Video: Bird dive bombs people in San Francisco

It looks like a scene crafted by Alfred Hitchcock. Unsuspecting, ordinary citizens walking the streets each day, when suddenly, and seemingly from nowhere, they are swooped upon by a blackbird who pecks at their head and musses their hair. They aren't sure what's causing this bird to behave this way, but a good guess is that the bird has a nest in close proximity and is defending her newly hatched birdlings.