It is also breed specific. If the breed is not listed as auto sexing, it is not going to be much more than 75% accurate.
Point to stress---DO NOT SELL THEM AS SEXED PULLETS unless they are an auto sexing breed or a sex link cross.
We have had posts about this!
Poultry Science?
You are applying sex-linked traits to a breed and not a cross....While it might be fun, it will not work better than a coin toss over time.
Wing, or feather sexing only works for breed crosses. It is a sex-linked thing:
There is a section about it and cross examples that work at the bottom of the first post on this thread:
https://www.backyardchickens.com/threads/sex-linked-information.261208/
It is unusual but California does have some odd climate pockets. We are getting snow in the Sierras this week and some rain in the Valley, which is odd since we had no rain in January to Feb. Which are our wettest months.
It will soon be over 100 F for more than 20 days in June through August...