[Posting this as an answer at original poster's request.]
I can't find a source to cite this unfortunately, but I recall watching a "behind-the-scenes" show years ago where someone (presumably the designer) mentioned that its major inspiration was a[n American] football player, with his head down/forward and padded shoulders. You can see this best from a head-on view, though you probably wouldn't live long enough to tell anyone about it. Qapla'!
(Photos aren't the greatest, but these are the best I could find.)
Actually, I just found a snippet from an interview with the designer here:
Besides Star Wars, you were also involved with Star Trek. In fact, you are credited as the inventor of the distinctive look of the Klingon Bird of Prey starship. How did you come up with this design?
The Klingon Bird of Prey was inspired by body builders' shape when they flex their muscles like crab. I had never seen Star Trek before, I didn't know there was already a bird of prey.
So, while I could swear I remembered him talking about football players, it could have been body-builders. Of course, both stances would have a similar look to them, so it may have been a bit of both, depending on who happened to be interviewing him at the time.
[Memory Alpha's entry for the Bird-of-Prey]4 now features the "muscleman" sketch that inspired the ship's design. It seems like Bill George, the model designer, took inspiration from both the muscleman sketch and the physique and equipment of a football player.
Ultimately, aspects of different designs were mixed to create a hybrid of the various looks. One of the sketches that Nilo Rodis gave Bill George, to help George in constructing the first prototype studio model of the ship, featured the image of a muscleman. Despite being somewhat surprised by Rodis' guideline of using this sketch as an influence, George was nevertheless inspired by the appearance of the pictured man while building the original prototype model. The muscular man's down-turned arms influenced the look of the ship's wings in their attack position and his large, distinctive shoulders and trapezius muscles inspired George to add the wiry meshes on the ship's aft hull. George also thought that the sketched male looked somewhat like a football player so he added the twin bulks below the meshes, believing they resembled shoulder pads, and a thin, tiny, brown pipe around the command bulb, as the protective-looking pipe's appearance was similar to a footballer's chin guard.