First up, please can you tell us where they are growing? Geographically, I mean. Secondly, can you tell us what bushes and trees are growing round about?

Pennsylvania, just west of Philadelphia. Mostly tall trees. The leaves and branches are very high up, so I am unable to see the leaves very well and cannot figure out which branches might have these berries growing on them. The berries seem like they're all over the yard, so it's had to tell if there is just one tree or several dropping these.

Prunus virginiana var. melanocarpa?

I'm in the exact same location as you are, and these are littering our driveway, coming from a black cherry tree in our neighbor's yard.