If they are very close to hatching, you may be able to hear a cheeping inside, or feel them moving.
I think if the parent hasn't come back by now, if you have an incubator, it wouldn't hurt to incubate them. You could open one and see what stage they are at.
I think once the bird has started sitting on them, they shouldn't be left for more than about 20 mins at a time, but it may be that the hen hadn't started to sit yet.
I think if she was sitting on them when you approached with the mower, you would have heard her squarking when she left the nest |