Terenty referred to the nightingale as a beautiful creature chosen by god. As the sparrow is comparable to a thief, a cunning and troublesome bird.

A Nightingale is a singing bird, without sin. He had a voice given to him  in his throat, to praise God and gladden the heart of man. He is like a pastor or a priest who share the words of God.
while on the other hand...
A Sparrow is a bad spiteful bird who is like a pickpocket in his own ways unlike the nightingale he doesn't like man to be happy. And it is said that when Christ was crucified it was the sparrow who brought nails to the Jews, and called 'alive!, alive!'
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