From the Super Mario Wiki
Golden Feathers, otherwise known as Feathers, are items that only appear in the Game Boy Advance remake of Donkey Kong Country 2: Diddy's Kong Quest. Their purpose is for strengthening Expresso's stats to increase his capability of participating in Expresso Racing.
When Diddy Kong and Dixie Kong first enter Cranky's Hut, and the player selects the "Cranky's Surprise" option, Cranky introduces Expresso and its racing mini-game, Expresso Racing. He tells the Kongs to find least four Golden Feathers for Expresso to join the first race, "Ship Deck Dash". Cranky gives them one feather to start out with. Diddy and Dixie can find one in each level, even before they first meet Cranky.
After the Kongs apply all four Golden Feathers, Cranky allows Expresso to participate in the first race "Ship Deck Dash". The later races are more challenging, meaning that the Kongs must collect and add more Golden Feathers to Expresso's stats.
There are a total of forty Golden Feathers. They all have a one-time use to add one point to one of Expresso's stats: Strength, Speed, Flight, or Boost. All four stats can be filled in up to ten points, meaning that all forty Golden Feathers fill up all of Expresso's stats entirely.