It is not possible for two species of bird to have ‘viable’ offspring. This means that if they did manage to breed, which I might add would probably be impossible, the offspring are more likely to abort in the eggs, have abnormalities, and are certainly not going to be fertile adults if they survived.

This is the physical barrier against hybrid production. The second barrier, the behavioral barrier, means that they would not mate because they would not find each other’s mating ritual attractive.