Which one of these sentences is correct:
A bat is not a bird, but a mammal.
Bats are not birds but mammals
The bats are not birds but mammals.
Both of the examples:
- A bat is not a bird, but a mammal.
- Bats are not birds but mammals
are correct. The choice of whether to use singular or plural forms is purely one of style. The meaning is identical. The third example:
- The bats are not birds but mammals.
is not incorrect. It uses "the bats" to refer to bats as a (specified) group. It is a bit more formal, and the extra words do not, in this case, add anything to the meaning. Some writers use a definite article with a singular form in such a construction, such as:
- The bat is not a bird, but a mammal.
This is not grammatically incorrect, but in my view is always a mistake. It implies that bats are a singular kind of thing, whereas in fact there are a number of different species of bats. This form disregards those differences, and indeed implies that they are unimportant. The late biologist and essayist Stephan Jay Gould wrote that the use of such forms as "The bat", "the whale", or "The monkey" when referring to entire wide groups was generally a mark of careless and lazy writing about nature. I agree, and advise avoiding such forms.