I don't know, I looked at the whole video, second by second at least ten times now (including going back and forth about 20 times at the 5 second mark) and I only see 11 from beginning to end. Also, there's some kind of blue cover on the "grate" that doesn't look like a duckling could fall through. I even looked at it full screen in as high a resolution as possible.
I would hope the cameraman would have gone and checked after he stopped filming.
I could see, though, where the confusion may come from at the 5 second mark because it looks like he panned over and there was one duckling that looked like it was off screen and there were 11 onscreen, but I think there was some editing or zooming done there and a little distortion.