They could be something wonderful. We would have to see the cocoon in order to identify what it is exactly.
Eastern tent caterpillar nests are very common in the landscape at this time. Perhaps this is what you are seeing. They are fond of cherry trees, but they are native and provide tremendous food source for birds, and are only there for a short time. They may eat a little of the leaves, but they won't kill an established tree. We recommend breaking up the nest with a stick to let the predators eat them, but otherwise just leaving them alone.
Here is our website fact sheet about them and how to handle them:
And more info: