John Lightfoot on Luke 12:6

[Are not five sparrows sold for two farthings?] Two sparrows were sold for one farthing, and five for two. We find that doves were sold in the Temple upon the account of women in childbed, and their issues of blood, by whom a pair of turtles and young pigeons were to be offered, if they had not wherewithal to present a more costly sacrifice. So probably the sparrows were likely to be sold upon the account of lepers, in the cleansing of whom they were made use of, Leviticus 14:4. I confess the Greek version in this place hath not two sparrows, but two little birds. And yet if you will believe the far-fetched reason that R. Solomon gives, you will easily imagine that they are sparrows that are pointed at: "The leprosy (saith he) came upon mankind for an evil tongue, that is, for too much garrulity of words: and therefore in the cleansing of it they used sparrows that are always chirping and chattering with their voice."

[And not one of them is forgotten before God.] "R. Simeon Ben Jochai standing at the mouth of his cave [wherein he lay hid for the space of thirteen years], he saw a certain man catching of birds. And when he heard Bath Kol out of heaven, saying, 'Mercy, mercy,' the birds escaped: but when he heard Bath Kol saying, 'The pain of death,' then was the bird taken. He saith, therefore, A bird is not taken without God, much less the life of a man." This passage is also recited in Midras Tillin, but the circumstances vary.