This sort of thing happens all the time. Over the last 7 years i have pulled several ducklings out of catch basins. The hen will always stick around flipping out while you retrive them. Once out they will run right back to her. We had to put portable gates on the catwalks on our secondary clairifiers to prevent the ducks from getting in. The Hen is able to fly out of the tank but the little ones can not. It takes a long time scooping them out with nets. As a side note be very careful entering catch basins as they may contain H2S gas which will kill you.