You are a duck!!

Somebody has written "You are a duck!!" on the other side of this card. These really are ducks but I suspect the recipient was not. You are a duck must have been a term of endearment. It is one of many such terms ...
Cupcake
Cutesy-pie
Cutie
Darling
Doll
Duchess
Duck
Dumpling
Gorgeous
Heart
Honey-bun

They have no rhyme or reason - and are just warm sentiments people in relationships use with each other rather than the names everybody else uses.