Thread: Sawing black ash bird house lumber.

Can't really comment on shrinkage of that timber but its more a concern as to the shrinkage in width
A 6" board could end up 5½" wide which would be more concern than a slight reduction in thickness.
I'd hazard a guess that ⅞" would dry to a fat ½", add another ⅛" for dressing.
¾" should be okay.
Caveat is the way you cut. ¼ sawn is traditionally the most stable. Back sawn could cup.
Wide boards that cup could reduce your thickness more than acceptable