Was he there before they came? Could be he didn't leave with them because they weren't his flock. I don't know about ducks, but chickens aren't always accepting of a newcomer or loner. And a new bird to the flock often chooses to be by herself.Maybe like someone here said he's just a moocher? ... but I'm still confused why he didn't leave with those 7 ducks... or I should say 6 ducks because one of them was severely injured and I took it to the sanctuary.