I like birds. I do a lot of hiking and I like to get out and about, and the idea of ornithology sort of appeals to the 8 year old inside me, whose chief career aspiration was to be a zoologist (I had a romantic image of myself being David Attenborough, essentially - travelling around the world studying exotic wildlife and making TV programmes about it; I'm still not convinced that this is beyond the realms of possibility).
What is most interesting about birds (this is also true of non-mammals in general, really, but birds are intelligent with it) is that their facial expressions don't change. They don't smile, they don't have whiskers or ears that they can move about, they don't move facial muscles. They have a fixed expression which is typically somehow both curious and disdainful.
This is also true of their behaviour. Birds are always on the alert, always moving, always looking for something, always ready to pounce. If you watch them closely there is something almost robotic about their actions - as if they are not really alive but are rather supremely detailed and exquisitely crafted clockwork automatons.
This is why avian-based monsters are chilling: their lack of expression gives them a kind of blank, totally cold, utterly remorseless gaze which, if fixed on you as prey, would strike you with the deepest sense of your own worthlessness. This is an animal that will kill you and eat you, and you will mean nothing to it. There is no animosity there, but nor is there any impression of feeling; while a tiger would kill you just as dead, at least it would convey to you a sense of aggression and ferocity in doing so. With a bird there would simply be that still, stony look - exactly the same look that it has when it is preening its feathers, taking a dust bath, or having a shit. You are simply nothing: a thing to be eaten and then instantly forgotten.