Question

why was the Nightingale a threat to the frog

Answer 1

Nightingale was a threat to the frog because she sang much better
the creatures in the blog loved her singing.
She drew everyone's attention.
:-)
hope it helps

Answer 2

The Nightingale was having a melodious voice which I had the power to sooth the heart of each creature living in the Bingle bog... creatures used to listen to her with intense love .. as we know that the frog had a horrible voice.. people use to pelt stones and sticks and some even requested him to give up singing but the frog didn't listen to anyone... people annoyed with the voice of frog and when the nightingale approached with the magnificent voice.... the creature starting neglecting frog and all the attention and limelight went to the nightingale and hence The Frog considered Nightingale as a threat to himself..

hope it helps