Consider the possibility that the woodlots where you have traditionally heard or seen Wood Thrushes have now been seriously degraded by deer that eat all the understory.  This is the first year in the past forty that we have no Acadian Flycatchers breeding behind our house.  At one point this past winter we counted 18 deer passing upstream.  It's an area-wide problem.

Paul O'Brien
Rockville, Mont. Co., MD