Ah, I was finally able to figure out the "joke," as I only got the "flat" earlier as I didn't know "huhu." I had just searched for "eulenwitz," and this thing popped up a million times. I knew "uhu" but I was too tired yet to notice the extra H to figure it was a pun. I knew one of you would lead me to the answer though!
For everyone not in on it:
Was sitzt auf einem Baum und winkt? — Ein "Huhu“!
What is sitting on a tree and waving? – A “Huhu”!
Explanation: in German, “huhu” is a greeting, while “Uhu” signifies an “owl”
From The Funniest One and Only Multi-Language Jokebook