The ornamental design for a birdfeeder, as shown and described.
FIG. 1 is a front view of a bird feeder showing my new design;
FIG. 2 is a right side 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 rear view thereof; and,
FIG. 7 is a 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.