It seems from Re'eh 14:18 that Torah considers BATS as birds. Though scientists classify them as mammals. I'm preparing a class on this Parsha and would like to hear comments about this difference.
The paragraph that probably answers your question is:
A system of classification has no independent reality. It is simply a means by which we measure and describes the animal kingdom, depending upon our purpose. For the purposes of science, the animal kingdom is evaluated on its own terms, based on anatomy. For the Torah’s system of classification, the animal kingdom is presented in terms the relationship between animals and human beings, and their perception by the common person. Neither system is more correct than the other; they are just serving different purposes. In the Torah, anything birdlike is classified as ohf - including bats. This is not a scientific error, just a different system of language.
The Passuk (Vayikra 11,13) uses the phrase ואת אלה תשקצו מן העוף לא יאכלו when describing all birds bats and insects
The word עוף essentially means "a being that flies"
This is proven from Tehilim where Dovid Hamelech says:
ואמר מי יתן לי אבר כיונה אעופה ואשכנה
And I said If only I would be given wings like a dove I would fly...
So if one classifies everything that flies into one category everyone would come to the same conclusion to include mammals that fly.
Torah considers BATS [to be] birds.
That doesn't make sense. Was the Torah written in English? If the Torah refers to something that is translated as "bat" with a word that is translated as "bird", that is a fact about the translation, not about the Torah. If you're asking why Ancient Hebrew had a word that refers to both birds and bats, that's not a question about Jewish law, that's linguistic question. Languages develop categories according to what their speakers find pertinent. Presumably, ancient Hebrews found flight to be an issue more pertinent than whether something is a mammal. Biologists, on the other hand, have a strong preference for classification based on common ancestry.