In which sentence is the word singing a gerund?

Question

The birds are singing a cheerful song.

The birds’ singing filled the park.

The hikers enjoyed the singing birds.

The birds kept singing.

Answer 1

The word "singing" is a gerund in the sentence:

The birds’ singing filled the park.

In this sentence, "singing" is used as a noun, describing the action of the birds. In the other sentences, "singing" functions as a verb or part of a verb phrase.