That's a hard decision of you think the mother bird might die soon, but if it was me since you do know the egg has a mother looking after it I think I would just leave it alone and let the mom care for it. If she dies then you could take it and try hatching it and raising it yourself. I would be too afraid it might not hatch if I took the egg! I don't know what kind of bird it is by the color though--what size it is--or did you already say?