The ouzel cock, so black of hue,
With orange-tawny bill,
The throstle with his note so true,
The wren with little quill –
…The finch, the sparrow, and the lark,
The plainsong cuckoo gray,
Whose note full many a man doth mark
And dares not answer "nay."
– William Shakespeare
A Midsummer Night’s Dream, Act 3, Scene 1. Bottom is singing loudly in an effort to keep his courage up and show that he isn’t afriad. He has just had his head transformed into an ass’s head, which has frightened away all of his fellow actors, leaving him alone in the forest. We can take it that Bottom’s singing is as bad as his acting, and probably more like braying since his transformation.