Well, I for one have no problem believing the "rural legend".
YMMV, but I'm not on my "first dog" here. I've worked with a lot of dogs over the years, and with a lot of dog trainers, and a lot of field-trial trainers and handlers, and there's more than one or two of us who've known dogs that appeared able to tell cocks from hens by scent. You don't find it in field-trial dogs hardly ever, because they're trained on all manner of species of birds, cocks & hens, and they've learned that any flush is a good flush.
Not so with a dog that's trained to hunt cockbirds only. He doesn't get the "reward" of a shot and a retrieve if he flushes a hen, and any dog I've owned or shot over that was worth hunting over figured out the difference in a hurry. Certainly by the time the dog is 4 years old he'll know the difference, if he's worth the price of his kibble. It helps if you train them carefully... I douse my live training pigeons with cockbird scent, and I don't train with hen pheasants, even though it costs more to buy cockbirds for training. It pays off in the hunting field.
When you have a dog whose flushes are 75-80% cocks, and the pheasant population is 75-80% hens, that gives you something of a clue.
The vets and the field-trial trainers can talk all they want, but none of them really knows what a dog can or can't smell. We can only guess about it by what they do with their sense of smell, and in my view, the good ones can do a lot more with their noses than most dog men give them credit for.
