Craft shops and exclusive boutiques are stocking a new accessory for the season: birdhouses. Birds and their human buddies flock to these high-style havens, and for good reason. Far from being run-of-the-mill rectangles with roofs, these are whimsical reflections of human architecture, more contemporary folk art than anything else. Used indoors, they offer an alternative to the recently popular bird cages as decorative accents. Used outdoors, they shelter the birds and can transform a corner of the garden into something rather remarkable.