SPOILER ALERT: Don't read if you want to see Magnolia.
The most surprising aspect of this movie is how long it takes for the birds to attack. When they finally do, it feels like they're interrupting a movie that would've been perfectly good without them, a complex and suspenseful screwball comedy.
Ultimately the thrills of the second act fall short, and not because of cheesy effects. Hitchcock and co. are remarkably resourceful within their technical limitations. No, what makes it fall short is that birds just aren't that scary.
M. Knight Shyamalan cites this movie as inspiration for The Happening. There's a thematic connection, but structure-wise a film that's far closer is From Dusk Till Dawn. Both films deliver in the first act and drop the ball when the tone switches. (And Shyamalan's film is inept but comical all the way through.)
I think I would like this movie more if it were called Weekend at Bodega Bay; calling the film The Birds is like referring to Magnolia as The Frogs.