Why did the birds attack? Mystery solved.

One of the inspirations for this film was an incident which took place in another coastal California town called Capitola:

In the summer of 1961 hundreds of birds attacked the town. Most of the birds were sooty shearwaters, a normally non-aggressive species that rarely comes to shore. Alfred Hitchcock was a regular visitor to nearby Santa Cruz and read about this episode. He went on to direct a film—The Birds —based on the idea of hundreds of birds attacking humans. The reason for this attack remained unknown for over 25 years until it was discovered that the birds had been affected by domoic acid, a toxin produced by red algae.
https://en.wikipedia.org/wiki/Capitola,_California#History

Further information for anyone interested:
https://www.mercurynews.com/2015/12/07/this-hitchcock-movie-was-inspired-by-crab-toxin-frenzy-in-capitola/