why were the birds attacking people

If you're looking for a scientific reasoning, there is none given in the film, but we could assume that some kind of disease overtook the birds and caused them to become hostile. We could always look at the dead birds lying around, flying into things and essentially killing themselves as signs of their illness (birds apparently do this if they are ill and disorientated).

As for why they would flock and attack simultaneously is a mystery, which is what Hitchcock intended it to be. It's the mystery that makes the film so much more intriguing, the fact that it's never explained. Really, the film is a dialogue driven and story driven piece and the birds, as others mentioned, is only a plot device, it's a survival story, I suppose, in some ways.



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We've become a race of peeping toms.

My personal opinion is that the detailed explanation of Norman Bates' motivations at the end of Psycho did not make it a better movie. Perhaps Hitch thought the same upon reflection and did not offer another pat explanation for the Birds.

perhaps its like sea world when you buy a small pack of bait fish to feed the orca and the seagulls dive bomb you until they get the fish

to avoid making the movie boring?

*Spoiler ahead, so if you don't want to know how the movie ends, or anything about specifically how the birds attacked, stop reading now*

They only started attacking when Melanie arrived in the village, and they seemed to stop attacking when Melanie left the village, which implies that it was something to do with her. We see at the start of the movie that Melanie supports the caging of birds, which implies that she may have this kind of "anti-bird" aura about her. The birds of the village probably picked up on this via some kind of sixth sense that they had, perhaps viewing Melanie as a threat to them.

So they probably started exhibiting their aggressive behaviour because they could sense the "disturbance" that Melanie had brought to the village, through her anti-bird attitude. But they probably couldn't quite decide on the source of the disturbance at first, which would explain why they attacked other people of the village first, before finally zeroing in on Melanie.

So why hadn't birds attacked Melanie before? Well, Melanie is clearly something of a city girl, and no bird collectors catch birds for caging from cities; they take them from more wild habitats. Melanie had probably spent most of her life in a city, hence not causing the threat of being caged to register in the birds she encountered then. But as soon as she entered a more rugged, wild location - exactly the sort of location that cage birds are taken from - birds coincidentally started attacking, because they probably subconsciously viewed her as a potential threat.

So in short, I think the birds started attacking because Melanie was a bird freak!