This is in response to a friend’s request for information on a natural-looking bluebird house. I have made several of these out of appropriate sized hollow tree trunks (just compare to a commercial bird house for approximate dimensions). After finding and cutting the hollow log into reasonable lengths, I use a large screwdriver to scrape out the interior (if needed) so as to have a decent cavity. The entrance hole is made with a one-inch diameter drill bit. I usually rough up the edges a bit to enlarge it slightly and to make it look more natural. The top and bottom of the house are made with rough lumber. I pre-drill and attach with wood screws. The roof has an overlap all the way around to provide some shelter from rain.
The mounting bracket is made with a one-inch strip of aluminum (Lowes, etc. carry all of these pieces) cut to a length that allows it to be doubled over for inserting into the mounting pole. Drill two holes for the bracket to attach it to the back of the house. I am careful to use the shortest possible wood screws so there is not a sharp screw tip inside the nest cavity that might injure the birds. Then bend the bracket so the free end slides down into the mounting pole. I usually compress the top of the mounting pole with a hammer so the bracket slides easily down into the narrow slit. One and a quarter-inch diameter electrical conduit is used for the long top mounting pole (I usually use 10 ft lengths and cut them in half). It then slides over a shorter, smaller diameter rod or piece of re-bar driven into the ground with a sledgehammer. Strike the conduit with a hammer a foot or so from the bottom to create a dent that will act as a stop when it hits the top of the ground pole as you slide the conduit down over it. The location of the dent allows you to adjust the height of the unit as needed. The boxes tend to last a few years depending on the durability of the hollow log. I rarely clean out these boxes but they are used many times (after all, no one is cleaning out the natural cavities out there and some studies of nest boxes have hinted that parasites of nest parasites increase in number, thereby reducing nest parasite populations, if next boxes are not cleaned out frequently). Before setting in the box, I often finish the pole system off by slipping a length of 4 inch diameter PVC pipe over the mounting poles. Some say this may help deter snakes from climbing into the nest box. If I use it I always put sticks or grass or other debris down into the top of the PVC to prevent any birds from getting stuck in that pipe.
All of this effort is to provide a natural-looking cavity for the birds and a photographic backdrop for me. I usually put a pop-up blind near the nest box and allow the birds to acclimate to it empty before I start entering and taking pictures. Below are a couple of images from one such nest box in late afternoon light.