Feather sexing baby chicks
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True feather sexing is reliant on a careful selection of parent stock so that you are crossing a slow feathering female and fast feathering male - it is then a sex linked trait with the male offspring feathering in slowly like the mother, and the females feathering in quickly like their father. The "reading" must take place in the first three days.
Anything other than the above is merely a matter of luck - it has a 50/50 chance of being accurate if used for sexing, just like any of the other common myths/wives tales/etc about sexing.