorbing nutrients and keeping the water clean.