So, what I have so far is my detective, a hard boiled type pushing the edge of parody without quite crossing the line, yet close enough to have some fun with the genre. I've got a victim, the woman eagle watcher. False suspect is the male eagle watcher. Obvious suspect is the husband, as marriage was under strain, and he was having his own affair. Misleading eyewitness testimony who sees a flash of color in the male watcher's car and believes it is the blond head of the woman when it turns out to be, in fact, a yellow lab's head.
What we need now is the actual killer. Some seeming unimportant character who is involved with the woman. Perhaps something about the eagle watching organization. She, this unimportant character, introduced quickly early in the story, is the treasurer skimming money. She is found out by the woman, who was in fact having a relationship with the treasurer's husband. Getting a bit complicated for a short story, but we'll see how it develops.