When Pixel first arrived here, she seemed ok with all the birds but one.
Shade didn't do anything to her. In fact, Shade is pretty easy going with all the birds here.
So where could this come from?
Of course, like with humans, not all birds will necessarily get along and this might just stem from a personality difference.
However, I do believe there might have been other reason(s), the biggest being competition in Pixel's eye for my attention. Shade is undeniably more my bird than she is my boyfriend's bird. Pixel, as of now, is still weary of my boyfriend (it's getting better). Pixel was particularly "aggressive" towards Shade when Shade would be on me or around me and she wanted the attention as well. But some of these display of aggression also occurred on play stands. When she acted out towards Shade, she got some time out in the bird room (as these always happened when everyone was out in the living room area) for a few minutes and then I would bring her back and put her on a stand further away from me and I would spend more time with Shade (as she has shown to be a bit jealous I believe).
This was definitively more pronounced earlier on. Although I don't believe these two will never be the best of friends, they do seem to have made some progress. Here's a picture of them hanging out in close proximity on the same stand. They were like this for quite some time and it only ended when Shade moved on to get some food elsewhere. There is definitively hope that they will at least learn to stand each other.