I think it's a great question for a problem that many have had. I must say that the level of knowledge on shock collars is appallingly low in this thread. They are not like electric fences. They aren't meant to cause pain when used properly and they won't "light up" the animal. I've used them extensively on dogs and I've tested them on myself with every level on several models. Electric fences give a much larger jolt, even when compared with a shock collar's highest setting. The lowest settings, I can barely feel one. It's certainly not pain, more of a tingle at worst. They have a great many features, including buzzing prior to shock, buzzing only, beeping to warn, and settings that most animals will not even pay any attention to they're so mild. Proper use of a shock collar does not include hurting an animal or having it yelp in pain or any other such nonsense that permeates these types of threads. They are meant to grab the animals attention. I can see how breaking the attention of a rooster might stop it from crowing.
The biggest problem you'll have to overcome is attaching it. Depending on the size of your rooster, I'm betting it could be attached to the body somehow - maybe with the strap going between the legs. It wouldn't have to be on tight, since feathers are not nearly as dense as fur on a dog. I'd pick a light-weight model and not worry about the range (you'd be right next to it). If it works, I'll bet you could patent it and make millions.
