In this context, the question is not necessarily “what is ‘en être’, ” (which has many uses) but rather what is the expression “Qu’en est-il de…”

Here,

“Qu’en est-il des ailes, par exemple ?”

could be translated as

“What about the wings, for example?”

in English.

For this paragraph, the author is questioning the description of bats in French (literally “bald mice”) in saying that the lack of hair on bats (hence “chauve“) is not really the first distinguishable attribute.

The author is suggesting an alternative trait that is seen more immediately recognizable in bats – the wings.