Believe me it happens I had a 6' black snake hiding in the loft of one of my coops and at night it came down and killed and tried to eat one of my pullets of course all he did was suffocate it since she was too big to swallow. So when things like this happen It's heartbreaking and we blame...
I don't have Calls but I have noticed that some of my females take longer to get their quack and once they get it they are very opinionated and like to let everyone know. lol
I don't kn ow Calls all that well so can't answer that question but maybe @WVduckchick can since she has Calls. Or @shawluvsbirds or @Pyxis
As for intros the best way is look no touch side by side and can't give a time line becasue it all depends on the ducks .A large dog crate inside the coop...