We claim the ornamental design for a bird feeder, as shown and described.
FIG. 1 is a front elevation view of a bird feeder showing my new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top perspective view thereof; and,
FIG. 8 is bottom perspective view thereof.
The broken lines and unshaded portions contained within broken lines depict unclaimed features of the bird feeder that form no part of the claimed design but are shown for purposes of illustration.