The demon, Legion or “Morgan Creek” (taken from the ancient Gibberish), had a bone to pick with brain-endowed stories, and made, as any devil ought, Blatty kill the poor bird; the scene being a byproduct of the bigger effort to imbue whiz-bang scares, as in reshooting half the film and slapping an exorcism on at the end to validate the film as a sequel to The Exorcist.
So, the WHY was Hollywood can be crafty and sinister when cliched trends make money, and the original thinking gets jettisoned.
And the HOW was, Blatty was contracted — so he shot the scene in such a way to suggest that the demon, sans a human body, swiftly picks off the poor avian creature during his day of devilry — and he keeps his signed promises. That doesn’t mean, he, us, or anyone but the studio are terribly happy with the film, or ought to be. It has glimmers of greatness, but those are mere drops in the bucket. Okay, tablespoons.
M.I.K.E.