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From a reproductive perspective, the last sperm in is the first sperm out. Hens have sperm storage tubules where they can store sperm cells and maintain fertility and embryo livability for up to 3 weeks. Once a hen has been inseminated (mated), the sperm cells are deposited and stored in the storage tubules near the vaginal region. Following passage of an egg down the oviduct the LAST sperm cells deposited will be released and will migrate to the upper regions of the oviduct to await ovulation of the next ovum or yolk and THAT egg will be fertilized. So, when mating occurs, either artificial or an observed natural mating, eggs laid two days later will be fertilized by the new male. So when a male is added to a group of hens on a Tuesday, those hens that were mated that day will begin laying eggs fertilized by the new male on Thursday. This of course, relies on successful mating and transfer of sperm cells to the hen.