The point is, she doesn't speak their language and they don't speak hers. They don't share her behaviors (swimming) either. So it would be like if you were plopped down in a group of Chimpanzees and had to live with only them for company. Would you follow them around and hang out with them while they did stuff? Probably, they're your only company after all, and the jungle is dangerous so you should probably stay with them for some protection. But you have NO idea what they're communicating with each other and a lot of their behaviors make no sense to you and you're always an outsider. You're still lonely.
This is the same with the duck. Her instincts tell her she needs to be part of a flock for safety and company, so she is hanging out with the only flock she has access to. But she never really understands them, they never really understand her, and she's never going to be as happy as she would be with another duck around.