Chickens fertilize eggs through a brief mating act where the rooster’s sperm enters the hen’s reproductive tract and later meets the egg before the shell forms.

How Do Chickens Fertilize Eggs?

Simple overview

  • A hen can lay eggs with or without a rooster, but only eggs that receive sperm are fertile.
  • Fertilization happens inside the hen, early in the egg’s formation, before the hard shell is added.

Step‑by‑step: From mating to fertile egg

1. Mating and the “cloacal kiss”

  • Roosters and hens both have a common opening called the cloaca; when they mate, the rooster mounts the hen and they press their cloacas together in what’s often called a “cloacal kiss.”
  • In that moment, the rooster releases sperm, which enters the hen’s cloaca and moves up into her reproductive tract (oviduct). This whole act only takes a few seconds.

2. Sperm storage inside the hen

  • The hen has tiny structures called sperm storage glands in the upper part of her oviduct, where sperm can be stored alive for about 1–2 weeks (sometimes up to 3–4 weeks).
  • Because of this storage system, a single successful mating can make a hen’s eggs fertile for many days afterward, often a dozen or more eggs in a row.

3. When and where fertilization actually happens

  • The hen releases a yolk from her ovary; this yolk is technically the “egg cell” before any white or shell is added.
  • Sperm waiting in the oviduct swim up to the top section called the infundibulum and meet the yolk there; this is where fertilization occurs, usually within minutes of ovulation.
  • After that union, the developing embryo travels down the oviduct while layers of egg white, membranes, and finally the shell are added, a process that takes around 24–26 hours before the egg is laid.

Do all eggs we eat have chicks inside?

  • Most store‑bought eggs come from large commercial flocks of hens that never see a rooster, so those eggs are not fertilized and can never develop into chicks.
  • Even a fertilized egg will not start developing into a chick unless it’s kept warm enough (by a broody hen or an incubator) for about 21 days.

A few extra neat facts

  • Roosters can mate many times a day, giving several chances for sperm to reach the hen’s storage glands.
  • Hens have some ability to eject or “reject” sperm from males they don’t prefer, which influences which rooster sires the chicks.

TL;DR: A rooster doesn’t fertilize the egg after it’s laid. His sperm enters the hen during a quick “cloacal kiss,” is stored in her reproductive tract, then meets the yolk inside her body before the shell forms, turning that future egg into a potential chick.

Information gathered from public forums or data available on the internet and portrayed here.