These Quicklock Mason Bee Stacking Trays are made from biodegradable corn plastic trays are interlocking and long lasting. Easy to assemble and clean, you can create your own shelter design (try a birdhouse look or a pipe) to protect the trays from weather. Set the stack of 5 (2part) trays horizontally into a dark container 5-8cm (2-3") deeper than the 15cm (6") trays. One set of Quicklock Mason Bee Stacking Trays provides 30 nesting sites.