"I am not a duck, but an owl!"

Translation: Ich bin keine Ente, sondern eine Eule!

[SydneyBlakem]

Why is "........aber eine Eule " wrong ?

[MortiBiRD]

In most cases, you use sondern when the first part is negated.

Also theres a huge tendency to use sondern, if both "contents" exclude each other.

[SydneyBlakem]

Thanks, aber( sondern??) Deutsch ist verrückt !

[az_p]

That would be aber, because you're not negating your thanks by saying German is crazy.