What do deck feathers refer to?

Deck feathers specifically refer to the central feathers in the train of a bird, particularly in falconry and ornithology. The train is the elongated tail feather portion, and the deck feathers contribute significantly to a bird's flight dynamics and overall tail shape. These feathers play a crucial role in aerodynamic control during flight, allowing for better maneuverability and stability.

In contrast, the other choices involve different parts of the bird. The tail feathers used for flight are part of the train but encompass more than just the central feathers, while the feathers on the wings pertain to the primary and secondary feathers that enable flying. Finally, the feathers surrounding the beak are related to the facial area of the bird and do not have any connection to the structural function of flight. Therefore, the definition of deck feathers is accurately captured by identifying them as the central feathers in the train.