Pigeon egg on balcony

Hi,

I live in a flat in Hackney, London and I have been growing some plants on my balcony. Over the past couple of weeks, I noticed we were being visited by quite a few pigeons, and over the weekend, two pigeons were particularly interested in a large container that has a spiky mahonia in it. I thought they might be looking for a spot for a nest, so I put a shiny cd and some foil to put them off. It didn't work! One egg was left there this morning. The pigeons don't seem to have returned.

What should I do? How long should I wait until I can assume the egg has been abandoned? I am going to water the tubs, but should I not water the tub with the egg in it?

Thank you!