Doesn't a parent usually consume an egg or dispose of it, if it doesn't hatch, so as not to attract predators? 
Not necessarily - sometimes they just leave the unhatched egg or eggs in the nest. Sometimes such eggs are collected when chicks are banded, and analyzed to see why they did not hatch.
Sometimes the eggs end up being damaged or disintegrating after a period of time, especially if they are infertile eggs. If an egg has been broken during incubation, a parent will quite often consume it.
