Another easy one. In the second hub world there is a level called Egg Roll. It's a bit left from the beginning point of the hub. Go into the level, cross the first bridge and turn left. Cross the partially broken bridge and you will find a small pen (fenced area). Nearby you will find 8 chickens, which you need to gather into the pen by hitting them with
. Animal cruelty, I know. It's very easy to find all the chickens, just gather all the coins etc. to make them appear. If you're struggling, there's a video below and the chicken gathering begins at 2:37. Credit to The 8-Bit Arcade for the video.
You will be rewarded with an achievement and the letter "U".