I used to believe that ducks grew up to be swans. I think this has to do with the fact that there is a pond near-by with ducks and swans on it. And also the tale of the "Ugly Duckling" where the duckling did actually become a swan.
A childhood belief about birds

others believed things like this
- My mother's name was Anna. She was called "Annemor"(Duck mama). I was convinced I was a duckling.— Christine Norway
- i used to believe that swans were mythical creatures like unicorns or a phoenix.
— amber - i belived that ducks would take over the worldAnon
- I thought ducks couldn't fly - thats why they swam.— G
- I used to believe that dumplings were made from baby ducks. — Sarah
- When I was little, I had tiny toes and beleived that ducks mistook them for kernels of corn and eat them.— Seguin