Maybe a sprinkling of extra strong curry powder where they wait? Not harmful, but a tad off putting.
I hope you don't mind me saying that i'm not so sure that that is a particularly kind method
Cats spend most of their time licking themselves (when they are not asleep.)
i think if the powder got stuck on their feet ,perhaps after dew ,frost or rain, it might cause them some considerable distress when licking their feet.
its not going to be particularly healthy for ground feeding birds or other mamals like squirrels, foxes, hedgehogs,etc.
i know cats can be a real nusience (I have two of my own) but I'm sure that there must be safer and less harmfull methods of protecting our garden birds:t:
We have put down 3x roof slates in our garden where they catch the sun. Not only are they brilliant protection for reptiles, amphibians,mammals and invertebrates, the local cats will spend hours sleeping on them when they warm up in the morning sun. Keeps them away from our feeding areas:t: