The ornamental design for a birdhouse, as shown and described.
FIG. 1 is a top perspective view of a birdhouse showing my new design;
FIG. 2 is an elevational view taken from one corner thereof, the other
corners being the same as the corner shown;
FIG. 3 is a top plan view thereof, the bottom being the same as the top
shown; and
FIG. 4 is a side elevational view thereof, the other sides being the same
as the side shown.
The fragmentary broken line representation of a support pole in FIGS. 1, 2,
and 4 if for purposes of illustration only and forms no part of the
claimed design.