“Trucker and New Yorker are both in the dictionary, not the made up word birther..”
Oh, for pete’s sake. It’s a made up word which follows a long-established tradition in real words. And no, it wouldn’t follow that birthers are those who give birth. Just as New Yorkers are people who use New York, although fryers are those who use frying pans. Birthers are those who care about birth, specifically Obama’s birth.