The birds are sick of humans. Humans put up utility poles that electrocute them, wind farms that behead them and solar panels that fry them. People hunt and kill birds for sport. And then they eat them. People munch on pumpkin seeds and don't share them with the birds. Worse yet, they invented rock and roll. And elected Donald Trump. Death to people!
Birds rule!

Those lovebirds were captured from a tribe that worshipped them. The shaman of that tribe was so angered that he summoned an evil spirit, and that spirit would reside in the lovebirds, attracting the wrath of other bird species. The only way to exorcise the malevolent spirit would be to release them from their cage, which they never did.

One of the many blessings of getting older was that I think I learned when to analyze and when to stop analyzing. My opinion has to be Hitchcock was smart enough not to try to explain it since bird attacks are not even a true phenomena (closest I've heard of are the drunken flights of songbirds which gorge on fermented berriesnot at all what occurred in the movie)
Be that as it may, there are viruses which make some animals change their behavior. I just heard a biologist say that cat parasite which can be found in 90% of all cats now can make mice and voles engage in risky behavior. We know research is full of holes,but she said there is a measurable correlation between exposure to this parasite and running red lights, starting fights, and other risky driving behavior

Ah so now I'm realising I should have posted this here:
http://www.imdb.com/board/10056869/board/thread/213184410?d=260218634#260218634

One theory is that the birds had some type of unknown disease that confused their minds and it caused them to attack people.
"Do All Things For God's Glory"-1 Corinthians 10:31
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One theory is that the world may have tilted slightly on its axis, which disturbed the birds. One upset in the balance of nature causes a chain reaction. Also the birds may have only been the beginning.
Another theory (although going into the sci-fi cliche territory) would be nuclear testing and fallout.
A third theory was a simple freak of nature. Here in the UK, we had snow in April before. That doesn't usually happen here. My grandparents who were born and raised in North Carolina remember it snowing every Wednesday for two months. It is just one of them things.

The attacking birds are for the most part a macguffin.

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The only reason I could come up with it was that Tippi Hedren had those caged lovebirds with her and they saw that and got pissed that their fellow birds were locked up for no reason but for the amusement of man and decided to join together against the human tyranny once and for all, regardless of breeds cause the birds shared one common enemey The humans!
I'm chill. I'm chill as a cucumber, man.

The "drunken old codger" was Charles McGraw, who played Marcellus, the gladiator trainer, in Spartacus (1960). Show some respect.
You got your mind right, Luke?

The drunken old codger at the diner was probably right
It's the end of the world!
Theories are nice, though I like the ambiguity.
Hey there, Johnny Boy, I hope you fry!

But these people did not know why they and many others were being attacked. The Bible plainly shows that when Armageddon happens those being destroyed will know it is happening and why. Jesus said "they(people of Noah's day) took no note until the flood came and swept them all away" When the rain started, they knew it was too late for them and why. Jesus likened Armageddon to what happened to them-Matthew 24:39.
"Do All Things For God's Glory"-1 Corinthians 10:31
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I prefer a more supernatural approach. The book of Revelation in the Bible speaks of the role birds will play during Armageddon, which lends itself nicely to the movie:
Revelation 19:17-18
Then I saw an angel standing in the sun, and he cried out with a loud voice, saying to all the birds which fly in midheaven, "Come, assemble for the great supper of God, so that you may eat the flesh of kings and the flesh of commanders and the flesh of mighty men and the flesh of horses and of those who sit on them and the flesh of all men, both free men and slaves, and small and great."
I kind of like the idea that our main characters were driving off into certain doom.

I'm sure it was caused by global warming.
Or, no, wait a minute. It was 1963. The birds had all read
Silent Spring
, published the year before. As a result, they were mad as hell and weren't going to take it any more.

I've also tried to interpret this movie from the standpoint of what was going on in the world at the time. WW2 had ended less than 20 years earlier, and the world was gripped in a Cold War and threat of nuclear war. Former colonies were demanding and fighting for their independence. There was also growing civil unrest within the United States and other countries.
The fact that Melanie was created as a daughter of a wealthy newspaper publisher might also be significant, since freedom fighters, protesters or anyone with a cause would certainly want to get the attention of the press.

I don't believe it was intended for there to be any explanation for it, but if I had to come up with one i'd say it was probably nature starting to fight back against humans. Human activity has disrupted the ecosystem and a balance needs to be restored, the birds began perceiving humans as a threat and started attacking them on sight. Actually I was half-expecting for someone in the movie to come up with this theory, and was surprised when nobody said it after Cathy specifically asked why the birds were attacking people.
Even this, though, I don't know if it's adequate. It's just what i'd expect, I guess. Humans have been a threat to the environment for a long while, so why only now ("now" being 1963)? Of course, at that time, humans were becoming an even bigger threat due to the rising level of industrial development and its destructive effect on the environment, as well as the development of more destructive weapons of war, but animals wouldn't perceive the latter change (well, animals in Asia would, since wars from the 1930s to 70s pretty much burned much of the region to the ground, but animals in California wouldn't notice a difference). And even then, why only birds? And very common birds that have weathered human activity fairly well too, such as seagulls and crows.
Of course there's the religious "end of the world" scenario, but even then, why only birds, and why only in that general area? Though you could say it's only just starting then and there.
Or maybe it's something more simple, like a local chemical plant or something releasing stuff into the air that caused the birds of the area to go insane.
The people, and the people alone, are the motive force in the making of history.
-Mao Zedong

Maybe the bird expert was a geneticist? Just performing studies on altering the bird's DNA for some reason (probably well-intentioned) only to cause them to become vicious carnivorous maniacs.
Y'know, Grayditch, basically.

It's me.Barait's always bloody Bara!

I think the bird expert in the diner had something to do with it. I mean, how many bird experts are there in the world? What are the odds that one would just happen to be in Bodega Bay the day the birds attack? I think she was ticked at the way people treats birds and somehow found a way to train a large group of them to attack people.

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I speculate it is because we taste like chicken!
In the frozen land of Nador they were forced to eat Robins minstrels. And there was much rejoicing.

Your "God Theory" does not work because God gave this earth to humans to be their home for eternity. Only the bad people will be off the earth-Psalms 115:16; Psalms 37:10, 11
"Do All Things For God's Glory"-1 Corinthians 10:31
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Hitchcock was "aiming for the fences" with this one. He had everything but a Best Director Oscar, and he seems to have felt that only with a "really big symbolic idea" driving "The Birds" could he get one.
I don't think Hitchcock ever really wanted an explanation for the birds doing what they are doing.
But I would suggest: it can't be for a "small" reason. It can't be because they have rabies or even because they want revenge for Thanksgiving and bird hunting.
Hitchocck SUGGESTS that Thanksgiving/hunting reason in his trailer for The Birds but ask thiswhy wouldn't all the OTHER animals we hunt and eat come after us? Cattle? Fish?
Hey, wait a minute. Really, birds are the main animals other than cattle and fish we DO eat. And outside of a shark or pirhana or barracudawe can't have killer fishthey can't come on land. And stampeding cattle can't get down into chimneys and up into attic rooms like the birds. (OK, we slaughter some other animal breeds, too pigs, deerbut, not all that many categories for domestic food.)
Still, "Something Big" is going on with the bird attacks and I daresay the big solution might well be the religious one:
God has begun the project of removing the human species from the planet.
For these killer birds seem to be driven by an "overriding intelligence" that commands them to group together, wait at rest for long periodsand then suddenly go berserk.
The birds here are presented very much like storms or hurricanes or tornadoes"climatic changes" that build and build and build and then unleash destructionand then end.
And there are those who have believed, over the centuries, that savage attacks "from nature" ARE directed by a God-like intelligence. He,she or it doesn't SAY anything.it just DOES things. Unleashes things. Thunderstorms, hurricanes, tornadoes..earthquakes, floods, wildfires.
To go to the "God theory" of the bird attacks loses our non-believers rapidly, but there is also the pseudonym of "nature" for patterns of behavior that we cannot explain in human scientific terms. If we can't explain why a skunk has THAT smell, or why a spider builds a web to trap prey, or why a caterpillar becomes a butterfly, or why birds fly in a certain "V" pattern well, those are REAL unexplainable things in REAL life.
"The Birds" just asks us to go a bit farther in comprehending what might happen if a "higher universal intelligence" decided it was time to exterminate the human species.
And WHY would that intelligence seek to exterminate humans? Because the humans now had nuclear weaponry and the ability to wipe out ALL species from the earth.
Hey, it a theory.

I think the birds are fighting back against mankind. There is the pet-shop part when we see caged birds. There is the cafe part when in the background someone is ordering fried chicken. That makes me think the birds are fed-up with humans.

I blame Obama.
Janet! Donkeys!