"That bird is a bad bird."
Translation:저 새는 나쁜 새다.
This is a little trick we call skipping syllables! 새이다 is what it is. I'd imagine they skip the 이 just because it's more natural for their mouth to say "sae-da" rather than "sae-ee-da"
Actually, it's not the infinitive form. It's the plain form. https://www.howtostudykorean.com/unit1/unit-1-lessons-1-8/unit-1-lesson-5/
Somehow being an 아가새, over the last few years I've actually had reason to say this sentence a number times