For some reason everyone thinks that this is about the birds. But it isn't, really, it's about the people. None of them can get their sh!t together.
Remember, in the resturant? They all argue about it! The 'don't scare the children' lady and the 'it's the end o' the world' guy and the wizened old crone who says that birds 'just don't do that!' And the 35-years-old man has an 11-year-old sister!
God, but he was good at showing the desolation. And without it, it would've been 'attack of the Killer Tomatoes.' It wasn't about birds at all.