I also know of this method of sexing. I think I posted it on the Pigeon Bulletin board some time ago. When I go to purchase pigeons,,,,,, If looking for a hen, I first pick out the smaller birds. I then
double-check with the pelvic opening. Has worked for me very well. The person at the place I was purchasing the pigeons thought me this trick.

It was at the Live poultry store. They sold pigeons that fanciers no longer wanted to keep. Often times they were homers that lost races.

That is what fanciers do to loo$er$ in my area. The wager amounts are kept in the club confidentially. It is Gambling in varying scales.