Rock pigeons

I'm new to bird watching and recently about 3 weeks ago a pigeon nest appeared on my balcony under two oldbikes I have been out a few time n this dose not bother the bird at all have been watching n now have 2 healthy chick's about 2 weeks old but this morning another egg appeared the original mum hasn't been seen since yesterday the egg is from another bird n the chick's looks like they have been kicked out or are they just spooked by a diffrent adult so I put a shoe box out there with they are now hiding under ...My question will the chicks mum come bk to feed them or will they b able to fly before they go hungry or will the new mum feed them too