That's a very creative idea, Curiosity, I would not have thought of that! Sounds very logical, and I will certainly bear this method in mind if I ever have problems settling in a new one.
Nosey, I would certainly agree with Curiosity with regard to the age of the duckling. I forgot to emphasise...
I used the "sneak it in at night" method. The duck was older than yours and about the same size as the others though. I just popped her in the coop at night and in the morning they all came out together and did not really notice having a stranger in their midst! The newbie had to learn her new...