I'm no expert on birds, but if it's well developed enough that you could see it by candling the egg, I think there's a pretty good chance that the circulatory system is working by now.

[She takes the egg just as carefully, holding it steady as she holds the stethoscope to the side of the shell. After listening thoughtfully for a moment, she nods.]

It's a little quiet, but there's definitely a regular heartbeat.