https://en.wikipedia.org/wiki/Duck_typing#Structural_type_sy...

> Duck typing is similar to, but distinct from structural typing. Structural typing is a static typing system that determines type compatibility and equivalence by a type's structure, whereas duck typing is dynamic and determines type compatibility by only that part of a type's structure that is accessed during run time.

With structural typing, the type system will check that the "duck" not only quacks but also looks like and walks like a duck, before it is allowed to be asked to quack.

EDIT: of course with small enough interfaces you can get structural typing pretty close to duck typing, if you have interfaces "QuacksLikeDuck", "WalksLikeDuck", "LooksLikeDuck", etc. instead of one big "Duck" interface.