The ornamental design for a shuttlecockfeather, as shown and described.
FIG. 1 is a perspective view showing my new design for a shuttlecock feather in a flexed configuration;
FIG. 2 is another perspective view thereof shown in association with a shuttlecock. The broken lines showing the shuttlecock and additional feather elements are for illustrative purposes only and form no part of the claimed design;
FIG. 3 is a front elevation view showing the design in a flattened configuration;