Giggle, Giggle, Quack

Farmer Brown is going on vacation. He asks his brother, Bob, to take care of the animals. "But keep an eye on Duck. He's trouble." Bob follows the instructions in Farmer Brown's notes exactly. Is that giggling he hears?
 
Giggle, giggle,
quack, giggle, moo,
giggle, oink...

Duck, the cows, the hens and the pigs are back!