Ambrose of Milan — as excerpted in the Catena Aurea on Luke 12:4-7

“Or else; A good sparrow is one which nature has furnished with the power of flying; for nature has given us the grace of flying, pleasure has taken it away, which loads with meats the soul of the wicked, and moulds it towards the nature of a fleshly mass. The five senses of the body then, if they seek the food of earthly alloy, cannot fly back to the fruits of higher actions. A bad sparrow therefore is one which has lost its habit of flying through the fault of earthly grovelling; such are those sparrows which are sold for two farthings, namely, at the price of worldly luxury. For the enemy sets up his, as it were, captive slaves, at the very lowest price. But the Lord, being the fit judge of His own work, has redeemed at a great price us, His noble servants, whom He hath made in His own image.”