Open a used 4 inch diameter tin can on both sides. Use a normal can opener, the ones that leave the lip intact.
Print both front and back part. Remove support.
Nail or screw the back part unto a tree or a wall about 1 to 3 meters above ground to attract eastern bluebirds.
More models for different type of birds will come.
I suggest to use ABS with a FDM machine to have a good clip!