Chihuahua, not only that, but many pet parrots may be supremely under-socialized with *other* parrots.
If you don't want a dog to be scared or aggressive with strangers then you need to socialize them with strangers at a young age. If you don't want a dog to be scared or aggressive with other dogs, you need to socialize them with other dogs (and puppies) at a young age.
The same should be said for parrots.... yet, we don't. One reason is disease. Two might not be knowing anyone else in the area with birds. Three might not be taking birds out of our homes to meet up with other birds, or allowing other birds into our homes to socialize.
Many birds then grow up not knowing how to interact with other birds as a flock, so they can easily end up being aggressive to a strange bird. I don't feel as if this is normal, and it's something I've even noticed within my own birds! Although the cockatiels consider each other as flock mates, I don't feel as if it's as tight as I've seen wild flocks of parrots act. (as limited as that is...)