There are people who are able to tell the sex of certain breeds of chicks by checking their vent. It has something to do with out far apart the bones are on either side of the vent. It's quite an art and the people who are good can earn thousands of dollars per week.
Look it up on You Tube and you'll find a clip from Dirty Jobs (TV show with Mike Rowe) demonstrating this being done at the McMurray Hatchery. I don't think it's something that is easily learned...the "experts" at the hatcheries are only about 85-90% accurate.
It is a way to determine gender in day old chicks by looking for a shiny bump in the vent (the all purpose exit on a chicken) or the absence of the bump. Shiny and/ or Bump = male. I have read accurate vent sexing is a very hard skill to master, and that is why hatcheries quote 90% guarantee.
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The space of the bones is an old wives tale. They check the vent to see the sex organ, its and art that most can learn, better have good eyes. Waterfowl are easier