Yup, hittade det nu och han/hon skriver exakt som jag trodde...
Alltså såhär:
"Cracking an egg to look at the yolk for fertility is really to see if the rooster and hen were successful at mating, so you can know if you are incubating eggs that are hatchable. Eat that egg, then with the next one laid, collect for incubating. From one successful mating, a hen will lay fertile eggs for about two weeks, sometimes more, even if the rooster never mates her again."