Back in the Spring of 2015, I proudly mounted two owl houses on wooden posts -- 12' in the air and equipped with snake baffles. I thought the design ensured the safety of future nesting birds. A Tufted Titmouse quickly took over one of the houses that Spring and produced a brood of 6. Late one night in April, the nestlings were preyed upon by a Flying Squirrel. I was sick. If you really want to feel ill also, you can watch the video footage of this attack -- but I don't encourage it.
I very much enjoy watching the Flying Squirrels and stick by my ground rule of "do no harm". What possible guard could one devise to protect a nest from a creature that glides through the air while retaining access for birds? I began designing. My first design, as you will see in the video, was to sheet the house in acrylic with large wings extending out from the top and sides. When that failed, I added "spikes" made of long finishing nails to the top of the post (to prevent the squirrels from landing above the house), and added a baffle directly below the house. After the failure of these designs, I concluded Flying Squirrels are smarter than I am.
A New Design
The next fall, I came up with a new design. It is pictured here. I understand it looks both mediaeval and evil, but so far it is performing its purpose. The "armor" consists of a single sheet of acrylic applied to the front of the owl houses, and closely-spaced carpet tacks applied in bands around the top and front. The application of the tacks is intended to discourage Flying Squirrels from stretching from top-to-side, or side-to-front, of the house. The tacks orientation away from the front of the house would prevent a fledging bird from coming in contact with the tacks.
The removable armor is only applied when a bird has shown intent to nest in the house -- or when the Northern Flicker has used a house for winter roosting. In this way, so far, Flying Squirrels happily use the dis-armed houses for nesting in the fall and winter -- and birds have an increased level of safety should they choose to nest/roost (armor applied).
I remain cautiously optimistic while humbly-aware ... flying squirrels may once again prove they are smarter than I am.