Why did the birds attack in Alfred Hitchcock's 'The Birds'?

Question

In Alfred Hitchcock's movie The Birds, why did the birds attack everyone?

What could be the reason?

And why wasn't there any attack from the birds at the final scene?

Answer 1

We don't know.

The motivation or cause of the birds' behaviour &/or attack is never explained either in the movie or the original short novel on which it is based.

The Birds is a 1963 American horror-thriller film directed by Alfred Hitchcock, loosely based on the 1952 story of the same name by Daphne du Maurier. It focuses on a series of sudden, unexplained violent bird attacks on the people of Bodega Bay, California over the course of a few days.

^Wikipedia

The source short story...

Inspired by her quiet, rural home town in Cornwall, the story is thought to be a metaphor for the air attacks on London during World War II, in particular The Blitz.

^Wikipedia

..bears little relation to the screenplay by Evan Hunter, who was told by Hitchcock to develop new characters and a more elaborate plot while keeping du Maurier's title and concept of unexplained bird attacks.

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The fact that the reason for the attacks is unexplained increases the sense of suspense and horror. If we can explain something, we fear it less.

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Hitchcock had his own interpretation which he gave in 1962 but this is never explained in the movie itself and, I feel, is more in the nature of plot explanation than actual motivation..but here's the relevant section.

AH: The story concerns the fact that birds... The plot is as follows: birds are tired of being killed, plucked and eaten by mankind, so they decide to avenge themselves. One day, they group together and swoop down on the people's towns and villages.

Q: Do they really kill people?

AH: They try to, anyway. But in reality, they don't succeed. Afterwards, events occur that also suggest that things can be worked out peacefully. But in any case, the warning will have been harsh.

^Source

Answer 2

I watched "the birds" many times growing up. I always sensed the movie was a statement about humans and their destructive abuse of the environment. The attacking birds saw the love birds in a cage as the final straw. Time to fight back! At the end, the birds showed compassion on the couple, child, and caged lovebirds because the point had been made.