Love, Catholicism and the Cuban Missile Crisis
The theme of love and reconciliation is subtext in The Birds. Keep in mind Hitchcock was a devout Catholic. Although the general theme of guilt, Catholic or otherwise, runs throughout all of his films, religion didn't crop up often. The few occasions it did were memorable. I Confess is probably Hitchcock's most overtly Catholic film, dealing with a priest who has his faith tested after a murderer confesses to him. If I recall, The Wrong Man had a bit of religious subtext as well. Think also of Mrs. Blaney in Frenzy, who prays (to little effect) as she is being killed by Rusk. She knew her time was up and used religion as a last refuge from Rusk's brute, unreasoning savagery.
For The Birds, I think the lovebirds are an important symbol. They never get aggressive or violent even though all the other birds do. I think this is significant. As for the human characters, the mother (Jessica Tandy) and Melanie (Tippi Hedren) start off on the wrong foot and are antagonistic. Mitch (Rod Taylor) and Melanie also start off very antagonistically. There is also tension between Annie (Suzanne Pleshette) and Melanie. By the end of the film, all of these characters are reconciled and come to care for each other. I think one of the key themes of the film is love and reconciliation in the face of a harsh and indifferent universe. When they drive off at the end of the film, we do see the birds have taken over (that's true), BUT there is also a shaft of sunlight coming through the clouds in the distance. This suggests at least the possibility of hope at the end of the film.
I'd also mention that the theme of the Apocalypse is woven into the story as well. We have the famous "It's the end of the world" refrain from the "Drunken Doomsayer" (Karl Swenson) in the diner. Remember, the Cuban Missile Crisis of 1962 had only (just barely) been resolved the year before the release of The Birds in 1963. Hitchcock of course made a film about the Cuban Missile crisis six years later in 1969 (Topaz), so it was clearly a topic of interest to him. Is it possible to read The Birds as a film about the danger of nuclear Armageddon and the need for love and reconciliation?