There is no motor driving it, no music to tether it, and nothing to hold it aloft apart from that up-draft of sensual atmosphere and existential dread. Hitchcock reportedly worried at length over how to wrap things up. He eventually ditched the scripted final scene in favour of a non-resolution, an open ending, although I would have liked better a more consistent ending.
3-4. I'm ambivalent about qualifying this one. It's curious that the story of Melanie's relationship with the family wasn't really taken anywhere. It seemed like prime territory for the sort of psycho thriller fun Hitchcock's been known for. But he deserves serious kudos for managing to make birds genuinely scary, when I'm sure most people don't particularly take them very seriously.
I have an irrational fear of birds and if anyone asks me why I just point them in the direction of this movie and ask them to imagine an 11 year old child watching it in 3D in Universal Studios and being so scared she cried for the entire time. Scarred for life.