The reason for the formation is that two eggs mature at the same time and are separated from follicles and brought into fallopian tubes, where they are wrapped by protein, shell membrane and eggshell in turn, thus forming double yellow eggs. Sometimes even multiple eggs mature at the same time and are brought into the fallopian tube together to produce multiple yellow eggs. The production of double yellow eggs by poultry is often related to the richness of food and the health of poultry.
Double-shell eggs are eggs that have formed a hard shell in the hen's uterus. Because of sudden fright or physiological abnormality, the fallopian tube moves reversely, which makes the eggs retreat from the uterus to the upper part of the fallopian tube. When the egg descends to the uterus for the second time, a second eggshell is formed, and the consumption of bivalve eggs is not affected.