This film is a classic and a work of true horror. Few films actually take advantage of the primal fears that humans are born with. There are two types of fears: primal fear and acquired fear. Primal fear is a fear that is natural and comprehensible like the fear of darkness, spiders, water, large dogs, rats, snakes, and heights. It is adapted for the means of self-preservation whereas acquired fear is shoved down our throats by the media and propaganda and is impractical and unrealistic like Freddy Krueger or Jason Voorhees. I recommend watching the short film "Of Darkness" on fearnet.com. It does a great job of hitting the ancient fears of the darkness and the unknown.
P.S. a group of birds as large and intelligent as crows and ravens (they can get pretty big) can kill a human. Have you ever seen a crow eat the eyes out of a cow carcass? Crows and ravens have the intelligence to manipulate their environment and take advantage of human-made conveniences. There was a video of a crow dropping a walnut in the middle of street and then after a car ran over it, cracking the shell, the crow waited for the traffic signal to change and went to retrieve its meal. How's that for intelligent! Given their intelligence and opportunistic nature, something like this doesn't seem very impossible.