The duck delivers the notes to Farmer Brown. This can be inferred from the context of the question, as the other options (cow, pig, and newspaper boy) are not typically associated with delivering notes. Additionally, in the popular children's book "Click, Clack, Moo: Cows That Type" by Doreen Cronin, the story revolves around the animals on Farmer Brown's farm, including a duck, who communicate with him through notes. Therefore, it is reasonable to conclude that the duck is the one delivering the notes to Farmer Brown.