Frass, Droppings, Spore, Poop... What ever you call it, this is a big stinking pile of privet seeds.
One privet bush can produce thousands of seeds every year. These hard round black seeds have a multitude of ways of finding fertile ground. First is the structural nature of the bush itself. It grows in long arching branches that sprout from a central root ball. These branches drop seeds away from the root ball onto new open ground. The branches are also very wiry so in high winds they can act like whips to fling seeds further a field. Due to the small round shape of the seeds, they are easily washed into storm drains and creeks, this being probably the most common form of seeding in urban areas. Birds and some animals can eat the seeds, which are resistant to the acids in the digestive system, so they are excreted later in perfect condition to sprout in new locations.
Please note that privet seeds are slightly poisonous to humans, but they will kill a horse if eatten them.