The ornamental design for a birdcage, as shown and described.
FIG. 1 is a front perspective view of the birdcage in accordance with the present invention, with a separate stand therefor being shown in broken lines;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a left side perspective view of the birdcage;
FIG. 4 is a right side perspective view of the birdcage;
FIG. 5 is a top plan view of the birdcage; and,
FIG. 6 is a bottom plan view thereof.
The broken lines are for purposes of illustration only and form no part of the claimed design.