In the theory of selfishness
We also keep parrots in cages, horses in pastures and a lot of other animals in smaller accommodations then really what they should be, but you can still do it without it being harmful to them and considered inhumane. If i was taking a bird from the wild and had not grown up in captivity, and put it in a small pen, then that would be selfish. Anyone that has animals confined is keeping them in a selfish way... in that theory no pen would be humane. IMO, But to keep any animal there is a loss to the animal in some way, we just try to give them the best care and space we can to substitute. You can have a bird in a 4x8 ft pen or in a 20x20 pen, but if the person with the 4x8 pen spends more time with there birds, buys them more treats and meal worms and all the things they like, spends time with them and truly enjoys them and the person with the 20x20 pen does none of those things............ it is just a pretty bird in a pen, which is less humane.