Treats are positive reinforcement - anything that the subject finds pleasant is positive reinforcement. For dogs that may be food, toys, play or human interaction. Chickens probably aren't into human interaction they way dogs are but that doesn't mean they can't be trained by positive...
The current thinking is that number of neurons is related to overall intelligence and the number of neurons is not directly related to the size of the brain or body size...
Yes - many dog trainers attend "chicken camp" which is a training course using clickers to train chickens as a way to hone dog training skills. It is easier for many people to learn to use a clicker on a chicken than it is on a dog:
https://www.legacycanine.com/chicken-workshops...