We put up several kestrel boxes in 2014. One has been occupied by kestrels 2 years in a row and I see that the male is around now (Ohio).
During cleanout In Jan, I found straw nests in 4 boxes - 3 kestrel boxes and one barn-owl box.
a) In 2 of the kestrel boxes and the barn owl box the nest was loosely woven about 4" high with a nest 3-4" diameter and 3" deep. All 3 nests looked similar.
b) in the other kestrel box the nest was 4" thick, packed, with a slight ~1/2" indent in the center.
None of the nests had feathers, sticks, egg fragments,
I've searched and looked at pics of OH cavity nesters but found no matches ie. other nests are distinct via mud, moss additions, mud-lining, ... or are too small.
Starlings have tried to use other boxes but their nests are just piles of sticks.
Any suggestions about what kinds of birds might be utilizing the boxes?