Answer:
The birds' singing filled the park.
Step-by-step explanation:
Gerund - a form that is derived from a verb but that functions as a noun, in English ending in -ing, e.g., asking in do you mind my asking you?.
A gerund is a verb ending in -ing that functions as a noun. In this sentence, singing functions as a noun by performing the action of the verb filled. Therefore, it is a gerund.