Nesting Pigeon

Hi. We have had a pair of pigeons build a nest in a tree in our garden. There is currently one egg in the nest which an adult but  is sitting on.  The problem is that they have located it in the low branches and we have a cat. I am 100% certain that if our car notices it he will either kill the adult bird sitting on the nest or any chicks that come along. I can’t bear the thought of that happening What is the kindest most ethical course of action please.