The Paranormal
There is something rather paranormal about a giant flock of birds who manage to organize themselves into an effective attacking force, yet this is the concept that the author presents to the reader in The Birds. However, the other stories in the collection are also heavily weighted in the paranormal; for example, "The Apple Tree" tells the story of a man who believes that the spirit of his wife is inhabiting an old, neglected tree in his yard. Much of the plot of "Kiss Me Again, Stranger" takes place in a cemetery, which is a classic gothic horror setting, linking present with paranormal activity. There is a suggestion of paranormal activity in "Monte Verita". Are the women disappearing because something paranormal is going on or because someone is making them disappear? The author leaves it up to us to decide but throughout the stories in the collection there is an element of the unexplained, without any attempt made to actually explain it.
Murder
The birds are definitely murderous avians, and the way in which their bodies are discovered by Nat is certainly similar to the way in which bodies are discovered after any kind of murderous rampage usually carried out by a human perpetrator. There is human murder too, in the collection, most notably in the story entitled "The Old Man", as a neighbor tells the story of the man living next door whom it is believed has killed his children. It is not a murder mystery per se - it's not a mystery at all, because we have the identity of the killer at the start of the story, but it is a mystery when it comes to the voracity of the suspicions, and whether or not this man is truly a murderer, or whether he is reclusive and odd, and an urban myth has built up around him because of that.
There is suggestion of murder in "Monte Verita" too; young women leave their towns and villages to go up to the mountain retreat and are literally never seen again. If this is not caused by paranormal activity then it must be caused by something rather murderous, although we never actually discover and are left to formulate our own scenario.
Flawed Humanity
"The Birds" actually shows humanity in a far better light than the rest of the stories in the collection; Nat is an honorable man, and his boss, Mr Trigg, an honest land owner trying to help returning veterans by employing them. However, the human characters in the remainder of the stories are considerably more flawed. For example, the Marquise in "The Little Photographer" is spoiled and bored and dissatisfied despite her obvious blessings and financial fortune. She looks to others to fulfill her rather than looking to make changes to herself. The widower in "The Apple Tree" has not been a good husband; whilst his wife was alive he neglected her terribly and now that her spirit is living in his apple tree, he is neglecting that as well, because he keeps meaning to cut the tree but never does. He is flawed because of his absolute self-centeredness and his lack of compassion or caring for his wife. "The Old Man" also has flawed human beings at its core; obviously the man who is said to have murdered his child is more than flawed if the rumor of his crime is actually true; the neighbor telling the story is also flawed because what she is reporting is more guesswork and hearsay than actual fact.
