The Rat King has constantly addressed the penguins as "birds" instead of addressing them specifically as "penguins", except only once. He seems to use the word 'bird' as an insult. Because of this he might think that birds are inferior, much like how Skipper thinks that mammals are inferior. This list shows every chronological quote hinting The Rat King's belief of bird inferiority.
You and me, bird. One on one."
I'm talking about no help from the flock!"
Is that the best you got? Bird that is some weak sauce."
How do like me now, flightless bird?"
Aw gee bird, you don't look happy to see us."
What do you say fellows? Shall we give these "birds" another whooping?"