What is the common noun in The bird could not fly?

The common noun in the sentence "The bird could not fly" is bird.

Here's why:

* Common noun: A general name for a person, place, thing, or idea. It is not capitalized unless it's the first word of a sentence.

* Proper noun: A specific name for a person, place, thing, or idea. It is always capitalized.

In this sentence, "bird" refers to any bird in general, making it a common noun.