I'm a little surprised with the lack of information on this group of parrots.
They are excellent pet birds. Reasonable size (depends on species), reasonably quiet, not overly active - but don't confuse this with inactivity, because they are NOT inactive. They have a reputation as being 'perch potatoes', but this is only when new, stressed or uncertain. Once comfortable with their surroundings they're as active as you could wish.
They also have a reputation for being dull coloured. Again, not true. The colours may be a bit more subtle than what novice owners would like, and photos do not do them justice. The colours of a well-feathered bird are quite breath-taking in real life...very iridescent...
They are not great talkers...but you can expect a few words from them. Some individuals do build up quite a vocabulary.
Related to the Amazon parrots...they live in the same areas and have that same stocky amazon build. However, their flight muscles and ability differ.
There are several species:
1. The white-capped pionus. Pionus senilis, more correctly called the white-crowned, although hardly anyone uses the correct popular designation. Again, they have a reputation for being fiesty that seems to put people off. I think this is because they are probably the most active of the pis...but they are still not more active than a Green-cheek conure or most of the other common parrots...so I'm not sure why this rumour persists.
2. The white-crowned pionus (or white-headed pionus). Pionus seniloides This is the bird that IS also correctly called the white-capped. Rather silly to try and change the popular designation with P. seniloides, but what can you do? Not seen in the pet trade.
3. The blue-headed pionus. Pionus menstruus . Maybe more than one sub-species. Quite common.
4. Maximilian's pionus (often called the Scaly-headed Parrot). Pionus maximiliani. One of the larger parrots. May also be the most commonly found of the pet species in North Amercian. could also be considered the least colourful. Very nice birds.