Have you discovered a pigeon's nest on your balcony? Tips for a harmonious solution. For example, if cats pose a danger to birds, you should contact the Wild Bird Center.
Keep the balcony clean to prevent the pigeons from getting too used to it and to prevent it getting too full of birdseed or grubs. If you spot a city pigeon nest near you, you can offer the birds an alternative nesting box. If the nest has already been built and there are already eggs in it, you could replace the eggs with plastic eggs.