What part of speech is fluttering?

To fly with light, swift wing movements or to move the wings lightly and swiftly up and down in flight, as a bird. similar words: flit

it means: (of a bird or other winged creature)

Explanation:

prime examples)

to fly with light, swift wing movements or to move the wings lightly and swiftly up and down in flight, as a bird. similar words: flit

to beat or pulsate swiftly or irregularly. Her heart fluttered when she heard the news