Ideally, as I have witnessed and observed the optimal size is approximately 22 x 22 x 24 with a nest hole cut approximately 6-8” high into the side. With a hole the proper size for a Barn Owl you can have a perch on the outside because a predator like a Great Horn Owl will not be able to get it’s bigger head in.

Also a thought would be some kind of perch on the inside at the hole for more than one baby to sit on and watch the world as they mature. Along with those suggestions I feel it is wise to have a front porch across the front of the nest box for all the babies to congregate along with some type of rough strapping on the roof since this is a point of take off, landing and wing flapping so they need to be able to grip with their talons while they do this activity.

Watching these nests I noticed how much room the young owlets need once they are ready to start moving around more with their jumping, taloning nesting pellet material and wing flapping behavior. The small boxes don’t allow for much movement at all, just simple wing spread/stretch that most times is getting impinged from the sides, roof and siblings.

Also noted is that in the heat of the day the owlets lay down, stretch out and pant to cool their selves. In the smaller nests it isn’t possible to do that and another big factor is no air circulation to help cool and the addition of the siblings body heat must make it miserable to endure the afternoons until the sun goes down to give them much needed relief which brings me to another point about ventilation.

Simply slapping a wooden box together does not make a safe or comfortable nest, there has to be some kind of significant air flow and drainage provided because if there isn’t then there is a great possibility for heat exhaustion, bacteria multiplying and other problems developing from stagnate air, decaying prey, pellets and poop (which BTW in avians is a combination of urine and feces).

In doing further research regarding the "perfect" nest box, I stubled upon some thing called a "Roosting Room" that makes a lot of sense, especially for the nests on poles in agricultural settings.   A roosting room for the parents to perch in during the day while the young occupy the nesting area. This room usually has the same size opening as the main cavity, and is crossed with perches with 14 inches or so of head space. No floor is recommended for this room so as to permit castings and fecal material to fall to the ground.  I found this info in a site that talks about using Barn Owls for Rodent control and think this is an excellent option to add to the Barn Owl nest box.  Besides being a place for the parents to roost during the day it would also be ideal for the young owlets to use before fledging to get a good picture and idea of what the real world has to offer like nothing under your feet.  Just a thought but I think worth mentioning and worth doing in agricultural or other places where there isn't a lot of perching opportunities.  This also provides a place for hiding when there are predators around to.

There are laws against leaving pets in closed vehicles, etc., but nothing to protect the wildlife and little lives that are depending on us using our brains. In open nests in trees the babies by instinct raise their little behinds up and all most over the side of the nest to excrete keeping the nest clean. That isn’t the case in these enclosed nest boxes and I have been so fortunate to know how bad they can smell.