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.
In the beginning of the Torah, the Torah describes various min - types or categories of animals. Each has a distinct definition - animals, birds, fish etc. The defining feature of all bird-like animals is that they fly and have wings. They come from the water but that's the definition. So if you find a winged creature that you don't know how to classify, you have to know if it flies and has wings. The question is really what animal has the potential to fly, and has wings - but doesn't - what is it's status. So an Ostrich is one such example. Then there is the duck-billed platypus that lays eggs is another example of an in-between species.
Rashi (Lev. 11:13) explicitly says that a single min includes various different species.
Acc. to Menachos 29a, it seems even Moshe Rabbeinu was not clear on which animal and species was what without prophetic help.
Often different types of animals share the same name because they have similar characteristics. (for example התנשמת) used in Lev. 11:18 and 11:30. See also Chulin 63a.
See also Rabbeinu Bachya Lev. 11:2 and Lev. 11:43 as well as the Malbim on Lev. 11:29.
So the Torah would easily consider bats to be birds for a number of reasons: 1. They both fly, and therefore can be considered to be under the same general category of winged-flying creatures. 2. They would have similar names for this reason. 3. We're not necessarily expert in the differences between animals and their names in the Torah, so it might well be that we're misunderstanding to which creature the name actually refers.