I have pictures of these worms who are falling out of our River Birch trees. These worms have been in other trees in our yard. These happen to be pictures of a cluster of these who at the base of the tree. They fall on the ground, on other bushes, and are crawling up the fence & along our concrete curbing. They have stripped the trees of their leaves. When you are within 15 yards of the trees you can hear objects hitting the ground. This may be their poop?
The caterpillars are migrating to find a place to spin their cocoons. They have done their damage eating the foliage and now will pupate and turn into moths and lay more eggs to hatch next year. I would spray the caterpillars with an insecticide called Sevin. This is a contact insecticide and will kill them preventing moth and eggs next year.
The trees will leaf back out next spring. Hardwoods are not killed by defoliation unless it happens several years in a row. Then you will get some dieback of branches. Next spring treat the trees with an insecticide called Bayer Advanced Tree and Shrub Insect Control. This is applied to the soil around the tree and the roots will transport it to the leaves and when the caterpillars hatch out and start to eat the leaf they are killed. It will protect the tree throughout the season. Here is a web link to this product. Check with your local nursery for these products.