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Previous Story: How Eggs Form?

Avian cloaca.jpg "Avian cloaca" by Jared Yelton, original uploader was Redtailed at en.wikipedia -  Licensed under Public domain via Wikimedia Commons."But how and when does a chicken egg get fertilized?" Josh asked.

"After sperm leave the rooster's testes," Rosh continued, "they enter his epididymis, where they learn to swim."

"Then they reach his vas deferens, where they are stored until the rooster mates with a hen. During mating, the sperm stored in his deferens get discharged through his cloaca."

"A cloaca is an opening in the backside of certain animal species. It serves as the only opening for their intestinal, reproductive, and urinary tracts, unlike most placental mammals, which possess two or three separate openings for these different functionalities."

"All amphibians, birds, reptiles, and monotremes have this hole, from which they excrete both urine and potty."

"Roosters and hens mate by rubbing their cloacas together, during which the sperm enters the hen's vagina."

"Through the anus?" said Josh incredulously, "intercourse by rubbing their assholes together?"

"Yea," said Rosh, "kind of. It is actually called the cloacal kiss. Birds that mate using this method touch their cloacae together."

"In some species, they only need a few seconds. That is sufficient time for sperm to transfer from the male to the female."

"Really?" Josh was discovering a whole new world, "Do birds have no dicks at all?"

"Some birds have them," answered Rosh, "and they use them too. Males of ostriches, cassowaries, kiwi, geese, and some species of swans and ducks, do not use cloaca for reproduction."

"They have a phallus. In these species, their penis helps ensure that water does not wash away the male's sperm during copulation."

"The rooster's sperm cannot enter the hen's ovaries immediately, because there are always developing eggs blocking the way. The sperm do not break through the eggshell when they are blocked by a developing egg."

"Instead they wait in the vagina, which has small membrane sacs for storing the sperm. They need to travel up the oviduct to the infundibulum to join with the egg yolk."

"The oviduct becomes free after oviposition (laying of the egg). The sperm can now swim on and continue their journey towards the ovaries. They come to the funnel-like end of the oviduct, the infundibulum, which has membrane sacs similar to the vagina. These glands can store more than half a million sperm."

"Here the sperm stay alive and fertile for a long time: 8-10 days in chickens, less than a week in geese and 3 weeks in turkeys. A hen has maximum fertility for only about 3-4 days after one mating. So male-to-female ratio in a flock must be enough to ensure mating of every hen at least once a week."

"Some poultry farmers choose not to keep roosters at all, because they can be noisy and aggressive. Others prefer to have them, because they do offer significant protection for the flock. Roosters guard against predators and sound the alert if there is any perceived danger."

"But because roosters are not needed to get eggs, some farmers prefer to keep an all-female flock. Urban or suburban homesteaders may not even have a choice due to zoning laws in many countries that forbid roosters."

"Generally male chicks have no commercial value since male birds don't lay eggs. But they do need chicken feed, which is expensive. So baby brothers of egg laying hens are routinely suffocated to death in trash cans, electrocuted, gassed, or ground up alive as soon as they break out of their shells."

"When the yolk matures inside the hen, it leaves the ovary. Within 20 minutes it is captured by the infundibulum, the first part of the oviduct. Hundreds of sperm waiting here make holes in the yolk and enter it."

"While only one sperm is needed to fertilize an egg, the chance of an egg being fertilized by just one sperm reaching and penetrating it is very low. Around 30 sperm must enter the egg near the germinal disc for a 95% chance of fertilization."

"After about 15 minutes, the yolk leaves the infundibulum (fertilized or not) and receives the egg white, shell membranes, and shell over the next several hours from the magnum, isthmus, and uterus sections of oviduct."

"So a rooster sperm has a 15-18 minute narrow window of opportunity to fertilize the yolk. If it's fertilized, an embryo is formed. The rest of the yolk then becomes its primary food source."

"The fertilized egg has a possibility of life, the unfertilized egg has none. When the hen lays a fertilized egg, the embryo inside it has already developed for about 25 hours into approximately 20,000 embryonic cells."

"It is a live, breathing organism. While the embryo is growing, the exchange of oxygen and carbon dioxide gases occurs through the eggshell."

"If a hen sits on this fertilized egg, or it is properly handled and incubated for 21 days under specific temperature and humidity conditions, the embryo inside it will develop fully and a healthy chick will hatch out of it. If it is not incubated properly, it is as good as unfertilized. No chick will hatch from it."

Next Story: How Eggs Hatch?

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