The ornamental design for a birdfeeder, as shown and described.
FIG. 1 is a left side view of a bird feeder showing my new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a right side 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.