Their article also states that just because you purchase, say, an African gray, don't necessarily expect it to talk. Personality plays a large role in whether any bird learns to speak, according to Pepperberg and Bright, and some birds never learn, no matter how long or intense their training. Others pick up seemingly everything with apparent ease, even undesirable things such as curse words or the sound of a baby crying. If you want to teach a bird to speak, choose a bird that appears alert and interested in what's going on around it, one that seems to listen when you speak to it and is vocal in general. The best-talking budgies are usually those that constantly babble and chirp before being trained to talk.
Males also tend to be better talkers than females, especially in budgerigar and cockatiel species.
It is important to spend time observing a bird's behavior before making a purchase. Young birds, especially hand-raised ones, are much more likely to talk than older birds. The best time to begin training is around 4 to 6 months of age with smaller birds like budgies, and between 6 and 12 months of age for larger birds like African grays and Amazons, although younger parrots can begin picking up words. One bird is easier to teach than two birds. Two birds are more likely to speak to each other in their own language than to speak English to you.