Learn Italian: Il volare silenzioso è essenziale per il gufo. - Silent flying is essential for the owl.

While you could say il volo silenzioso, using the substantivized infinitive il volare emphasizes the continuous action or physical process of flying. It draws attention to the act of doing it, rather than just the abstract concept of a flight.

Since volare is a verb, shouldn't it be modified by an adverb like silenziosamente?

Normally verbs take adverbs, but here volare is acting entirely as a noun (substantivized by the article il). Because it is functioning as a noun, it must be modified by an adjective instead. All substantivized infinitives are treated as masculine singular, which is why we use silenzioso.

Could il gufo refer to owls in general here, rather than one specific bird?

Yes, and it most likely does! In Italian, using the singular definite article plus a noun (like il gufo) is a standard way to make a generalization about an entire species or category. In English, we usually default to the plural without an article ("owls") for this same meaning.