In addition, I've just now noticed some dark(er) spots visible from the outside that I don't think were there yesterday. They look almost like bruises and definitely seem to be on the inside of the egg. Still can't see anything more from candling though... Hope the little guy is alive in there!!

We had a mallard duck nesting on our property with 12 eggs. 11 of the eggs hatched and she left the nest, leaving the 1 remaining egg behind. We have candled the egg - about 1/3 is taken up by the air sack - the rest is completely black (can't see into the dark area) and there is no sign of movement. I also did the float test and the egg floated with approx 25% above water. I have read that this means it's probably alive, but how much later might a duckling hatch compared with the rest of its siblings? It's been approx. 24 hours at this point. Could the duckling be dead and still "pass" the float test or is it likely still alive in there?

I'll give it a couple of days regardless but would love some feedback.

Is there anything else I can do to confirm if it's dead or alive?