Trink, Trink, R├Ąttelein, Trink

rats in the beer garden

Linguistic purists will argue that rats do not take an umlaut, so the word is properly Rattchen or Rattlein. In reply I direct your attention to other irregular declensions such as

I am firm, you are stubborn, he is a pigheaded so-and-so

or, as it were,

I am a purist, you are a spoilsport, he is a boring old poop.

The song they are singing is Ach Du Lieber Augustin. Do not ask me how I know this.