The original ending of The Birds was supposed to be a lot more elaborate. Mitch and family were supposed to drive through the town and see devastation and dead people. There were supposed to be more bird attacks—a proper Hollywood blowout. (For more on this version of the ending, head over to our "Screenwriter" section.)
As it is, though, the family drives away in their little car, with birds everywhere as far as the eye can see. Not much resolution or drama there. Not much triumph, either. Just, you know, birds. Hitchcock didn't even want the title card saying, "The End"; the studio tacked it on, though it's sometimes dropped in current version of the movie.
The ending may seem anti-climactic—but that's surely the point. Hitchcock doesn't want to resolve everything for you. You don't know where the birds came from, and you never find out. Will Melanie recover from her shock and marry Mitch and settle down to live happily ever after? Will the whole family be killed on their way out of town? If so, you'll never see it. The film ends with the birds in possession of the field, and the humans slinking off, hoping they'll be allowed to escape.
We're left wondering if we'll be the next victims.