It's because the bird is not what it appears to be, because its outward appearance does not reflect what comes from inside. And isn't that often where some of the best stories are found? In something unexpected.
As you can see from the picture over there <---- the nightingale isn't the loveliest of birds. It's just a common brown bird. Even its feet and beak are brown. But to hear it sing! Listen here:
http://www.scricciolo.com/eurosongs/canti.htm (scroll down to the scientific names Luscinia luscinia and Luscinia megarhyncos)
I first became enchanted with the nightingale in the form of a children's story, in which the nightingale is jealous of all the other birds for their beautiful plumage, their speed, strength, magnificence, but learning in the end that what he has to offer the world--his song--is at least as good and beautiful and strong and magnificent as all of these.