Woodcrete.

Anybody who has known me for any length of time will know that birds (no not that sort) figure fairly largely in my life.  I know there is no such thing as a free lunch but this is as near a free nest box as you can get.  I took up an offer for reduced price membership of the BTO and this was "thrown in" as an extra.  Quite a throw as they are about £20 to buy.

Woodcrete is an interesting material it makes a sort of nuclear bunker for birds.  It feels like concrete and I think has concrete or at least cement in it but has sawdust as a bulking material making it warmer and breathable. 

The nail at the bottom of the picture is for hanging and is the largest aluminium nail I have ever seen.  Aluminium so that if the tree ever makes its way to a saw mill (or meets a chain saw) the saw blade isn't damaged as it would be on a steel/iron nail.