Mottled egg yolks (yolks that are not uniformly in color but seems to have a "mottled" appearance, an uneven distrubution of the pigment) can be the cause of an increase in early mortality, as the vitelline membrane of these eggs is not in optimal condition. This drop in hatchability due to an increase in early mortality or appearant infertility can be quite severe.
The reason for this mottling is not completely clear, but disturbance of the breeder flock by stressors as blood sampling, over mating etc is believed to be a factor of influence. When eggs are stored under improper conditions or too long, the level of mottling will increase.
Medication of the flock against worms or traces of Nicarbazin in the feed can induce mottling, and is probably the most frequent cause.
Nutritional factors as cottonseed meal and raw soya beans are known to increase mottling. Also unsufficient levels of calcium, too high levels of tannin and the presence of certain weeds like Sheperd's weed (often present in corn) can cause mottling.
Too high temperature in the laying nest or not frequent enough collecting of the eggs during warm periods might also be a factor that can result in an increase in mottling and an increase in early deads, resulting in a drop in hatchability.
Viral infections are suspected to play a role as well, but until sofar there has not been scientific evidence for this.