OK, I know I'm in the minority here but I just can't help but feel that this is the decidedly weakest of Argento's early films. Sure enough, there's lots to admire. Morricone's score and Storaro's camerawork is top-notch and the film has a very solid story and a great central puzzle. I'm also impressed by the cast, including the great Mario Adorf and Enrico Maria Salerno, who is more likeable and smart than most cops we see in gialli. There's also a bit of humor that actually works.
Still, I can't help but feel Argento's inexperience here - he would improve on a lot of the themes seen her in his next couple of films. Here, he doesn't quite have the necessary experience or timing to successfully execute all of his suspense scenes. Most of the victims are unimportant characters who are introduced and killed right afterwards, which means there is little emotional impact since we never establish a relationship with these characters. The one scene that really should make an emotional impact is the one where the killer attacks Suzy Kendall, whose character we do sympathize with. But unfortunately, this scene falls flatter than a pancake and isn't able to deliver any thrills whatsoever. I thought it was amazing to see Argento handle a suspense sequence with such uncertainty! He also doesn't quite know how to direct his actors - as evidenced by Suzy Kendall's unconvincing hysterical acting in this scene. Kendall is a good actress but doesn't express her fear in a very satisfying manner here. It seems as if neither she or Argento had any clue how to do this scene.
So for me, Argento was just cutting his teeth on this one. He would improve vastly as a director with his next to films, which have much more emotional impact as well as leading characters that we actually care about (the relationship between Karl Malden and his niece in CAT is much warmer and convincing than any of what we see in BIRD). That's not to say that BIRD is a poor film but for me the only really GREAT thing it has going for it is the central plot element of what the protagonist saw (or rather thought he saw), which really re-defined the whole giallo genre at the time. The rest of the film, however, is pretty routine, and this is one Argento film I rarely feel any urge to revisit.