I have put up many Kestral boxes. In the past I put them up in Power plant sub stations to discourage house sparrows from nesting in transformers. It saved consumers hundreds of thousands of dollars a year. It also encouraged a native specie to thrive. Most boxes are made of 1x lumber. 1x10"or 12" work great. Kestrals need at least 10" deep below the hole. A 2.5" to 3" wide hole is all you need. Put just 1 handful of saw dust in the bottom. No more. Drill a couple of small vent holes in the sides at the top. Try to put a hinge so you can clean it out some where. Door or roof. Kestrals are blasters the young will paint the inside white with fecal material. Put the box in the open on a pole if you have one. 10' to 15' up faceing south our East. A dead tree works. Some times you can get success with the side of a barn too. A hing door helps to dump squirrels or starlings. If you get a pair of Kestrils and their four offspring, at one mouse a day they could eat 135 pounds of mice and grasshoppers each year. How much is your time worth setting traps, buying mouse traps, Or loss of grain eaten by mice and house sparrows. A screech owl is the night time equivalent. Same box but in a tree Apple orchard is ideal. Neither would ever go after a medium for full size chicken. maybe on a chick but you would have them in a pen any way. Why not get night and day protection! both are awesome animals and a perfect addition to any chicken yard!