During our experimental boxes we also have discoverd that an oval hole is very feasible for the nestlings when they are near fledge. Making it easier for 2 fledglings to peer out at the new world with lessening the situation of accidental falling from the nest box. This size in no way has increased predator infiltration.
The oval opening size is as follows. 4 inches wide & 3 inches high. Photo included.
Over the years we have increased our box size to accommodate our larger Kestrel families with no impacted on their nesting procedures. So we agree larger box sizes are needed.
Thank you all for watching our live stream and all the nice comments,
Nu-Sun Cinema Winnipeg, Manitoba, Canada
Link to KESTREL KORNER http://www.nu-sun.com/html/kestrel_korner.html