'i.e., "Me Conservative, I good - you Liberal, you bad". I rarely hear people who would self-describe as "Liberal" saying, "You Conservative shit-bird, you".'

You've never listened to Randi Rhodes or Ed Schultz or Al Franken have you? You've never even read posts here at Mudcat about Ann Coulter or the Dick Cheney or conservatives generally. But you're correct, I have never heard Liberals (L or l) use the word "shit-bird".