Sucker...
The little stinker. We were fortunate to avoid this habit but suddenly in the past month he decided it would be a wonderful thing and now we have a fingersucker. He tried the thumb but seemed to prefer his index finger. He has also developed the habit of digging in your clothes while he sucks his finger. He will randomly come up to you and dig in your sleeve or your pant leg, looking for the warmth of your skin. Funny little stinker...
But he's so gosh-darn cute we're gonna keep him!