It looks legit to me. The size is about right for the book paintings, and the composition is right. There's a technique in there that I don't see in any of his other paintings that we have, and that made it look off to me at first glance too. But looking at it more closely, it seems believable.
Wow - tough one indeed. I'm inclined to say genuine (of course it's real, you've seen it), but a few points:
The painting appears to have possibly been trimmed (or simply has had the notebook ring hole strip removed). Typically, the art in books (for which this was clearly intended, assuming it's genuine) had notebook ring at the top, but not always. I believe that this practice was more common in later paintings, but I'm not positive.
If this painting is exactly 6" x 8 3/4", it's not inconsistent with the typical size of the book signatures post South of No North. I look at that painting and think: unusual color choices for Buk, not terribly unusual subject matter, but not common. Technique seems right, albeit somewhat atypical, signature seems consistent, but something just doesn't look right, as Bill mentioned.
All said, I'd attribute my skepticism to its departure from Buk's typical style in terms of color. The triangular beak and wide eyes, along with the areas of extreme attention to coloring areas contrasted with other areas that lack extreme attention (see the orange-red areas around the wings in contrast to the brown chest) tell me it's the real deal. Date? Mid-late '70s as a guess.
If this is legit, it really IS odd.
The palette is very unusual (as Purple has stated already).
The style is equally as strange für a Bukowski.
As is the technique - I believe, I've never seen such a messy brushwork on a Bukowski watercolor:
THIS is what a Bukowski watercolor-brushwork should look like.
could be a thick watercolor or a thin acrylic.
What leads me to believe in the first is that the paint looks too soft. When he used oil or acrylics he usually had a very harsh way of using the brush, so you could see the texture of each stroke. This doesn't appear here.