Many designs of boxes can be made with this mixture. It has the advantages of being fairly predator-proof and of being easy for use in mass production. The disadvantages include the potential difficulty of making the initial mould. The sawdust/cement mixture can be sawn, screwed or nailed once it has set and hardened although initial attempts with these boxes often end in a heap of fragments. The material is suitable from House Martin and Swallow boxes, free hanging tit boxes and many other designs. This material is widely used on the continent from factory, rather than home, produced boxes. Production of these boxes require practice.
Method: Make a mound from wood or plastic. Soak fine sawdust overnight in a solution containing a wetting agent (any not too frothy detergent). Squeeze surplus moisture out of the sawdust and allow it to dry. Mix the dried sawdust with fresh cement in the proportions 1:3½ by volume cement:sawdust. Add water to make a fairly wet mixture. The mould should be wetted with a strong soap or detergent solution. Pour the mixture into the mould, press down very firmly and allow to dry. Fitting (Hooks, locking tabs etc.) to the box may be attached using a strong adhesive. Fittings should be of a rust proof material. It is essential that the sawdust is thoroughly wetted and that the mixture is very firmly pressed into the mould. Failure to ensure these two details will make a porous box, which cracks when exposed to frost – if not before!
I would recommend you by the Nest Box book by Chris Du Feu, Available from the BTO – BTO Guide 23