The first part of this quote, from Act III, scene i, is a song that Bottom is singing in the forest to try to keep his courage up.  He has just had his head turned into that of an ass, which scared all of his fellow actors away from him, leaving him alone in the forest.  His singing is what awakens Titania, the fairy queen, who promptly falls in love with him because of the love potion that was put into her eyes.

You can see a side-by-side translation of these lines at the link below (midsummer-text/3403), but it's important to realize that this is an example of Shakespeare's familiarity with rural animals and themes, which he used extensively in his plays.  He was not always a Londoner - He grew up in a more rural area of England, and so was familiar with the flora and fauna of that region.

Bottom's song is quite appropriate and ironic for him to sing as he is comparing the qualities of a variety of birds, ending with the line,

"Who would give a bird the lie, though he cry 'cuckoo' never so?"

Indeed, who would give much attention at all to such a silly bird (or man, like Bottom) as the cuckoo?

Be sure to check the links below! :)