Bush Cherries are very small bushy trees that bear quite heavily, and most years. The fruit are the size and shape of sour cherries. I have Jan, Joy and Joel , which where bred by Elwyn Meader of UNH fame (Prunus japonica x Prunus jacquemontii). He worked on these for 35 years before releasing them. Joy is self fertile. Joel seems to be the most vigorous here in Zone 4. The flavor is tart, but I like them.
I also have Nanking Cherry (Prunus tomentosa), another very vigorous shrub. This plant is the one that got me started on Bush Cherries, seeing one absolutely covered in fruit in a year when my standard cherry trees were bare… The really interesting thing about all these cherries is the deer don’t browse them! All the other cherries I have get browsed to stubs.