I haver heard you can tell if an egg is fertile by tracing the air sack and then the next day candling the egg and seeing if the air sack got bigger. Is this true?
I think that works and I know weighing the egg at setting and every week works too. A developing egg will lose about 12-14% of it's starting weight by hatch day. I did weights on some dark eggs and it worked great!
You probably won't notice a difference from Day 1 to Day 2 by the air sac. Through the process you can see the air sac changing, yes, but if you're looking to tell earlier than other methods, like looking for veining, you'd probably be out of luck. In a nice light colored egg, you eventually can be pretty sure by Day 3 through veins on a fertile egg.
