I think lissener is correct to perceive a psychosexual subtext to the film. Hitchcock was forever working out his personal kinks in his films, from Psycho to Vertigo to Frenzy. In a strange way, when it comes to Hitchcock, the less perverse the subtext, the less successful the movie (e.g. Topaz). With respect to The Birds, that doesn’t necessarily mean it should be considered the foreground element; it just provides a clue to the operation of the film’s suspense, and why we find it so disturbing without really being able to explain why.
I would also argue strongly in favor of the deliberate-unknown interpretation behind the attacking-bird MacGuffin. I take the position in this thread that a big part of what makes good fright movies so effective, for me at least, is the quality of incomprehensibility. Most often, this manifests as a decision not to show some or all of the monster, because if all it’s doing is killing people then there’s very little mystery to preserve. Other horror movies, though, present a monster with very different methods and objectives, and can get away with showing it to the audience.
I’d suggest that The Birds is effective in the latter fashion, but, for reasons I delve into in the other thread and will explore again here with a slightly different focus, it won’t be effective for everybody. Some people need explanations; they need to know what they’re supposed to be scared of, and are distracted by ambiguity (classic example: why couldn’t the kids find their way out of the woods in Blair Witch). Other people fill in the ambiguity with their own paranoia, and are distracted by explanations that don’t make any logical sense (e.g., the alien weakness revealed at the end of Signs).
The way The Birds is supposed to work, I believe, is that the lack of a rational foundation increases fear for some people. Let’s compare the two approaches.
Consider, for example, a hypothetical scenario in which the film makes clear that the birds went crazy because of sunspots, or nuclear waste, or some new high-frequency radar used by the military, or a brain parasite, or whatever. Doesn’t matter what the explanation is, exactly, just that there is one. Audience member A will be satisfied with the neat package of the film, will be freaked out by the violence, and will go home with a little thrill of fear, but won’t really think about the movie again. Audience member B, by contrast, will be unable to give over to the fear, being nagged by thoughts about the explanation: Is there a chance the brain parasite will cross over to other creatures? What’s the infection vector? Why does it have such a specific behavioral effect? And in any case, looking around the world outside the movie, it’s clear that sunspots don’t really do that to animals or there isn’t nuclear waste in the neighborhood or whatever and consequently there isn’t any reason to carry the fear and paranoia outside the film.
Now contrast the above with the movie as we have it. Audience member A sits through the movie, wondering, why is this happening? And when it’s over, A goes home frustrated because the movie seemed to be a tease, and he calls it stupid and incompetent and unsatisfying. Audience member B, on the other hand, has the same reaction, wondering why this might be happening, but instead of being frustrated with the lack of explanation, he walks out of the movie with a half-dozen possible theories and no guarantee that any one of them is correct. The idea here is that the next time he sees a row of starlings sitting on a telephone wire, he’ll get a frisson of uncertainty because his subconscious is still wondering what conditions could cause the birds to freak out and attack him.
The point is, in my opinion, movies designed to scare people fall into two basic types, and each one works for half of the audience but not at all for the other half. Neither side is right or wrong; it’s just a different preference. The Birds, I think, is designed to stick with people like a splinter of doubt. Some don’t like that, and reject the film. Some do, and enjoy it. All you need to know is which type of moviegoer you are, and pick your films accordingly.