Hens ovulate three eggs at a time or mature prematurely. After ovulation, they meet in the fallopian tube and are wrapped by protein, forming "three yellow eggs". The "three yellow eggs" rate of primiparous hens is higher than that of old hens. This is because primiparous hens have strong physique, strong vitality and strong ovulation ability. Premature chicks are also prone to produce "three yellow eggs" because the organs of chicks are not fully developed and mature, and normal ovulation cannot be completely controlled.