This greedy cockatoo wasn’t content just nibbling some seeds off the sunflower, he picked the whole head. And from the ease of his attack, I suspect he’s done this before.
I’m guessing that he’s a he and the 2nd bird is a she based on purely sexists bigotry. Check out this picture where “he” eats contentedly after flying to a nearby roof while “she” watches with silent longing. Selfish rat.
“She” did try to get her own sunflower head but the others were too far for her to reach (next photo). 
Which is, I suppose, why “she” stooped to begging from Mr. Greedy.
Cockatoos are bigger than crimson rosellas so I believe she rightly deduced she couldn’t stand on the flower and eat as my previous photo-op had.
In a moment of weakness I went and tied a second sunflower head to the fence so the “female” could have a go. Of course the male came back and picked that one too!
I picked a 2nd head for myself and it’s hanging under the house to dry. I’m hoping between that one and the flower I have still in a vase (its stalk broken courtesy of the crimson rosella) I’ll have some seeds to plant.
Confession: I’m 2 seeds down since Frank and I each tasted one. Yummy, but I suspect the birds appreciate them more than I do.
Laura Rittenhouse's Gardening Journal 