Abandoned nest

Hello!

I've had a pigeon nesting in my garden for quite some time, maybe around 4/5 weeks.

A couple of days ago I noticed they had left the nest but I think I can see the top of an egg, which I'm assuming is unhatched as I can't see/hear a nestling or anything in there. They were quite undisturbed, I think I only saw them leave the egg on its own once for a short time (my office desk looks out over the wisteria they nested in).

Is it possible that they gave up on the egg as it might not have been viable?

Thanks!

Cheryl