。
Bats aren't birds — they're mammals.
Literal
Bat [topic-は] bird is-not, mammal is.
A zoology fact. 蝙蝠 (こうもり) is 'bat' — the kanji is considered difficult, often written in hiragana in modern text. 哺乳類 (ほにゅうるい) is 'mammal' (literally 'nursing class'). The construction 'A ではなく B' ('not A but B') is the formal contradictive pairing, common in educational writing.